wake turbulence.PDF

7/25/2019 wake turbulence.PDF

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SECTION 2

Pilot and Air Traffic Controller

Guide to Wake Turbulence


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2.iii

Pilot and Air Traffic Controller Guide toWake Turbulence

Table of Contents

SECTION 2

2Section Page

2.0 Introduction .......................................................................................................................... 2.1

2.0.1 Preview .......................................................................................................................... 2.1

2.0.2 The Goal........................................................................................................................ 2.1

2.0.3 Participants and Review Process ................................................................................... 2.1

2.1 Objectives ............................................................................................................................. 2.2

2.2 Historical Examination of the Wake-Turbulence Hazard ..................................................... 2.3

2.3 Review of Accidents and Incidents ...................................................................................... 2.5

2.4 Description/Characteristics of the Wake-Turbulence Hazard .............................................. 2.82.4.1 Wake-Turbulence Formation......................................................................................... 2.8

2.4.2 Velocity Flow Field ....................................................................................................... 2.9

2.4.3 The Hazard .................................................................................................................. 2.10

2.4.4 Vertical Motion of the Wake ....................................................................................... 2.14

2.4.5 Horizontal Motion of the Wake................................................................................... 2.15

2.4.6 Decay Process ............................................................................................................. 2.16

2.4.7 Gaps in Our Knowledge.............................................................................................. 2.16

2.5 Future Wake-Turbulence Detection Technology ................................................................ 2.16

2.6 Air Traffic Control Responsibilities for Maintaining Aircraft Separation ......................... 2.17

2.6.1 Wake-Turbulence Cautionary Advisories ................................................................... 2.17

2.6.2 Radar/Approach Controllers ....................................................................................... 2.18

2.6.3 Tower Controllers ....................................................................................................... 2.18

2.6.3.1 Wake-Turbulence Separation for Departing Aircraft ........................................... 2.18

2.6.3.2 Wake-Turbulence Departure Separation Criteria ................................................. 2.18

2.6.4 Visual Separation ........................................................................................................ 2.19

2.6.4.1 Visual Separation-Terminal Area ......................................................................... 2.19

2.6.4.2 Visual Separation - En Route ............................................................................... 2.19

2.6.4.3 Visual Separation - Nonapproach Control Towers .............................................. 2.19

2.7 Pilot Responsibilities for Maintaining Wake-Turbulence Separation ................................ 2.19

2.7.1 Who Does What and When......................................................................................... 2.19

2.7.2 Communications ......................................................................................................... 2.20

2.8 Wake Turbulence Recommended Visual Avoidance Procedures ....................................... 2.21

2.8.1 Specific Procedures ..................................................................................................... 2.212.8.1.1 Landing Behind a Larger Aircraft - Same Runway ............................................. 2.21

2.8.1.2 Landing Behind a Larger Aircraft - Parallel Runway

Closer Than 2500 Feet .............. ............... .............. .............. .............. .............. .... 2.22

2.8.1.3 Landing Behind a Larger Aircraft - Crossing Runway) ...................................... 2.22


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Table of Contents (continued)

Section Page

SECTION 2

2.8.1.4 Landing Behind a Departing Larger Aircraft - Same Runway ............................ 2.23

2.8.1.5 Landing Behind a Departing Larger Aircraft - Crossing Runway ....................... 2.23

2.8.1.6 Departing Behind a Larger Aircraft ..................................................................... 2.24

2.8.1.7 Intersection Takeoffs - Same Runway ................................................................. 2.25

2.8.1.8 Departing or Landing After a Heavy Aircraft Executing a

Low Approach, Missed Approach or Touch-and-Go Landing.............. .............. . 2.26

2.8.1.9 En Route Within 1000 Feet Altitude of a Large Aircraft's

Altitude ................................................................................................................ 2.26

2.8.2 Avoiding Helicopter Outwash Vortices ....................................................................... 2.27

2.9 Pilot Difficulty in Visually Maintaining Separation........................................................... 2.28

2.9.1 Flightpaths................................................................................................................... 2.28

2.9.1.1 Use of ILS Glideslope ......................................................................................... 2.29

2.9.1.2 Visual Illusions .................................................................................................... 2.29

2.9.1.3 Darkness/Reduced Visibility................................................................................ 2.29

2.9.2 Instrument to Visual Situation ..................................................................................... 2.29

2.10 Pilot Techniques for Visually Maintaining Separation....................................................... 2.29

2.10.1 General ........................................................................................................................ 2.29

2.10.2 Visual Cues for Estimating Leaders Flightpath ......................................................... 2.30

2.10.3 Using ILS Glideslopes for Vertical Separation ........................................................... 2.31

2.10.4 Using ILS Localizer for Lateral Separation................................................................ 2.31

2.10.5 Longitudinal Separation .............................................................................................. 2.32

2.10.5.1 Air Traffic Control Assist .................................................................................... 2.32

2.10.5.2 On-board Radar ................................................................................................... 2.32

2.10.5.3 Time and Distance Methods ................................................................................ 2.32

2.10.6 Establishing Longitudinal Separation ......................................................................... 2.32

2.10.7 Radio Communications............................................................................................... 2.32

2.10.8 Estimating Movement of Wake Turbulence ................................................................ 2.33

2.11 Pilot Responses Upon Encountering Wake Turbulence ..................................................... 2.33

2.12 Cooperative and Efficient Management of Capacity ......................................................... 2.33

2.13 Air Traffic Considerations When Applying Separation ..................................................... 2.34


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2.1

Pilot and Air Traffic Controller Guide toWake Turbulence

SECTION 2

22.0 IntroductionThe Pilot and Air Traffic Controller Guide toWake Turbulence is one part of the WakeTurbulence Training Aid. The other partsinclude Section 1, Wake Turbulence - Over-view for Training Aid Users; Section 3, Ex-ample Pilot and Air Traffic Controller WakeTurbulence Training Program; Section 4, WakeTurbulence Training Aid - Background Data,and a video.

2.0.1 Preview

This Pilot and Air Traffic Controller Guide toWake Turbulence is a comprehensive docu-ment covering all the factors leading to ashared awareness and understanding of waketurbulence. A review of the history of wake-turbulence studies, from the introduction ofturbo-jet aircraft to todays environment, isthe starting point. A description of typicalaccidents and incidents allows a look at trendsand lessons learned from history. With his-tory as a basis, a thorough description is givenof the wake-turbulence hazard. This includes

the formation, effects, and dissipation of thewake vortex phenomenon. A description isgiven of our ability to predict, detect, andmeasure the wake-turbulence hazard. Thisincludes future planned improvements inthese areas.

Given our knowledge of wake turbulence, thebest solution is to avoid the wake-turbulencehazard. This document reviews the existingavoidance guidance and both air traffic con-trol and pilot responsibilities. A discussion isoffered regarding the difficulty for pilots to

visually maintain separation and offers rec-

ommended techniques. A brief discussion ofpilot responses to encountering wake turbu-lence precedes a section that stresses the nec-essary cooperation of pilots and air trafficcontrollers to safely and efficiently managethe busy airport environment and avoid wake-turbulence encounters. Lastly, the impor-tance of air traffic control considerationsassociated with assisting pilots in avoidingwake turbulence is discussed.

2.0.2 The Goal

The goal of the Wake Turbulence TrainingAid is to reduce the number of wake-turbu-lence related incidents and accidents by im-proving the pilots and air traffic controllersdecision making and situational awarenessthrough increased and shared understandingand heightened awareness of the factors in-volved in wake turbulence. This can be ac-complished by the application of knowledge,techniques and training applied to everydayoperations.

2.0.3 Participants and Review Process

The Wake Turbulence Training Aid is theresult of many hours of effort on the part of alarge industry team. This team consisted of:Air Line Pilots Association, Air Traffic Con-trol Association, Airbus Industrie, Airbus Ser-vice Company, Inc., Allied Pilots Association,American Airlines, Aircraft Owners and Pi-lots Association, Air Transport Association,Boeing Commercial Airplane Group, DeltaAir Lines, Inc., Federal Aviation Administra-tion, Flight Safety Foundation, General Avia-


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2.2

tion Manufacturers Association, Hydrolin Re-search Corporation, Independent Pilots As-sociation, International Civil AviationOrganization, McDonnell Douglas AircraftCompany, National Aeronautics and SpaceAdministration, National Air Traffic Control-lers Association, Inc., National Air Traffic Ser-vices (CAA), National Air TransportationAssociation, Inc., National Business AircraftAssociation, National Transportation SafetyBoard, Regional Airline Association, South-west Airlines, The Communications Com-pany, U.S. Department of Transportation, andUnited Airlines.

The team worked on this project over a periodof nine months. During this period the WakeTurbulence Training Aid and associated video

was developed. In all, a total of four reviewcycles were conducted, during which the com-ments and recommendations of all partici-pants were considered for inclusion in thefinal material. Three industry review meet-ings were held along with a final draft/finalvideo industry buy-off process. The FederalAviation Administration is responsible forthe final reproduction and distribution of theWake Turbulence Training Aid. As signifi-cant material is developed and changes arerequired to this document, a review will beconducted by the industry team and appro-

priate updating of the material will be devel-oped and distributed.

2.1 Objectives

The objectives of the Pilot and Air TrafficController Guide to Wake Turbulence are tosummarize and communicate key wake-tur-bulence related information relevant to pilotsand air traffic controllers. It is intended to beprovided to air traffic controllers and pilotsduring academic training and to be retained

for future use.

The Pilot and Air Traffic Controller Guide toWake Turbulence will:

educate pilots and air traffic controllerson wake turbulence and avoidance of thephenomenon.

increase the wake turbulence situationalawareness of pilots and air traffic control-lers (situational awareness being definedas an accurate perception by pilots and airtraffic controllers of the factors and con-ditions currently affecting the safeoperation of the aircraft and the crew).

provide usable information to develop aground training program.

The most important success tool availabletoday to pilots and air traffic controllers toreduce wake-turbulence accidents and inci-dents is awareness and education. One of theobjectives of this training aid is to educatepilots and air traffic controllers on wake tur-bulence and avoidance of the phenomena.This can be done by updating the basic under-standing of wake turbulence to help reduceand clear up common misconceptions andgenerate respect for the hazard. This educa-tion will expand the awareness of pilots andair traffic controllers of their mutual involve-

ment in the avoidance of wake turbulence.Additionally, education will generate base-line knowledge for instructors and thosepeople involved with developing trainingprograms.

Another clear objective is to increase the wake-turbulence situational awareness of pilots andair traffic controllers. This aid will providerecommendations to improve situationalawareness involving wake turbulence andtechniques for detection, avoidance and re-covery. This should lead to shared awareness

and cooperation among air traffic controllersand pilots. Improved situational awareness


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2.3

will better prepare pilots and air traffic con-trollers for future improvements and newtools to cope with wake turbulence.

Lastly, this Pilot and Air Traffic ControllersGuide to Wake Turbulence aims to provideusable information for the development ofground training programs. There are manysources of information about wake turbu-lence. This aid attempts to compile thosesources to provide information for trainingdevelopers. Since simulation capability islimited, the ground training material is devel-oped into written modules, exams, and astand-alone video.

2.2 Historical Examination of the Wake-

Turbulence HazardWake turbulence is a natural by-product ofpowered flight, but was not generally regardedas a serious flight hazard until the late 1960s.Upsets or turbulence encounters associatedwith other aircraft were usually accredited topropwash and later on, with jet wash.Interest in this phenomenon greatly increasedwith the introduction of large, wide-body tur-bojet aircraft during the late 1960s and a con-cern about the impact of greater waketurbulence. This was the impetus to conduct

research to gain additional information anddetermine what safety considerations werenecessary as more and more large aircraftentered the industry fleets.

An investigation of the wake-turbulence phe-nomenon, conducted by Boeing in mid 1969as part of the FAA test program, includedboth analysis and limited flight test and pro-duced more detailed information on wakevortices. The flight tests provided a directcomparison between the B-747 and a repre-sentative from the then current jet fleet, a B-

707-320C. The smallest Boeing jet transport,the B-737-100, was used as the primary wake-turbulence probing aircraft along with an F-86 and the NASA CV-990. Smoke generating

towers were also used to observe the waketurbulence generated by aircraft as they flewby. Several observations were made.

The strength of the wake turbulence isgoverned by the weight, speed and wing-span of the generating aircraft.

The greatest strength occurs when thegenerating aircraft is heavy, at slow speedwith a clean wing configuration.

Initial flight tests produced sufficient infor-mation about the strength, duration and move-ment of wake turbulence to come toconclusions and recommendations on how toavoid it. The wake was observed to movedown initially and then level off. It was neverencountered at the same flight level as thegenerating aircraft or more than 900 feet be-low the generating aircraft. Therefore, a fol-lowing aircraft could avoid the waketurbulence by flying above the flightpath ofthe leading aircraft. While this can be accom-plished in visual conditions, an alternativewas developed for instrument meteorologi-cal conditions. Aircraft were placed into cat-egories determined by their gross weight. Itwas noted that a division based on the wing-span of the following aircraft was a moretechnically correct way to establish catego-

ries; however, it did not appear to be an easilyworkable method. Since there is a correlationbetween aircraft gross weight and wingspan,gross weight was selected as a means of cat-egorizing aircraft and wake-turbulencestrength. Minimum radar-controlled wake-turbulence separation distances were estab-lished for following aircraft. The separationdistances depend on the weight of both theleading and following aircraft. Adjustmentsin separation distances were made as moreinformation on the wake-turbulence phenom-enon was gained during the 1960s, 1980s and

1990s, but the basic concept of using aircraftweights remained constant.


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2.4

Initially, the turbojets that were being pro-duced fit cleanly into distinct categories withlogical break points. For example, heavy air-craft such as the Boeing B-747, Lockheed L-1011 and the Douglas DC-10 were clearly in aclass by themselves. There were very fewregional or business support size aircraft.Today, there is almost a continuum of aircraftsizes as manufacturers developed the air-craft family concept and produced manynew transport and corporate aircraft. With

improved technology, heavier aircraft are pro-duced with better aircraft performance allow-ing them the use of shorter runways thatpreviously could only be used by smalleraircraft. Additionally, a hub and spoke mix of

regional aircraft with heavy jets, coupled withan already active private and recreational air-craft population, results in a range of wake-turbulence strengths produced and potentiallyencountered by a large variety of aircraft, asillustrated below (Figure 2.2-1).

3.00

2.50

2.00

*Relative

strength 1.50

1.00

0.50

0.00B747 MD11 B777 A340 767ER A300 A310 B767 B757 B727 A320 MD80 B737 F100

Aircraft type

Empty weight

Max landing weight

* Relative strength is the strength variation between maximum landing weight and empty weightrelative to a B-737 of a weight midway between its maximum weight and its empty weight.

Figur e 2.2-1

Calculated init i al

vort ex strength


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2.5

The wake-turbulence separation criteria, whilenecessary, are currently a limiting factor inseveral airport capacities. The FAA is work-ing with NASA to develop and demonstrateintegrated systems technology for addressingseparation criteria. The thrust of the work isto develop wake-turbulence prediction capa-bility, sensors for detecting wake-turbulencehazards on final approach and an automatedsystem to maximize operating efficiency whilemaintaining safety standards.

The effort to gain more information aboutwake turbulence continues.

2.3 Review of Accidents and Incidents

National Transportation Safety Board datashow that between 1983 and 1993, there wereat least 51 accidents and incidents in the UnitedStates that resulted from probable encounterswith wake turbulence. In these 51 encounters,27 occupants were killed, 8 were seriouslyinjured, and 40 aircraft were substantiallydamaged or destroyed. Numerous other en-counters have been documented in the NASAAviation Safety Reporting System (ASRS).Since participation in ASRS is voluntary, thestatistics probably represent a lower measureof the true number of such events which oc-

curred. The following are accounts of realevents.

1. A pilot of a medium transport (60,000+pounds) was told to expedite the takeoff be-hind a large transport (150,000+ pounds) onrunway 32L at Chicago. He began his takeoffroll as the large transport rotated. The largetransport went straight ahead and the pilot ofthe medium transport was instructed to turnto 180 degrees. He started the turn at 300 feetAGL with 15 degrees of bank angle. The bankangle violently increased to 30 degrees from

the apparent wake turbulence of the largetransport.

The takeoff was initiated about 30 or 40 sec-onds after the first aircraft.

2. A Cessna Citation 550 crashed while on avisual approach. The two crew members andsix passengers were killed. Witnesses re-ported that the aircraft suddenly and rapidlyrolled left and then contacted the groundwhile in a near-vertical dive. Recorded ATCradar data show that at the point of upset, theCitation was about 2.78 nautical miles (about74 seconds) behind a B-757. The flightpathangle of the Citation was 3 degrees and theflightpath angle of the B-757 was 4.7 degrees.Standard IFR separation (greater than 3 nau-tical miles) was provided to the pilot of theCitation. About 4.5 minutes prior to theaccident while following the B-757 at a dis-tance of 4.2 nautical miles, the pilot requestedand was cleared for a visual approach behindthe B-757. After the visual approach clear-

ance was acknowledged, the speed of theCitation increased while the speed of the B-757 decreased in preparation for landing. Thecontroller informed the Citation pilot that theB-757 was slowing and advised the pilot thata right turn could be executed to increaseseparation.

Although radar data indicate that, at any in-stant, the Citation was at least 600 feet higherthan the leading B-757 during the last 4 milesof the approach, the flightpath of the Citationwas actually at least 300 feet below that of the

B-757.

3. The pilot of a Cessna 182 was executing avisual flight rules approach to runway 32 atSalt Lake City International Airport, Utah.The pilot reported that he was instructed byATC to proceed direct to the numbers ofrunway 32 and pass behind a Boeing thatwas on final approach to runway 35. TheCessna pilot reported that while on final ap-proach, the aircraft experienced a burble,and then the nose pitched up and the aircraftsuddenly rolled 90 degrees to the right. The

pilot immediately put in full-left deflection ofrudder and aileron and full-down elevator inan attempt to level the aircraft and to get thenose down. As the aircraft began to respondto the correct attitude, the pilot realized thathe was near the ground and pulled the yoke


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back into his lap. The aircraft crashed short ofthe threshold of runway 32, veered to thenortheast, and came to rest in the approachend of runway 35. The pilot and the twopassengers suffered minor injuries, and theaircraft was destroyed. The wind was 5 knotsfrom the south.

The approach ends of runways 32 and 35 areabout 560 feet apart. Radar data show that theCessna was at an altitude of less than 100 feetabove ground level (AGL) when it crossed theflightpath of the B-757. The B-757 had passedthe crossing position about 38 seconds priorto the Cessna 182.

4. A Gulfstream IV departed New Jersey on aroutine night trip to Florida with a crew of 3and 2 passengers. The weather was clear withunlimited visibility and smooth air. During aslow descent for landing at approximatelyFlight Level 250, ATC advised the pilot that hemight see traffic crossing from right to left.The Gulfsteam pilot sighted the traffic farahead. At about 15,000 feet and 300 knots, theGulfstream pilot reported that he felt like hehad hit a 20 foot thick concrete wall at 300knots. The flight attendant and passengerswere injured. The passengers were jettisonedto the ceiling and slammed to the floor. Theaircraft was checked for damage and landed

uneventfully.

5. A McDonnell Douglas MD-88 was execut-ing a visual approach while following a B-757to the airport. The crew of the MD-88 re-ported that the aircraft suddenly rolled rightabout 15 degrees and the pilot rapidly de-flected both the wheel and rudder pedal tocorrect the uncommanded roll. Data from thedigital flight data recorder indicate that atabout 110 feet AGL the roll angle reached 13degrees right wing down and the ailerons andrudder were deflected about one-half of full

travel, 10 degrees and 23 degrees respectively.The crew regained control and the approachwas continued to an uneventful landing. Re-corded radar data show that at the point ofupset, the MD-88 was about 2.5 nautical miles(65 seconds) behind the Boeing 757 while theflightpath of the MD-88 was slightly belowthat of the B-757. The flightpath angle of bothaircraft was 3 degrees.

The MD-88 flight crew had been issued avisual approach clearance when the aircraftwas 4.5 nautical miles from the leading air-craft. However, the separation quickly re-duced to 2.5 nautical miles. The MD-88 flight

crew told investigators that they thought theyhad a 4 nautical mile separation at the time ofthe encounter.

6. An Israel Aircraft Industries Westwindcrashed while on a visual approach. The twocrew members and three passengers werekilled. Witnesses reported that the aircraftrolled and the cockpit voice recorder (CVR)data indicate that the onset of the event wassudden. The aircraft pitch attitude was about45 degrees nose down at ground contact. Re-corded radar data show that at the point of

upset, the Westwind was about 1200 feet abovemean sea level and 3.5 nautical miles from therunway. The Westwind was about 2.1 nauti-cal miles (60 seconds) behind a B-757 and ona flightpath that was about 400 feet below theflightpath of the B-757. The flightpath angleof the Westwind was 3 degrees and theflightpath angle of the B-757 was 5.6 degrees.CVR data indicate that the Westwind pilotswere aware they were close to a Boeing air-craft and the aircraft appeared high. Theyanticipated encountering a little wake andintended to fly one dot high on the glideslope.

While receiving radar vectors to the airport,the crews of both aircraft were flying gener-ally toward the east and would have tomake right turns to land to the south. Radardata and ATC voice transcripts show thatthe Westwind was 3.8 nautical miles north-east of the B-757 when cleared for a visualapproach. The Westwind started its rightturn from a ground track of 120 degreeswhile the B-757 ground track remained atabout 90 degrees. The resultant closureangle started at 30 degrees and became

greater as the Westwind continued its turn.About 23 seconds later, the B-757 was clearedfor the visual approach. The average groundspeeds of the Westwind and the B-757 wereabout 200 and 150 knots, respectively. TheWestwind was established on course 37 sec-onds ahead of the B-757. Although thecombination of the closure angle and thefaster speed of the Westwind reduced sepa-


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2.7

ration distance from about 3.8 nautical milesto about 2.1 nautical miles in 46 seconds, theprimary factor in the decreased separationwas the converging ground tracks. Theonly way the pilot of the Westwind could

have maintained adequate separation wasto execute significant maneuvers.

Based on radar data, at the time the visualapproach clearance was issued, the separa-tion distance was rapidly approaching the 3nautical miles required for IFR separation. Toprevent compromise of the separation require-

ment, the controller would have had to takepositive action to change the Westwinds track,or to issue the visual approach clearance andreceive confirmation that the pilot acceptedthe visual approach within 29 seconds.

These cases are extreme wake-turbulence en-counters. In all cases, it was possible to avoidthe encounters if the pilots and air trafficcontrollers had sufficient knowledge of waketurbulence and applied proper avoidance pro-cedures and techniques. Hopefully, this train-ing aid will help prevent similar occurrences.


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2.8

2.4 Description/Characteristics of theWake-Turbulence Hazard

2.4.1 Wake-Turbulence Formation

The phenomenon that creates wake turbu-

lence results from the forces that lift the air-craft. High pressure air from the lower surfaceof the wings flows around the wingtips to thelower pressure region above the wings. A

pair of counter-rotating vortices are thus shedfrom the wings, the right wing vortex rotatescounterclockwise, and the left wing vortexrotates clockwise as shown in Figure 2.4-1.This region of rotating air behind the aircraftis where wake turbulence occurs. The strengthof the turbulence is predominantly determinedby the weight, wingspan and speed of theaircraft.

The wake turbulence associated with helicop-ters also results from high pressure air on the

lower surface of the rotor blades flowingaround the tips to the lower pressure regionabove the rotor blades. A hovering helicoptergenerates downwash from its main rotor(s) as

shown in Figure 2.4-1A. In forward flight apair of downward spiraling vortices are shed

from the rotor blades, as shown in Figure 2.4-1B. This region of rotating air below thehelicopter is where wake turbulence occurs.

Figu re 2.4-1

Wake-turbulence

format ion

Figur e 2.4-1B

Formati on of

heli copter w ake

turbulence

(forw ard f l ight)

Figur e 2.4-1A

Formati on of

heli copter w ake

tur bulence (hover)


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2.9

The early theories, pre-1970, describing air-craft wake vortex characteristics were verysimplistic. They stated that:

1) The vortex strength depended on the size,weight, and speed of the aircraft;

2) The pair of vortices generally descendedafter generation and would separate whenthey approached the ground;

3) The vortex motion was substantially af-fected by the ambient wind.

The lack of field testing prior to 1970, espe-cially of vortices near the ground, precludedan in-depth understanding of vortex behav-ior, and in particular of the decay process.Now, two decades later, the industry recog-

nizes that there are more factors associatedwith wake turbulence.

This section briefly summarizes the currentknowledge of the behavior of wake vortices.Much has been learned about the characteris-tics of vortices, but there are still gaps in ourunderstanding. The weight, wingspan and

speed of the aircraft determine the initialstrength and motion of the vortices; however,the ambient atmosphere (wind, stability, tur-bulence, etc.) eventually dictates the motionand decay rate of the vortices.

2.4.2 Velocity Flow Field

The general flow field of a vortex is approxi-mately a circular flow and composed of thefollowing regions:

The core region of the vortex can rangefrom a few inches in diameter to severalfeet. The outer edge of the core has themaximum rotational velocity of the vor-tex. The maximum core velocity mayexceed 300 ft/sec. The greatest maxi-

mum strength occurs when the aircrafthas a clean wing.

The outer region of the vortex is charac-terized by a decreasing velocity profile.As seen in Figure 2.4-2, this region maybe as large as 100 feet in diameter.

Figure 2.4-2Velocity profil e

Cor

e

May exceed300 ft/sec

Upto100feet


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2.10

2.4.3 The Hazard (Figure 2.4-3)

The usual hazard associated with wake tur-bulence is that the induced rolling momentcan exceed the roll control of the encounteringaircraft. To evaluate the induced rolling mo-

ment, the overall profile of the vortex must becombined with the aerodynamic characteris-tics of the encountering aircraft. Duringflight tests, aircraft were intentionally flowninto the vortex of a heavy aircraft. These testsshowed that the capability of an aircraft tocounteract the roll imposed by the vortex

primarily depends on the wingspan and thecontrol responsiveness of the encounteringaircraft.

Counter control is most effective and inducedroll minimal where the wingspan of the en-countering aircraft is outside the rotationalflow field of the vortex. Counter control ismore difficult for encountering aircraft withwingspans that are relatively shorter thanthat of the generating aircraft. Pilots of shortspan aircraft and high performance aircraftmust be especially alert to vortex encounters.

Wakevo

rtex

flow

fie

ld

Countercontrol

Figure 2.4-3

Induced rol l


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2.11

The response of an aircraft to the usual wake-turbulence encounter is illustrated below inFigures 2.4-4 thru 2.4-9.

Pilots have also reported brick wall en-counters where the aircraft experiences arather abrupt displacement. These encoun-ters seem to occur en route when the encoun-tering aircraft crosses through the wake of thegenerating aircraft.

When approached from above, the down-ward flow between the vortices pulls the air-craft through the wake. This creates anuncommanded descent (See Figures 2.4-4and 2.4-5).

Glideslope

Figure 2.4-4

Ai rcraft reacti on to

w ake-turbul ence

encount er, appr oach

fr om above-cent er

(rear vi ewdepiction)

Figure 2.4-5


w ake-turbul ence


from above-right

(rear vi ew

depiction)

Glideslope


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2.12

When approached from the side, the upwardflow at the outside of the wake will cause theaircraft to bank away from the wake. A rapidapproach from the side may result in theaircraft passing through the wake (See Fig-ures 2.4-6 and 2.4-7).

Glideslope

Glideslope

Figu re 2.4-7

Ai rcraf t react i on to

w ake turbulence

encounter, rapi d

approach from t he

side (rear v i ew

depiction)

Figur e 2.4-6

Air craf t react i on to

w ake turbulence

encounter,

approach from the

side (rear v iew

depiction)


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2.13

When approached from below, the down-ward flow through the wake pushes the air-craft down and away from the wake. Ifapproached at a rapid enough rate, the air-craft will pass through the wake (See Figures2.4-8 and 2.4-9).

Figure 2.4-9


w ake-turbul ence

encount er, rapid

approach from

below (rear vi ew

depiction)

Figure 2.4-8


w ake-turbul ence


frome below ri ght

(rear vi ew depicti on)

Glideslope

Glideslope


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2.14

2.4.4 Vertical Motion of the Wake

The wake of an aircraft has behavioral charac-teristics which can help the pilot visualize thewake location and thereby take avoidanceprecautions. The initial descent rate of the

wake is adequately described by classicaltheory; the descent rate is determined by theweight, flight speed and wingspan of the gen-erating aircraft. Generally, vortices descendat the initial rate of about 300 to 500 feet per

minute for about 30 seconds. The descent ratedecreases and eventually approaches zero atbetween 500 and 900 feet below the flightpath.Flying at or above the flightpath provides thebest method for avoidance. Maintaining avertical separation of at least 1000 feet whencrossing below the preceding aircraft may beconsidered safe. This vertical motion is illus-trated in Figure 2.4-10.

low this height the wake does not completelyform into concentrated vortices and the tur-bulence in the wake is weakened. Thus, the

turbulence level is reduced, but may still be afactor to aircraft in the touchdown area. Thisis illustrated in Figure 2.4-11.

On approach and takeoff the wake descendsbelow the flightpath until it enters groundeffect whereupon the vortices slow their down-

ward descent and move laterally as shownbelow. Typically, the wakes descent will bearrested within approximately 1/2 wingspan(50-100 feet for the B-747) of the ground. Be-

50 feet 50 feet

Approach path

Wake

turbulence

No strong wake

6

1600 feet

1200

feet

500 to 900 feet

Flightpath

Levels off in approximately

5 nm in approach configuration

Figu re 2.4-10

Vert i cal moti on out

of ground effect

Figur e 2.4-11

Verti cal moti on in

ground eff ect


19/40

SECTION 2

2.15

crosswind, the two vortices move apart toclear the flightpath. Crosswinds of 1 to 5knots can cause one vortex to remain near theflightpath. A light quartering tailwind re-quires maximum caution. However, a pilot

does not have the tools to determine that aperfectly zero crosswind condition exists.Crosswinds greater than 5 knots cause thevortices to move quickly across the flightpathand to break up. This is illustrated in Figure2.4-12 below.

Figure 2.4-12

Horizontal motion100

T = 0 sec

T = 10 secT = 20 sec

100

T = 0 sec

T = 10 sec

T = 20 sec

100

T = 0 sec

T = 10 sec

T = 20 sec

3-knot crosswind

0 crosswind

6-knot crosswind

Altitu

de,

ft

300 200 100 0 100 200 300

300 200 100 0 100 200 300

T = Time

T = Time

300 200 100 0 100 200 300

T = Time

Ground plane, feet

Ground plane, feet

Ground plane, feet

2.4.5 Horizontal Motion of the Wake

The horizontal motion of vortices is dictatedby the ambient wind and the proximity of thevortices to the ground.

At altitude, the wakes horizontal motion isdetermined by the velocity of the wind. Onapproach and takeoff, the wake descends be-low the flightpath until it enters ground effectwhereupon the vortices decrease their down-ward descent and move laterally. With no


20/40

SECTION 2

2.16

Vortices have been found to move laterally asmuch as 1500 feet under certain conditions,but with seemingly weak strengths at thelarger lateral distances. Additionally, undersome crosswind conditions, vortices have been

observed to bounce (i.e., descend towardthe ground and then later begin to rise upsomewhat).

2.4.6 Decay Process

The decay process of the wake is complex andis strongly influenced by the atmospheric con-ditions. The decay process is driven by thefollowing factors:

Atmospheric Turbulence. Atmosphericturbulence plays a significant role in the

decay of the vortex. Atmospheric tur-bulence imparts viscous forces on thewake. These forces extract energy fromthe vortex, thus reducing its strength.The heavier the turbulence, the quickerthe wake decays.

Viscous Interactions. The viscosity ofthe atmosphere slowly extracts energyfrom the vortex, thus reducing itsstrength.

Buoyancy. An upward force acts on the

vortex as a result of the density insidethe vortex system being lower than thedensity outside the vortex. This forcealso slowly extracts energy from thevortex; thus, reducing its strength.

Vortex Instability. A small amount ofturbulence in the atmosphere can createan instability in the vortex pair thatcauses the vortices to link. When thevortices link, the strength of the pairdecays rapidly.

2.4.7 Gaps in Our Knowledge

The initial behavior of the wake is well de-scribed by theory. However, the long-termbehavior is strongly dependent on meteoro-logical conditions. Work continues to fully

understand the effects of meteorological con-ditions on the decay process.

2.5 Future Wake-Turbulence DetectionTechnology

There are many sensors/systems that havehad or may have application in forecasting ordetecting wake turbulence. These range incomplexity from simple sensors, such as pro-peller anemometers, to complex systems, suchas the FAAs Integrated Weather Sensing Sys-tem (ITWSS). There is a general consensusthat it would be desirable to use sensors/systems which already exist (such as the LowLevel Windshear Alert System). However,there is currently nothing in operational usewhich meets all of the requirements for wake-turbulence sensing. There is not even com-plete agreement on what the requirements forwake-turbulence sensing should be.

Wake-turbulence sensor research is currentlybeing conducted in the United Kingdom,France, Canada, Germany, and the UnitedStates. The U.S. research is the most extensiveand includes research in most, if not all, of theareas of interest to other countries.


21/40

SECTION 2

2.17

In addition to the major sensor technologies,there is a continuous stream of ideas for newsensors based on new technologies or combi-nations of old technologies. During 1995 and1996, the FAA/NASA Wake Vortex Programwill evaluate vortex technology and select themost promising technology with the goal ofdeveloping and demonstrating an operationalsystem by the year 2000.

2.6 Air Traffic Control Responsibilitiesfor Maintaining Aircraft Separation*

Air traffic controllers play a large role in as-suring that aircraft avoid wake turbulencesince pilots are unable to visually apply avoid-ance procedures during IMC. Controllers,

while providing radar vector service, are re-sponsible for applying the wake-turbulencelongitudinal separation distances between IFRaircraft and wake-turbulence advisories toVFR aircraft.

2.6.1 Wake-Turbulence CautionaryAdvisories

Air traffic controllers are responsible for pro-viding cautionary wake-turbulence informa-tion to assist pilots prior to their assumingvisual responsibility for avoidance. Control-

lers must issue wake-turbulence cautionaryadvisories and the position, altitude if known,and direction of flight of heavy jets or B-757sto:

a. VFR aircraft not being radar vectored,but which are behind heavy jets or B-757s.

b. VFR arriving aircraft that have previ-ously been radar vectored and thevectoring has been discontinued.

c. IFR aircraft that accept a visual approachor visual separation.

Air traffic controllers should also issue cau-tionary information to any aircraft if, in theiropinion, wake turbulence may have an ad-verse effect on it. When traffic is known to bea heavy aircraft, the word heavy should beincluded in the description.

The primary areas of research are Radar, Li-dar (Laser Radar), Sodar (acoustic Radar),Infrared sensors, and combinations of thesetechnologies. A high-power radar has dem-onstrated the capability of detecting and track-ing wakes, but not at the much lower powerlevel which might be practical in a terminalarea. Radar is not able to resolve whether awake is hazardous or not as there is even someuncertainty over the source of the signal re-turn. Radar research is continuing because ithas a number of advantages as an operationalsensor, even though technical results have notbeen as promising as for other sensors.

Laser systems have a long, successful historyas research instruments for wake-turbulencemeasurements. They can detect, track, and

measure wake strength. Research is continu-ing to improve their range and all weathercapability. Because of their complexity, theprimary challenge is to develop a safe, stand-alone system for operational use. Researchsystems have been used in several countriesto develop a wake-turbulence database.

Acoustic systems have also proven successfulin wake-turbulence research. Older systemsrequired several sensors to track wake turbu-lence but new systems are being developedwhich can detect, track, and measure strength

with a single sensor. Acoustic systems haveprovided most of the airport wake-turbulencestrength measurements in the U.S. database.These systems are simpler and cheaper thanLasers but are limited in range (1000 feet orless).

Infrared sensor research for wind shearprompted tests of an infrared sensor for waketurbulence. These tests showed that therewas an infrared signature associated with thepassage of an aircraft. However, it is not clearif the signature is due to the temperature

profile in the atmosphere or some characteris-tic of wake turbulence. This situation is sounclear that presently, infrared sensors arenot considered promising.

*Information provided in Section 2.6 is compatible with FAA air traffic directives.


22/40

SECTION 2

2.18

2.6.2 Radar/Approach Controllers

Within the terminal area, IFR aircraft are sepa-rated by 3 miles when less than 40 miles fromthe terminal antenna. A 2.5 nautical mileseparation is authorized between certain air-

craft which is established on the final ap-proach course within 10 nautical miles of thelanding runway when:

a. The leading aircrafts Weight Class is thesame or less than the following aircraft;

b. Heavy aircraft and the B-757 are permit-ted to participate in the separationreduction as the following aircraft only;

c. An average runway occupancy time of50 seconds or less is documented;

d. Bright Radar Indicator Tower Equipment

displays are operational and used forquick glance references;

e. Turnoff points are visible from the con-trol tower.

Wake-turbulence procedures specify in-creased separation minima required for cer-tain classes of aircraft because of the possibleeffect of wake turbulence. Refer to Appendix4-F for FAA, United Kingdom and ICAO IFRradar controlled wake-turbulence separationcriteria.

2.6.3 Tower Controllers

Tower controllers are responsible for runwayseparation for aircraft arriving or departingthe airport. Tower controllers do not providevisual wake-turbulence separation to arrivalaircraft; that is the pilots responsibility. Towercontrollers do provide wake-turbulence sepa-ration for departing aircraft by applying timeintervals. Pilots may request a waiver to thewake-turbulence departure separation andthe tower controller will then issue a cautionwake turbulence advisory and clear the air-craft for takeoff provided no other traffic con-flict exists.

2.6.3.1 Wake-Turbulence Separation forDeparting Aircraft

Air traffic controllers are responsible for ap-plying appropriate wake-turbulence separa-tion criteria for departing aircraft. They will

inform the pilot when it is necessary to holdan aircraft to provide the required wake-tur-bulence separation. The proper communica-tion phraseology is hold for waketurbulence. Pilots may request a waiver todeviate from the criteria. A pilot request fortakeoff does not initiate a waiver request un-less it specifically includes a request to devi-ate from the required wake-turbulenceinterval.

2.6.3.2 Wake-Turbulence DepartureSeparation Criteria

Separation criteria (listed by aircraft wake-turbulence weight categories and runway situ-ation) are as follows:

Same or parallel runways separated lessthan 2500 feet:

- Small/large/heavy behind heavy - 2minutes (same direction).

- Small/large/heavy behind heavy - 3minutes (opposite direction or inter-section departure).

Same runway:

- Small behind large - 3 minutes (oppo-site direction or intersectiondeparture).

Note: Aircraft conducting touch-and-go andstop-and-go operations are consid-ered to be departing from an intersec-tion.

Intersecting runways:

- Small/large/heavy behind heavy - 2minutes (projected flightpaths crossor departure will fly through airbornepath of arrival).


23/40

SECTION 2

2.19

2.6.4 Visual Separation

Aircraft may be separated by visual meanswhen other approved separation is assuredbefore and after the application of visual sepa-ration. To ensure that other separation will

exist, air traffic controllers should consideraircraft performance, wake turbulence, clo-sure rate, routes of flight and known weatherconditions. Reported weather conditions mustallow the aircraft to remain within sight untilother separation exists. Controllers shouldnot apply visual separation between succes-sive departures when departure routes and/or aircraft performance preclude maintainingseparation.

2.6.4.1 Visual Separation-Terminal Area

Visual separation may be applied betweenaircraft under the control of the same facilitywithin the terminal area provided:

a. communication is maintained with atleast one of the aircraft, involved or thecapability to communicate is immedi-ately available; and the aircraft arevisually observed by the tower control-ler and visual separation is maintainedbetween the aircraft by the tower con-troller.

b. a pilot sees the other aircraft and is in-

structed to maintain visual separationfrom the aircraft as follows:

(1) The pilot is informed by the ATC ofthe other aircraft, including posi-tion, direction and, unless it isobvious, the other aircrafts inten-tion.

(2) Acknowledgment is obtained fromthe pilot that the other aircraft is insight.

(3) The pilot is instructed to maintainvisual separation from the other air-

craft.(4) The pilot is advised if the radar tar-

gets appear likely to converge.

(5) If the aircraft are converging, theother aircraft is informed of the traf-fic and that visual separation is beingapplied.

The tower controller shall not provide visualseparation between aircraft when wake-tur-bulence separation is required or when thelead aircraft is a B-757.

2.6.4.2 Visual Separation - En Route

Air traffic controllers may use visual separa-tion in lieu of radar separation in conjunctionwith visual approach procedures. Refer toSection 2.6.4 for those procedures.

2.6.4.3 Visual Separation - NonapproachControl Towers

Nonapproach control tower controllers maybe authorized to provide visual separationbetween aircraft within surface areas or des-ignated areas provided other separation isassured before and after the application ofvisual separation. This may be applied by thenonapproach control tower providing theseparation or by a pilot visually observinganother aircraft and being instructed to main-

tain visual separation with that aircraft.

2.7 Pilot Responsibilities forMaintaining Wake-TurbulenceSeparation

Pilots and air traffic control share the respon-sibility for assuring that aircraft avoid waketurbulence.

2.7.1 Who Does What and When

There is clear delineation of who and whenresponsibility is assumed for avoiding waketurbulence. The pilot is responsible for avoid-ing wake turbulence when:

a. flying in VFR and not being vectored byATC.

b. maintaining visual separation.

c. cleared for a visual approach.

Air traffic control (ATC) assumes wake-tur-bulence responsibility while providing the

pilot instrument flight rules (IFR) control ininstrument meteorological weather conditionsand when vectoring VFR aircraft. [A discus-sion of ATC procedures is included in theATC responsibility Section, 2.6.] A discussionof several situations will help to clarify apilot's responsibility.


24/40

SECTION 2

2.20

When the pilot is being radar controlled byATC, the aircraft will be spaced, for waketurbulence, behind a preceding aircraft at adistance determined by the weights of thetwo aircraft. Based on the known movementsof wake turbulence, this separation has beensuccessful in preventing wake-turbulence en-counters. The minimum separation is de-signed not only to allow time for the waketurbulence to begin to dissipate, but also toallow time for it to descend below the follow-ing aircraft's flightpath. Longitudinal separa-tion is but one element of avoidance. If VFRweather conditions exist when ATC is pro-viding radar control, the pilot is not relievedof the responsibility for assuring the flightpathwill avoid an encounter with wake turbu-lence. If instrument meteorological condi-

tions (IMC) exist, only the ATC establishedseparation distances are available to preventwake-turbulence encounters, since the pilot isunable to visually apply avoidance proce-dures.

When it is operationally beneficial, ATC mayauthorize the pilot to conduct a visual ap-proach to an airport or to follow anotheraircraft in VFR weather. The pilot must havethe airport or an identified preceding aircraftin sight before the clearance is issued. If thepilot has the airport in sight but cannot see the

aircraft he or she is following, ATC may stillclear the aircraft for a visual approach; how-ever, ATC retains both normal separation andwake-turbulence separation responsibility.When the pilot is able to visually follow apreceding aircraft, and accepts the visual ap-proach clearance, this transfers responsibilityfor avoiding wake turbulence to the pilot. Tosummarize this point, the pilot accepts wake-turbulence avoidance responsibility when:

a. ATC instructions include traffic infor-mation.

b. Instructions to follow an aircraft are givenand the pilot is able to comply.

c. The pilot accepts the visual approachclearance.

ATC is also responsible for assuring properwake-turbulence separation before issuingclearance for takeoff by applying time anddistance intervals. Pilots, after consideringpossible wake-turbulence effects, may spe-cifically request a waiver to the interval. Con-trollers may acknowledge this request asacceptance of responsibility for wake-turbu-lence separation. If traffic permits, takeoffclearance will be issued. A wake-turbulencecautionary advisory will be given.

During cruise flight in VFR weather, altitudeseparations could be as little as 500 feet be-tween IFR and VFR aircraft. In this situationthe same principle applies: pilots must useproper avoidance procedures.

2.7.2 Communications

To aid other pilots and ATC within FAAcontrolled airspace, pilots of heavy aircraftshould always use the word Heavy in theirradio communications. Radio communica-tions are usually country specific, thereforepilots should check appropriate regulationsregarding wake turbulence prior to opera-tions outside FAA controlled airspace.

ATC is required to provide a "CAUTIONWAKE TURBULENCE" advisory when VFR

aircraft are not being radar vectored and arebehind heavy jets or B-757s and to IFR aircraftthat accept visual separation or a visual ap-proach. ATC controllers may also issue awake-turbulence caution when, in their opin-ion, wake turbulence may have an adverseeffect on an aircraft following another air-craft. Because wake-turbulence movement isvariable, the controller is not responsible foranticipating its existence or effect. Althoughnot mandatory during ground operations, con-trollers may use the words jet blast, prop-wash, or rotorwash, in lieu of wake turbulence,

when issuing a caution advisory.


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SECTION 2

2.21

2.8 Wake Turbulence RecommendedVisual Avoidance Procedures

It would be easy to avoid wake turbulence ifit could be seen. Although under certainatmospheric or artificially generated condi-

tions it is possible to see wake turbulence, thisis not the normal situation. Therefore, pilotsmust rely on their knowledge of the behavioror characteristics of wake turbulence to visu-alize the wake location so that they may imple-ment avoidance procedures. Theseprocedures have been developed for varioussituations. It is important to note that theprocedures require pilots to adjust their op-erations and flightpath to preclude wake en-counters. Aircraft performance should beconsidered during the decision process of

applying the procedures. Generally, the pro-cedures were developed to assist pilots in

Figure 2.8-1

Landi ng behind a

lar ger aircraft

- same runw ay

avoiding the area below and behind the gen-erating aircraft. A go around may be theappropriate solution in some situations.

2.8.1 Specific Procedures

2.8.1.1 Landing Behind a Larger Aircraft -Same Runway (Figure 2.8-1)

Stay at or above the larger aircrafts finalapproach flightpath.

Note its touchdown point.

Land beyond the touchdown point, run-way length permitting.

If unable to land safely beyond the touch-down point, go around.

Wind

Touchdown point oflarger aircraft

Touchdown point oflarger aircraft

Planned touchdown pointof following aircraft

Planned touchdown pointof following aircraft

6


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SECTION 2

2.22

2.8.1.2 Landing Behind a Larger Aircraft -Parallel Runway Closer Than 2500Feet (Figure 2.8-2)

Consider possible wake-turbulence driftto your runway.

Stay at or above the larger aircrafts finalapproach flightpath.

Note its touchdown point.

Touchdown points

Offset Runway Situation

Parallel Runway Situation

9-L

9-R

Less than 2500 feet

Wind

Touchdown points

9-L

9-R

Less than 2500 feet

9

3

6

Aircraft crossing overwake turbulence

2.8.1.3 Landing Behind a Larger Aircraft -Crossing Runway (Figure 2.8-3)

Cross above the larger aircrafts flightpath.Consider lateral and vertical motion ofwake turbulence.

If unable to land safely, go around.

Figure 2.8-3

Landi ng behin d a

depart ing lar ger

aircraft - crossing

runway

Figure 2.8-2

Landi ng behin d a

larger aircraft -

paral lel runw ay

closer th an 2500

feet


27/40

SECTION 2

2.23

2.8.1.4 Landing Behind a DepartingLarger Aircraft - Same Runway(Figure 2.8-4)

Note the larger aircrafts rotation point.

Land before the rotation point, or goaround.

Figure 2.8-4Landi ng behind a

depart ing l arger

aircraft - same runw ay

2.8.1.5 Landing Behind a DepartingLarger Aircraft - Crossing Runway(Figures 2.8-5,-6)

Note the larger aircrafts rotation point. Ifpast the intersection, continue the ap-proach and land before the intersection.

If larger aircraft rotates before the inter-section, avoid flight below larger aircraftsflightpath. Abandon the approach unlessa landing is assured well before reachingthe intersection.

Figure 2.8-6

Landi ng behind a

depart i ng l arger

air craf t - crossing

runway

Figure 2.8-5

Landi ng behind a

depart ing l arger

air craft - crossing

runway

Rotation pointPlannedtouchdownpoint

9

15

Rotation point

Touchdown here

or abandon approach

Rotation point08

12


28/40

SECTION 2

2.24

Continue climb above the larger aircraftsclimb path until turning clear of its wake.Caution: This may not be possible be-cause of the larger aircrafts performance.

Avoid subsequent headings which willcross below and behind a larger aircraft.

Be alert for any critical take-off situationwhich could lead to a wake-turbulenceencounter.

Large aircraft

Small aircraft

Critical take-off situation

Large

aircraftSmall aircraft

2.8.1.6 Departing Behind a Larger Aircraft(Figures 2.8-7,-8,-9)

Note the larger aircrafts rotation point.

Delay, do not begin take-off roll unless

your rotation point will be prior to thelarger aircrafts rotation point.

Climb displaced upwind of larger air-craft.

Figure 2.8-8

Departi ng behind a

larger aircraft -

crossin g depart ure

courses

Figur e 2.8-7

Departi ng behind a

larger ai rcraft -same runw ay

Figure 2.8-9

Departi ng behind a

larger aircraft -

opposit e direction


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SECTION 2

2.25

2.8.1.7 Intersection Takeoffs - SameRunway (Figure 2.8-10)

Be alert to adjacent larger aircraft opera-tions, particularly upwind of yourrunway.

If intersection take-off clearance is re-ceived, avoid headings which will crossbelow a larger aircrafts path.

Ensure your rotation point is before largeraircrafts rotation point, or delay takeoff.

Rotation point Figure 2.8-10Int ersecti on tak eoffs -

same runw ay


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SECTION 2

2.26

2.8.1.8 Departing or Landing After aHeavy Aircraft Executing a LowApproach, Missed Approach orTouch-and-Go Landing(Figure 2.8-11)

Ensure that an interval of at least twominutes has elapsed before your take offor landing.

Wind

Wind

Take-off or landing hazard

2.8.1.9 En Route Within 1000 FeetAltitude of a Large Aircraft'sAltitude (Figure 2.8-12)

Avoid flight below and behind a largeaircrafts path.

If a larger aircraft is observed above andon the same track (meeting or overtak-ing), adjust your position laterally,preferably upwind.

Figure 2.8-11

Departi ng or

landi ng after a

heavy ai rcraft

executi ng a low

approach, m issed

approach or t ouch-

and-go landi ng

Figure 2.8-12

En rout e VFR (1000

foot al t i t ude plus

500 feet )


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2.27

2.8.2 Avoiding Helicopter OutwashVortices

In a slow hover taxi or stationary hover nearthe surface, helicopter main rotor(s) generatedownwash producing high velocity outwash

vortices to a distance approximately threetimes the diameter of the rotor. When rotordownwash contacts the surface, the resultingoutwash vortices have behavioral character-istics similar to wingtip vortices of fixed-wingaircraft. However, the vortex circulation isoutward, upward, around and away from themain rotor(s) in all directions. Pilots of smallaircraft should avoid operating within threerotor diameters of any helicopter that is in aslow-hover taxi or stationary hover (Figure2.8-13).

In forward flight, departing or landing heli-copters produce a pair of strong, high-speedtrailing vortices similar to wingtip turbulenceof larger fixed-wing aircraft (Figure 2.8-14).Pilots of small aircraft should use cautionwhen operating behind or crossing behindlanding and departing helicopters. Addition-ally, it is possible for the wake turbulencefrom a helicopter that hovers upwind of arunway to drift towards the runway.

In certain situations, ATC will use the phrase,caution, wake turbulence. Pilots must beaware that whether or not a warning has beengiven, they are expected to adjust their opera-tions and flightpath as necessary to precludeserious wake encounters.

Figure 2.8-13

Heli copter hover-

produced

downwash

Figure 2.8-14

Helicopter forw ard-

fl i ght-produced w ake

turbulence


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2.28

2.9 Pilot Difficulty in VisuallyMaintaining Separation

2.9.1 Flightpaths

A review of accidents and incidents involving

wake turbulence reveals a recurring problemthat pilots routinely must solve during arrivaland landing. Traffic and airspace as well asother considerations require the establishmentof flight patterns for sequencing aircraft forlanding. These patterns are designed to ac-commodate arrivals from several directions,as well as approaches and landings under IFRand VFR weather conditions. Pilots may flyvisual approaches when weather conditionspermit and authorized by ATC at controlledairports. The pilot is then solely responsiblefor avoiding the wake turbulence when other

aircraft are present by staying at or above theflightpath of any aircraft they may follow.The task of maintaining a proper visual rela-tionship with the lead aircraft becomes greaterand more complicated when aircraft of differ-

ent sizes and speeds, approaching from vari-ous altitudes and directions, are involved.These complexities increase the difficulty inmaintaining the appropriate flightpath.

Even though the leader aircraft is currentlybelow you, do not assume that the flightpathof the leader aircraft is below you. It is quitepossible that the leader aircraft varied its de-scent rate, especially during the initial portionof its approach (Figure 2.9-1).

6

Actual flightpath(leader)

Visual determination that the leaderaircraft is lower; therefore, wronglyassumes it is above the flightpath ofthe lead aircraft

Figure 2.9-1

Steeper fli ghtpat h

by l eader ai rcraft


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SECTION 2

2.29

2.9.1.1 Use of ILS Glideslope

When available to the pilot, the ILS glideslopecan be a starting point for assistance in deter-mining the flightpath of a leader aircraft; how-ever, it is not foolproof. In fact, the leader

aircraft may have intercepted and flown abovethe glideslope for wake-turbulence avoidanceor other reasons.

2.9.1.2 Visual Illusions

Pilots can experience visual illusions for sev-eral reasons. Different aircraft sizes can makeit difficult for pilots to determine distances orrates of closure with a leader aircraft. Addi-tionally, the body attitudes of some aircraftsignificantly change as airspeed is reduced.The change in aircraft body attitude can give

the illusion of a change in flightpath. Aircraftapproaching from different directions andaltitudes while turning to final approach isanother situation where it is difficult for pilotsto determine what the leaders flightpath wasor will be when becoming aligned behind theleader.

2.9.1.3 Darkness/Reduced Visibility

Determining the leader aircrafts flightpathduring darkness can be difficult for pilots.Depth perception is inhibited and pilots may

have to rely only on the leader aircrafts light-ing when ascertaining its flightpath. It is alsodifficult to determine flightpaths during re-duced visibility caused by weather condi-tions.

2.9.2 Instrument to Visual Situation

Changing from an instrument approach to avisual approach and landing, when condi-tions permit, is routinely accomplished. Thepilots situational awareness up until the timeof transition from IMC to VMC is usually

limited to information received from radiocommunications. While ATC will issue infor-mation and cautionary instructions, the pilotmust be prepared to react to the traffic situa-tion and apply proper avoidance procedures.

2.10 Pilot Techniques for VisuallyMaintaining Separation

2.10.1 General

The wake-turbulence avoidance procedures

discussed in Section 2.8 are effective whenproperly used. To properly apply avoidanceprocedures and techniques, it is important forpilots to know and understand the character-istics and movement of wake turbulence dis-cussed in Section 2.4. Normally, it is notpossible for pilots to know the precise loca-tion of wake turbulence. Pilots must thereforeavoid the area below and behind larger air-craft flightpaths, especially at low altitudewhere even a momentary wake encountercould be hazardous. While this is not alwayseasy to do, there are some techniques thatmay be used. Pilots should always considertheir aircraft performance when avoidingwake turbulence since several procedures andtechniques may require some adjustments toroutine operations. Notification of ATC mayalso be necessary.

For pilots to be able to avoid wake turbulenceby staying on or above the flightpath of theleader aircraft, trailing pilots must make someassumptions on where the leader has flownsince there is no available visual reference.

The use of visual glideslope indicators such asVASI or PAPI or instrument precision ap-proach aids, when possible, will assist in es-tablishing and maintaining a normal approachflightpath* and runway centerline course. Ifexternal aids are not available and obstaclesare not a factor, a descent rate of 300 feet pernautical mile traveled approximates a 3-de-gree flightpath. The aircraft should be stabi-lized on a flightpath not later than 500 feetAGL. Air traffic controllers and pilots mustunderstand that accomplishing a steep de-scent may have serious ramifications for trail-

ing aircraft with regard to wake turbulence.

*Heavy wide-body aircraft pilots routinely fly the upper two rows of VASI lights.


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2.30

2.10.2 Visual Cues for EstimatingLeaders Flightpath

One way to determine the flightpath that theleader has flown is to extend an imaginaryline from your position to the runway normal

touchdown point (Figure 2.10-1A). If theleader aircraft is above this line, you are below

its flightpath. Conversely, if the leader air-craft is on or below the imaginary line, you areon or above its flightpath. This techniqueassumes the leader has flown a consistentflightpath and is using a normal runway touch-

down point.

While following an aircraft, extending animaginary line from your aircraft through theleader to the runway should end at the nor-

mal runway touchdown point (Figure 2.10-1B). If it ends at a point down the runway, the

6

Normal touchdown point

Above leaders flightpath

Below leaders flightpath

6

Visual sightangle of T/Dif followingaircraft is aboveleader flightpath

Normaltouchdownpoint

Visual sightangle of T/Dif followingaircraft is belowleader flightpath

Above

Below

trailing aircraft is probably below theflightpath of the leader. If the imaginary lineextension is prior to the touchdown point,

e.g., in the overrun, the trailing aircraft isprobably above the leaders flightpath.

Figur e 2.10-1A

Determining

fl ightpat h of leader

using im aginary

li ne extension

method

Figure 2.10-1B

Determi ning if

foll ow er is above or

bel ow leader


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Miles from touchdown (nm) 5 4 3 2 1

One-dot (1/4 degree) deviation 130' 104' 78' 52' 26'

Two-dot (1/2 degree) deviation 260' 208' 156' 104' 52'

Miles from touchdown (nm) 5 4 3 2 1

One-dot (1-1/4 degree) deviation 838' 706' 573' 441' 308'

Two-dot (2-1/2 degree) deviation 1677' 1412' 1147' 882' 617'

2.10.3 Using ILS Glideslopes for VerticalSeparation

When ILS approaches are being used, consid-eration may be made by the pilot of the trail-ing aircraft to fly at or above the ILS glideslope.

This assumes the leader aircraft is positionedon the glideslope. Be alert! This assumptionis not always valid. A nose high pitch attitude

of the leader aircraft should not be used as anindicator of glideslope position because pitchattitudes vary among aircraft types and manu-facturers. Table 2.10-1 provides distance infeet for degrees in deviation from the glides-lope and illustrates position relative to theglideslope.

2.10.4 Using ILS Localizer for LateralSeparation

During crosswind conditions, pilots may con-

sider flying offset on the upwind side of thelocalizer centerline as a means of avoiding theleaders wake turbulence. This assumes the

Tabl e 2.10-2

Localizer deviat ion

leader is flying on the localizer course. Table2.10-2 can be used to determine offset dis-tance in feet for degrees in deviation from thelocalizer course.

Tabl e 2.10-1

Deviat i on from

standa rd 3-degree

glideslope

Note: The relative distance from the glideslope becomes quite insignificant closeto the runway.


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2.10.5 Longitudinal Separation

Pilots may also establish longitudinal separa-tion from a leader aircraft so as to allow timefor the wake turbulence to move or dissipate.Judging in-flight distances is not always easy

to do because different aircraft sizes can bevisually deceiving to the pilot.

2.10.5.1 Air Traffic Control Assist

Air traffic controllers are able to provide sepa-ration distance information to pilots whenworkload permits and they have radar dis-plays in the control tower. They can provideairspeed differential between aircraft and mayadvise pilots following another aircraft whenthey are overtaking the preceding aircraft.

2.10.5.2 On-board Radar

Aircraft equipped with radar may have thecapability to determine separation distancesfrom other aircraft. Caution: Be careful not tofocus attention on the radar at the expense ofoutside visual scans.

2.10.5.3 Time and Distance Methods

A technique available for the pilot of the fol-lowing aircraft is to start timing the leaderaircraft when it or its shadow passes a recog-

nizable geographical reference point. Radiocall points can also be used for timing refer-ences. Determine the amount of time it takesfor the following aircraft to pass over the samepoint. Convert that time into distance. Forexample, if it took three minutes and thefollowing aircrafts ground speed was 120knots (two miles per minute), then the dis-tance between the two aircraft is six miles.

Most heavy and large aircraft produce somesmoke from the tires during touchdown onlanding. Pilots of trailing aircraft, upon ob-

serving the smoke, can estimate their ownposition from touchdown as well as deter-mining a point to land beyond. Knowing thedistance from the runway to an instrumentfinal approach fix or an available landmarkcan be helpful in determining relative dis-tances.

2.10.6 Establishing LongitudinalSeparation

There are several ways to increase separationdistances while following an aircraft on finalapproach. Several factors should be consid-

ered before implementing these techniques:aircraft performance, in-flight visibility, othertraffic in the pattern as well as those that aretaking off or preparing to take off, notificationof ATC, etc.

Airspeed reduction is an obvious choice ofmost pilots, but usually is limited to smallchanges because of aircraft performance orATC restrictions. Pilots must not reduce air-speed below the aircrafts minimum safe op-erating speed. Also, recovery from aninadvertent wake-turbulence encounter is

more difficult at slower airspeeds. For plan-ning purposes, most transport category air-craft final approach speeds are between 120knots to 150 knots.

Flying S turns is another way to gain sepa-ration.

A 360-degree turn will greatly increase thedistance from the leader, but the impact onother aircraft may preclude its use.

The decision to abort the approach or landing

and go around is always an alternative foravoiding wake turbulence.

2.10.7 Radio Communications

Listening to all radio communications (notjust those directed to you) can be helpful inproviding information that can improve wake-turbulence situational awareness. Prior toentering a visual traffic pattern or initiatingan instrument approach, radio communica-tions between ATC and other aircraft can alertpilots to where they may fit in the landing

sequence or what type aircraft they may fol-low. Takeoff and landing clearances for otheraircraft provide pilots information that can beuseful for spacing considerations as well asanticipating the location of generated waketurbulence. Do not overlook any informationthat can aid planning and flying an approach,landing or go-around.


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2.10.8 Estimating Movement of WakeTurbulence

Basic surface wind indications can aid pilotswith estimating the movement of wake tur-bulence. Blowing dust, smoke or wakes on

lakes and ponds provide indications that maybe used in determining wind direction whichmay be applied to wake-turbulence move-ment. Use any on-board avionics equipmenti.e., inertial reference, Doppler radar, globalpositioning system, etc. to determine winddirection. Aircraft drift angles will also givethe pilot an indication of wind direction.

2.11 Pilot Responses Upon EncounteringWake Turbulence

An encounter with wake turbulence usuallyresults in induced rolling or pitch moments;however, in rare instances an encounter couldcause structural damage to the aircraft. Inmore than one instance, pilots have describedan encounter to be like hitting a wall. Thedynamic forces of the vortex can exceed theroll or pitch capability of the aircraft to over-come these forces. During test programs, thewake was approached from all directions toevaluate the effect of encounter direction onresponse. One item that was common to allencounters, without a concerted effort by thepilot the aircraft would be expelled from thewake. Refer to Section 2.4, Figures 2.4-4through 2.4-9, for the effects on an aircraftwhen encountering wake turbulence fromseveral directions. While this informationprovides a better understanding of wake tur-bulence, its usefulness is limited since wake-turbulence encounters are inadvertent andpilots will not be aware of their entry location.

Counter control is usually effective and in-duced roll is minimal in cases where the wing-

span and ailerons of the encountering aircraftextend beyond the rotational flow field of thevortex. It is more difficult for aircraft withshort wingspan (relative to the generatingaircraft) to counter the imposed roll inducedby the vortex flow. Pilots of short span air-craft, even of the high performance type, mustbe especially alert to wake-turbulence encoun-ters.

It may be difficult or impossible for pilots todifferentiate between wake turbulence andturbulence generated from another source.Apply appropriate corrective action if waketurbulence is encountered. A wake-turbu-lence encounter at low altitude is much morehazardous than an encounter at cruise alti-tude or early during the approach phase offlight.

2.12 Cooperative and EfficientManagement of Capacity

The worldwide number of aircraft continuesto increase each year for reasons that reachfrom the desire for greater recreational use toresponding to commercial demand. As this

number increases, so must the necessary sup-port or infrastructure. The critical or limitingfactor of this infrastructure continues tochange. For example, in the early years ofaviation, the small number of runways oftenlimited where a pilot could land. As morerunways were built, adverse weather becamethe critical element which was slowly over-come with the advent of better and betterterminal approach aids and air traffic sys-tems. We have evolved from few pilots tomany pilots; from few air traffic controllers tomany air traffic controllers. Most of the limit-

ing factors have gradually been mitigatedthough improved technology. Currently,wake turbulence and the application of exist-ing IFR separation and avoidance proceduresare a limiting factor at many major airports.This situation, coupled with high air trafficdensity, creates an environment that requirespilots and air traffic controllers to cooperateto safely and efficiently conduct flight opera-tions.

Air traffic controllers should understand thatmany times the pilots situational awareness

is limited to information provided by ATCuntil the pilot enters visual meteorologicalconditions. This means that initially it may bedifficult for pilots to visually detect whetherthey may be overtaking the leader aircraft orwhere they are, relative to the leadersflightpath. Any pertinent information thatcan be given to the pilot during a radar con-trolled arrival, will help the pilot transition toa visual approach and landing.


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Delaying a pilots descent increases the cock-pit workload and difficulty in accomplishinga normal approach for landing. A higher thannormal approach can impact trailing aircraft.The leader aircraft may not be aware of trail-ing aircraft or of their position.

Pilots can assist ATC in several ways. Oneway is to understand that ATC is continuallychallenged in sequencing arrivals with de-partures, planning for different aircraft withdifferent performance characteristics and ap-plying wake-turbulence separation criteria.A pilot who initiates an unusual request ormakes a change in his/her flight operationsfrom what is normally expected by ATC, willprobably increase an already high workloadfor most controllers at major airports. Early,

precise and disciplined radio communicationswith ATC improves the flow of vital informa-tion.

Wake turbulence is one of many factors thatpilots and air traffic controllers must over-come to fly safely. It takes cooperation amongpilots and air traffic controllers and under-standing of each others requirements to safelyavoid wake turbulence.

2.13 Air Traffic Considerations When

Applying Separation

Air traffic control is responsible for the safe,orderly and expeditious flow of all aircraft intheir area of responsibility. The primary con-siderations that affect the controller's abilityto do this are:

Type of approaches available (IFR orVFR)

Mix of traffic (turbojet, propeller, helicop-ter)

Traffic density

Wake-turbulence separation

Noise abatement procedures.

The terminal approach control can safely land

and depart more aircraft if the weather is VFRand visual approaches are being used. Typi-cally, aircraft flying visual approaches willhave approximately 1-1/2 miles between land-ing and arriving aircraft. Under IFR weatherconditions, aircraft require a minimum of2-1/2 miles inside the final approach fix andif wake-turbulence separation is required, theseparation may be extended up to 4, 5, or 6miles between aircraft. Traffic density is themajor factor in the amount of aircraft that canbe safely, orderly and expeditiously landed ordeparted. The busiest airports schedule air-craft takeoffs and landings based on weatherconditions. At almost any busy airport, whenthe weather is IFR, there are extensive delaysand even cancellations if the IFR weatherpersists for an extended period of time.

Visual conditions and visual separation allowair traffic to handle more aircraft in the sys-tem. When controllers clear pilots to maintainvisual separation or to fly a visual approach,they can concentrate their efforts on separat-ing the other IFR aircraft they are handling.The quicker an approach controller transfersthe responsibility of separation to the pilot,the better service he or she can provide to theother aircraft that still require IFR control.

There are several factors a controller should

consider before clearing a pilot to maintainvisual separation or to fly a visual approachwhen wake-turbulence separation must beapplied. First, winds have a significant effecton wake turbulence. A smaller aircraft up-wind from a larger aircraft is unlikely to en-counter any wake turbulence. However, it isnot always practical or possible to have asmaller aircraft follow a larger aircraft on theupwind side. Traffic patterns, runway con-figurations, and expeditious handling some-times do not make it practical to sequenceaircraft based on crosswinds. Another con-

sideration controllers need to make is theflightpath of the preceding aircraft comparedto the flightpath of the following aircraft. Steepdescents of larger aircraft for any reason couldcreate a hazard for smaller following aircraftflying a normal descent to the same runway.This is because the smaller aircraft at sometime could be below the glidepath of the largeraircraft.


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Many more fast, small jet powered aircraft arebeing manufactured. It is no longer a "smallaircraft fly slower than large aircraft" envi-ronment. Faster small jets following slowerlarge jets could create a serious wake-turbu-lence problem since the smaller aircraft couldget too close behind the larger jet. Intersectingrunways also create a hazard when a small jetis cleared to land on a runway and its flightpathwill take it through the flightpath of a largerjet that was landing or departing on a differ-ent runway.

The best prevention for avoiding wake turbu-lence is both pilot and controller awareness.Controllers must be aware of where waketurbulence could occur and how it will affect

other aircraft following. Crosswinds, steepdescents, different airspeeds and crossing run-ways are factors controllers should consider.Pilots also have to be made aware of wherethe potential hazards exist. Sometimes givinga cautionary wake-turbulence advisory is notenough. The pilot needs to know if the air-craft he/she is following is on a steeper thannormal descent, is flying slower, or if thepreceding aircraft has departed or is landingon another runway. If the controllers areaware of potential wake-turbulence hazards,then they need to inform the pilots of thosehazards and allow the pilot to adjust his/herflightpath accordingly.


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