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CHAPTER 12. AIRCRAFT AVIONICS SYSTEMS

SECTION 1. AVIONICS EQUIPMENT MAINTENANCE

12-1. GENERAL. There are several meth-ods of ground checking avionics systems.

a. Visual Check. Check for physicalcondition and safety of equipment and com-ponents.

b. Operation Check. This check is per-formed primary by the pilot, but may also beperformed by the mechanics after annual and100-hour inspections. The aircraft flightmanual, the Airman’s Information Manual(AIM), and the manufacturer’s informationare used as a reference when performing thecheck.

c. Functional Test. This is performed byqualified mechanics and repair stations tocheck the calibration and accuracy of the avi-onics with the use of test equipment whilethey are still on the aircraft, such as the trans-ponder and the static checks. The equipmentmanufacturer’s manuals and procedures areused as a reference.

d. Bench Test. When using this methodthe unit or instrument is removed from theaircraft and inspected, repaired, and calibratedas required.

e. Electromagnetic Interference (EMI).For EMI tests, refer to chapter 11 para-graph 11-107 of this AC.

12-2. HANDLING OF COMPONENTS.Any unit containing electronic componentssuch as transistors, diodes, integrated circuits,proms, roms, and memory devices should beprotected from excessive shocks. Excessiveshock can cause internal failures in an of thesecomponents. Most electronic devices aresubject to damage by electrostatic discharges(ESD).

CAUTION: To prevent damage dueto excessive electrostatic discharge,proper gloves, finger cots, orgrounding bracelets should be used.Observe the standard procedures forhandling equipment containing elec-trostatic sensitive devices or assem-blies in accordance with the recom-mendations and procedures set forthin the maintenance instructions setforth by the equipment manufactur-ers.

12-3. 12-7. [RESERVED.]

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SECTION 2. GROUND OPERATIONAL CHECKS FOR AVIONICS EQUIPMENT(ELECTRICAL)

12-8. GENERAL. When the operating orairworthiness regulations require a system toperform its intended function, the use of theTechnical Standard Order (TSO) equipment orthe submission of data substantiating theequipment performance is strongly recom-mended. An operation check of avionics is theresponsibility of the pilot in command. How-ever, it is recommended that after replacementof equipment during 100 hour or annual in-spections, an operational check of avionicsequipment be performed. The accomplish-ments of these checks must be done in accor-dance with the recommendations and proce-dures set forth in the aircraft’s flight manualinstructions published by the avionics equip-ment manufacturers.

12-9. INSPECTION OF AVIONICSSYSTEMS.

a. The inspection shall include the fol-lowing:

(1) Inspect the condition and security ofequipment including the proper security ofwiring bundles.

(2) Check for indications of overheatingof the equipment and associated wiring.

(3) Check for poor electrical bonding.The bonding requirements are specified byequipment manufacturers. Installation cablingshould be kept as short as possible, except forantenna cables which are usually precut orhave a specific length called out at installation.Proper bonding on the order of 0.003 ohms isvery important to the performance of avionicsequipment.

(4) Check to assure that the radios andinstruments are secured to the instrumentpanel.

(5) Check that all avionics are free ofdust, dirt, lint, or any other airborne contami-nates. If there is a forced air cooling system, itmust be inspected for proper operation.Equipment ventilation openings must not beobstructed.

(6) Check the microphone headsetplugs and connectors and all switches andcontrols for condition and operation. Checkall avionics instruments for placards. Checklightening, annunciator lights, and cockpit in-terphone for proper operation.

(7) The circuit breaker panel must beinspected for the presence of placarding foreach circuit breaker installed.

(8) Check the electrical circuit switches,especially the spring-load type for proper op-eration. An internal failure in this type ofswitch may allow the switch to remain closedeven though the toggle or button returns to theOFF position. During inspection, attentionmust be given to the possibility that improperswitch substitution may have been made.

b. Inspect antennas for:

(1) broken or missing antennainsulators

(2) lead through insulators

(3) springs

(4) safety wires

(5) cracked antenna housing

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(6) missing or poor sealant at base ofantenna

(7) correct installation

(8) signs of corrosion, and

(9) the condition of paint/bonding andgrounding.

(10) Check the bonding of each antennafrom mounting base to the aircraft skin. Tol-erance: .1 ohm, maximum.

(a) Test Equipment:

1 1502B Metallic Time DomainReflectometer or equivalent.

2 Thruline Wattmeter.

(b) Perform the antenna evaluationcheck using the domain reflectometer to de-termine the condition of the antenna and coaxcables. Refer to manufacturer’s maintenanceprocedures.

(c) Use thruline wattmeter as neededfor addition evaluation. Refer to manufac-turer’s maintenance procedures. Check for thefollowing:

1 Resistance.

2 Shorts.

3 Opens.

c. Inspect the static dischargers/wicksfor:

(1) physical security of mounting at-tachments, wear or abrasion of wicks, missingwicks, etc.,

(2) assurance that one inch of the innerbraid of flexible vinyl cover wicks extends be-yond the vinyl covering,

(3) assurance that all dischargers arepresent and securely mounted to their base,

(4) assurance that all bases are securelybonded to skin of aircraft in order to preventthe existence in voltage level differences be-tween two surfaces,

(5) signs of excessive erosion or dete-rioration of discharger tip,

(6) lighting damage as evidenced bypitting of the metal base, and

(7) megohm value of static wick itselfas per manufacturer’s instructions. It shouldnot be open.

d. Subsequent inspection must be madeafter a maintenance action on a transponder.Refer to Title 14 of the Code of FederalRegulations (14 CFR) part 91, sections 91.411and 91.413.

e. Inspection of the emergency locatortransmitter operation, condition and date ofthe battery.

f. Perform a function check of the radioby transmitting a request for a radio check.Perform a function check on navigationequipment by moving the omni bearing selec-tion (OBS) and noting the needle swing andthe TO/FROM flag movement.

12-10. COMMUNICATION SYSTEMS.Ground operation of communication systemsin aircraft may be accomplished in accordancewith the procedures appropriate for the airportand area in which the test is made and themanufacturer’s manuals and procedures.Check system(s) for side tone, clarity of

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transmission, squelch, operations using headphones, speaker(s), and hand microphone. If areceiver or transmitter is found to be defective,it should be removed from the aircraft and re-paired.

12-11. VHF OMNI-DIRECTIONALRANGE (VOR). A VOR operates within the108.0 to 111.85 MHz, and 112.0 to 117.95MHz frequency bands. The display usuallyconsists of a deviation indicator and aTO/FROM indicator. The controls consist of afrequency selector for selecting the groundstation and an OBS, which is used for courseselection. An ON/OFF flag is used to deter-mine adequate field strength and presence of avalid signal. There are numerous configura-tions when integrated into flight directorsand/or when using a slaved compass system,which uses an additional indicator that pointscontinually to the selected omni station re-gardless of OBS selection. In order to deter-mine the accuracy specified in a functionalcheck, a ground test set must be used in accor-dance with the manufacturer’s specifications.For the purpose of this inspection/maintenanceactivity, the following operational check canbe accomplished to determine if the equipmenthas the accuracy required for operation in in-strument flight rules (IFR) environment. Ver-ify audio identification, OBS operation, flagoperation, radio magnetic indicator (RMI) in-terface, and applicable navigation (NAV)switching functions. The operational check isalso published in the AIM, section 1-1-4. Thischeck is required by 14 CFR part 91, sec-tion 91.171 before instrument flight opera-tions.

12-12. DISTANCE MEASURING EQUIP-MENT (DME). The operation of DME con-sists of paired pulses at a specific spacing, sentout from the aircraft (this is what is called in-terrogation), and are received by the groundstation, which then responds with paired pulsesat the specific spacing sent by the aircraft, but

at a different frequency. The aircraft unitmeasures the time it takes to transmit and thenreceive the signal, which then is translated intodistance. DME operates on frequencies from962 MHz to 1213 MHz. Because of the cur-vature of earth, this line-of-sight signal is reli-able up to 199 nautical mile (NM) at the highend of the controlled airspace with an accuracyof 1/2 mile or 3 percent of the distance. DMEinspection/maintenance on the aircraft is mostcommonly limited to a visual check of the in-stallation, and if there have been previouslyreported problems, the antenna must be in-spected for proper bonding and the absence ofcorrosion, both on the mounting surface, aswell as the coax connector. Accuracy can bedetermined by evaluating performance duringflight operations, as well as with ground testequipment. If a discrepancy is reported andcorrected, it is good practice to make the accu-racy determination before instrument flight.Tune the DME to a local station, or use theproper ground test equipment to check audioidentification, and DME hold function verifycorrect display operation.

12-13. AUTOMATIC DIRECTIONFINDER (ADF). The ADF receivers are pri-marily designed to receive nondirectional bea-cons (NDB) in the 19 to 535 kHz amplitudemodulation (AM) broadcast low band. Thereceivers will also operate in the commercialAM band. The ADF display pointer will indi-cate the relative bearing to a selected AM bandtransmitter that is in range. An ADF systemmust be checked by tuning to an adequateNDB or commercial AM station. Verifyproper bearing to station, audio identificationand tone/beat frequency oscillator (BFO), cor-rect operation in closed circuit (LOOP) andsense modes. Note the orientation of the se-lected station to the aircraft using an appropri-ate chart. Observe the ADF relative bearingreading, and compare to the chart. Slew theneedle and observe how fast (or slowly) it re-turns to the reading. ADF performance may

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be degraded by lightning activity, airframecharging, ignition noise and atmospheric phe-nomena.

12-14. INSTRUMENT LANDINGSYSTEMS (ILS). The ILS consist of severalcomponents, such as the localizer, glide slope,marker beacon, radio altimeter, and DME.Localizer and glide slope receivers and markerbeacons will be discussed in this section.

a. Localizer receiver operates on one of40 ILS channels within the frequency range of108.10 to 111.95 MHz (odd tens). These sig-nals provide course guidance to the pilot to therunway centerline through the lateral dis-placement of the VOR/localizer (LOC) devia-tion indicator. The ground transmitter issighted at the far end of the runway and pro-vides a valid signal from a distance of 18 NMfrom the transmitter. The indication gives afull fly left/right deviation of 700 feet at therunway threshold. Identification of the trans-mitter is in International Morse Code and con-sists of a three letter identifier preceded by theMorse Code letter I (two dots). The localizerfunction is usually integral with the VOR sys-tem, and when maintenance is performed onthe VOR unit, the localizer is also included.The accuracy of the system can be effectivelyevaluated through normal flight operations ifevaluated during visual meteorological condi-tions. Any determination of airworthiness af-ter reinstallation before instrument flight mustbe accomplished with ground test equipment.

b. The glide slope receiver operates onone of 40 channels within the frequency range329.15 MHz, to 335.00 MHz. The glide slopetransmitter is located between 750 feet and1250 feet from the approach end of the runwayand offset 250 to 650 feet. In the absence ofquestionable performance, periodic functionalflight checks of the glide slope system wouldbe an acceptable way to ensure continued sys-tem performance. The functional flight test

must be conducted under visual flight rules(VFR) conditions. A failed or misleadingsystem must be serviced by an appropriately-rated repair station. Ground test equipmentcan be used to verify glide slope operation.

c. Localizer/Glide Slope (LOC/GS) mayhave self test function, otherwise the properground test equipment must be used. Refer tomanufacturer’s or aircraft instruction manual.

12-15. MARKER BEACON. Marker bea-con receivers operate at 75 MHz and sense theaudio signature of each of the three types ofbeacons. The marker beacon receiver is nottunable. The blue outer marker light illumi-nates when the receiver acquires a 75 MHzsignal modulated with 400 Hz, an amber mid-dle marker light for a 75 MHz signal modu-lated with 1300 Hz and, a white inner markerlight for a 75 MHz signal modulated with3000 Hz. The marker beacon system must beoperationally evaluated in VFR when an ILSrunway is available. The receiver sensitivityswitch must be placed in LOW SENSE (thenormal setting). Marker audio must be ade-quate. Ground test equipment must be used toverify marker beacon operation. Marker bea-con with self test feature, verify lamps, audioand lamp dimming.

12-16. LONG RANGE NAVIGATION(LORAN). The LORAN has been an effectivealternative to Rho/Theta R-Nav systems. Hy-perbolic systems require waypoint designationin terms of latitude and longitude, unlikeoriginal R-Nav (distance navigation) systems,which define waypoints in terms of distance(Rho) and angle (Theta) from established VORor Tacan facilities. Accuracy is better than theVOR/Tacan system but LORAN is more proneto problems with precipitation static. Properbonding of aircraft structure and the use ofhigh-quality static wicks will not only produceimproved LORAN system performance, butcan also benefit the very high frequency

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(VHF) navigation and communications sys-tems. This system has an automatic testequipment (ATE).

NOTE: Aircraft must be outside ofhangar for LORAN to operate.

Normally self test check units, verification ofposition, and loading of flight plan will verifyoperation verification of proper flight manualsupplements and operating handbooks onboard, and proper software status can also beverified.

12-17. GLOBAL POSITIONINGSYSTEM (GPS). The GPS is at the forefrontof present generation navigation systems. Thisspace-based navigation system is based on a24-satellite system and is highly accurate(within 100 meters) for establishing position.The system is unaffected by weather and pro-vides a world-wide common grid referencesystem. Database updating and antenna main-tenance are of primary concern to the GPSuser.

NOTE: Aircraft must be outside ofhangar for ground test of GPS.

12-18. AUTOPILOT SYSTEMS. Auto-matic Flight Control Systems (AFCS) are themost efficient managers of aircraft perform-ance and control. There are three kinds ofautopilot; two axes, three axes, and three axeswith coupled approach capability. Attentionmust be given to the disconnect switch opera-tion, aural and visual alerts of automatic andintentional autopilot disconnects, overrideforces and mode annunciation, servo opera-tion, rigging and bridle cable tension, and con-dition. In all cases the manufacturer’s inspec-tion and maintenance instructions must befollowed.

12-19. ALTIMETERS. Aircraft conductingoperations in controlled airspace under instru

ment flight rule (IFR) are required to havetheir static system(s) and each altimeter in-strument inspected and tested within the previ-ous 24 calendar months. Frequent functionalchecks of all altimeters and automatic pressurealtitude reporting systems are recommended.

a. The tests required must be performedby:

(1) The manufacturer of the aircraft onwhich the tests and inspection are to be per-formed.

(2) A certificated repair station properlyequipped to perform those functions andholding:

(a) An instrument rating Class I.(b) A limited instrument rating ap-

propriate to the make and model of applianceto be tested.

(c) A certified/qualified mechanicwith an airframe rating(static system tests andinspections only). Any adjustments shall beaccomplished only by an instrument shop cer-tified/qualified person using proper testequipment and adequate reference to themanufacturer’s maintenance manuals. Thealtimeter correlation adjustment shall not beadjusted in the field. Changing this adjustmentwill nullify the correspondence between thebasic test equipment calibration standards andthe altimeter. It will also nullify correspon-dence between the encoding altimeter and itsencoding digitizer or the associated blind en-coder.

b. Examine the altimeter face for evi-dence of needle scrapes or other damage.Check smoothness of operation, with particu-lar attention to altimeter performance duringdecent.

c. Contact an appropriate air traffic fa-cility for the pressure altitude displayed to thecontroller from your aircraft. Correct the re-ported altitude as needed, and compare to the

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reading on the altimeter instrument. The dif-ference must not exceed 125 feet.

12-20. TRANSPONDERS. There are threemodes (types) of transponders that can be usedon various aircraft. Mode A provides a(non altitude-reporting) four-digit coded reply;Mode C provides a code reply identical toMode A with an altitude-reporting signal; andMode S has the same capabilities as Mode Aand Mode C and responds to traffic alert andcollision avoidance system (TCAS)-EquippedAircraft.

a. Ground ramp equipment must beused to demonstrate proper operation. Enoughcodes must be selected so that each switchpo-sition is checked at least once. Low and highsensitivity operation must be checked. Identi-fication operation must be checked. Altitudereporting mode must be demonstrated. Dem-onstrate that the transponder system does notinterfere with other systems aboard the aircraft,and that other equipment does not interferewith transponder operation. Special consid-eration must be given to other pulse equip-ment, such as DME and weather radar.

b. All transponders must be tested every24-calendar months, or during an annual in-spection, if requested by the owner. The testmust be conducted by an authorized avionicsrepair facility.

12-21. EMERGENCY LOCATORTRANS- MITTERS (ELT). The ELT mustbe evaluated in accordance with TSO-C91a,TSO-C126 for 406 MHz ELT’s, or later TSO’sissued for ELT’s. ELT installations must beexamined for potential operational problems atleast once a year (section 91.207(d)). Therehave been numerous instances of interactionbetween ELT and other VHF installations.Antenna location should be as far as possiblefrom other antennas to prevent efficiencylosses. Check ELT antenna installations inclose proximity to other VHF antennas forsuspected interference. Antenna patterns of

previously installed VHF antennas could bemeasured after an ELT installation. Testsshould be conducted during the first 5 minutesafter any hour. If operational tests must bemade outside of this time frame, they shouldbe coordinated with the nearest FAA ControlTower or FSS. Tests should be no longer thanthree audible sweeps.

12-22. INSPECTION OF ELT. An in-spection of the following must be accom-plished by a properly certified person or repairstation within 12-calendar months after the lastinspection:

a. Proper Installation.

(1) Remove all interconnections to theELT unit and ELT antenna. Visually inspectand confirm proper seating of all connectorpins. Special attention should be given to co-axial center conductor pins, which are prone toretracting into the connector housing.

(2) Remove the ELT from the mountand inspect the mounting hardware for properinstallation and security.

(3) Reinstall the ELT into its mount andverify the proper direction for crash activation.Reconnect all cables. They should have someslack at each end and should be properly se-cured to the airplane structure for support andprotection.

b. Battery Corrosion. Gain access to theELT battery and inspect. No corrosion shouldbe detectable. Verify the ELT battery is ap-proved and check its expiration date.

c. Operation of the Controls and CrashSensor. Activate the ELT using an appliedforce. Consult the ELT manufacturer’s in-structions before activation. The direction formounting and force activation is indicated on

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the ELT. A TSO-C91 ELT can be activated byusing a quick rap with the palm. A TSO-C91aELT can be activated by using a rapid forward(throwing) motion coupled by a rapid revers-ing action. Verify that the ELT can be acti-vated using a watt meter, the airplane’s VHFradio communications receiver tuned to121.5 MHz, or other means (see NOTE 1).Insure that the “G” switch has been reset if ap-plicable.

d. For a Sufficient Signal RadiatedFrom its Antenna. Activate the ELT usingthe ON or ELT TEST switch. A low-qualityAM broadcast radio receiver should be used todetermine if energy is being transmitted fromthe antenna. When the antenna of the AMbroadcast radio receiver (tuning dial on anysetting) is held about 6 inches from the acti-vated ELT antenna, the ELT aural tone will beheard (see NOTE 2 and 3).

e. Verify that All Switches are ProperlyLabeled and Positioned.

f. Record the Inspection. Record the in-spection in the aircraft maintenance recordsaccording to 14 CFR part 43, section 43.9. Wesuggest the following:

I inspected the Make/Model ______________ELT system in this aircraft according to appli-cable Aircraft and ELT manufacturer’s in-structions and applicable FAA guidance andfound that it meets the requirements of sec-tion 91.207(d).

Signed: ______________________Certificate No. ______________________Date: ______________________

NOTE 1: This is not a measuredcheck; it only indicates that theG-switch is working.

NOTE 2: This is not a measuredcheck; but it does provide confidencethat the antenna is radiating with suf-ficient power to aid search and rescue.The signal may be weak even if it ispicked up by an aircraft VHF receiverlocated at a considerable distancefrom the radiating ELT. Therefore,this check does not check the integrityof the ELT system or provide the samelevel of confidence as does the AM ra-dio check.

NOTE 3: Because the ELT radiateson the emergency frequency, the Fed-eral Communications Commission al-lows these tests only to be conductedwithin the first five minutes after anyhour and is limited in three sweeps ofthe transmitter audio modulation.

12-23. FLIGHT DATA RECORDER. Theflight data recorder is housed in a crush-proofcontainer located near the tail section of theaircraft. The tape unit is fire resistant, andcontains a radio transmitter to help crash in-vestigators locate the unit under water. In-spection/Operational checks include:

a. Check special sticker on front of theflight data recorder for the date of the next tapereplacement, if applicable.

b. Remove recorder magazine and in-spect tape for the following:

(1) broken or torn tape,

(2) proper feed of tape, and

(3) all scribes were recording properlyfor approximately the last hour of flight.

c. Conditions for tape replacement (asapplicable):

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(1) There is less than 20 hours remain-ing in the magazine as read on thetape remaining indicator.

(2) Tape has run out.

(3) Broken tape.

(4) After hard landings and severe airturbulence have been encountered as reportedby the pilots. After the same tape has been inuse 1 year (12 months), it must be replaced.

(5) Ensure that a correlation test hasbeen performed and then recorded in the air-craft records.

d. Refer to the specific equipment manu-facturer’s manuals and procedures.

e. The state-of-the art Solid-State FlightData Recorder (SSFDR) is a highly flexiblemodel able to support a wide variety of aero-nautical radio, incorporated (ARINC) configu-rations. It has a Built-In Test Equipment(BITE) that establishes and monitors the mis-sion fitness of the hardware. BITE performsverification after storage (read after write) offlight data and status condition of the memory.These recorders have an underwater acousticbeacon mounted on its front panel which mustbe returned to their respective manufacturer’sfor battery servicing. For maintenance infor-mation refer to the equipment or aircraft manu-facture’s maintenance instruction manual.

12-24. COCKPIT VOICE RECORDERS(CVR). CVR’s are very similar to flight datarecorders. They look nearly identical and op-erate in almost the same way. CVR’s monitorsthe last 30 minutes of flight deck conversationsand radio communications. The flight deckconversations are recorded via the microphonemonitor panel located on the flight deck. Thispanel is also used to test the system and erasethe tape, if so desired. Before operating the

erase CVR mode, consult the operational man-ual of the manufacturer for the CVR.

a. Playback is possible only after the re-corder is removed from the aircraft.

b. Refer to the specific equipment manu-facturer’s manuals and procedures.

c. The Solid State Cockpit Voice Re-corder system is composed of three essentialcomponents a solid state recorder, a controlunit (remote mic amplifier), and an area mi-crophone. Also installed on one end of therecorder is an Under water Locator Beacon(ULB). The recorder accepts four separateaudio inputs: pilot, copilot, public ad-dress/third crew member, and cockpit area mi-crophone and where applicable, rotor speedinput and flight data recorder synchronizationtone input. For maintenance information referto the equipment manufacturer’s maintenancemanual.

12-25. WEATHER RADAR. Ground per-formance shall include antenna rotation, tilt,indicator brilliance, scan rotation, and indica-tion of received echoes. It must be determinedthat no objectionable interference from otherelectrical/electronic equipment appears on theradar indicator, and that the radar system doesnot interfere with the operation of any of theaircraft’s communications or navigation sys-tems.

CAUTION: Do not turn radar onwithin 15 feet of ground personnel, orcontainers holding flammable or ex-plosive materials. The radar shouldnever operate during fueling opera-tions. Do not operate radar systemwhen beam may intercept larger me-tallic objects closer than 150 feet, ascrystal damage might occur. Do notoperate radar when cooling fans areinoperative. Refer to the specific Ra

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dar System equipment manufacturer’smanuals and procedures.

12-26. RADOME INSPECTION. Inspec-tion of aircraft having weather radar installa-tions should include a visual check of the ra-dome surface for signs of surface damage,holes, cracks, chipping, and peeling of paint,etc. Attach fittings and fastenings, neopreneerosion caps, and lightening strips, when in-stalled, should also be inspected.

12-27. DATA BUS. Data Buses provide thephysical and functional partitioning needed toenable different companies to design differentavionics boxes to be able to communicate in-formation to each other. It defines the frame-work for system(s) intergration. There areseveral types of data bus analyzers used to re-ceive and review transmitted data or to trans-mit data to a bus user. Before using an ana-lyzer, make sure that the bus language is com-patible with the bus analyzer. For further in-formation refer to ARINC specifications suchas 429 Digital Information Transfer System,Mark 33 which offers simple and affordableanswers to data communications on aircraft.

12-28. ELECTRIC COMPATABLITY.When replacing an instrument with one whichprovides additional functions or when addingnew instruments, check the following electrical( where applicable) for compatibility:a. Voltage (AC/DC).b. Voltage polarity (DC).c. Voltage phase (s) (AC).d. Frequency (AC).e. Grounding (AC/DC).f. System impedance matching.g. Compatibility with system to which con-

nected.

12-29. 12-36. [RESERVED.]

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SECTION 3. GROUND OPERATIONAL CHECKS FOR AVIONICS EQUIPMENT(NON ELECTRICAL)

12-37. COMPASS SWING must be per-formed whenever any ferrous component ofthe system (i.e. flux valve compensator, orStandby Compass) is installed, removed, re-paired, or a new compass is installed. Themagnetic compass can be checked for accuracyby using a compass rose located on an airport.The compass swing is normally effected byplacing the aircraft on various magnetic head-ings and comparing the deviations with thoseon the deviation cards. Refer to CFR14,23.1327, 14 CFR 23.1547, and the equipmentor aircraft manufacturer's manual.

a. A compass swing must be performed onthe following occasions:

(1) When the accuracy of the compassis suspected.

(2) After any cockpit modification ormajor replacement involving ferrous metal.

(3) Whenever a compass has been sub-jected to a shock; for example, after a hardlanding or turbulence.

(4) After aircraft has passed through asevere electrical storm.

(5) After lighting strike.

(6) Whenever a change is made to theelectrical system.

(7) Whenever a change of cargo islikely to affect the compass.

(8) When an aircraft operation ischanged to a different geographic location witha major change in magnetic deviation. (e.g.,from Miami, Florida to Fairbanks, Alaska.)

(9) After aircraft has been parked onone heading for over a year.

(10) When flux valves are replaced.

b. Precautions.

(1) The magnetic compass must bechecked for accuracy in a location free of steelstructures, underground pipes or cables, orequipment that produces magnetic fields.

(2) Personnel engaged in the compen-sation of the compass shall remove all mag-netic or ferrous material from their possession.

(3) Use only nonmagnetic tools whenadjusting the compass.

(4) Position the aircraft at least 100yards from any metal object.

(5) All equipment in the aircraft havingany magnetic effect on the compass must besecured in the position occupied in normalflight.

c. Compass Swing Procedures.

(1) Have the aircraft taxied to theNORTH (0°) radial on the Compass Rose. Usea hairline sight compass (a reverse readingcompass with a gun sight arrangementmounted on top of it) to place the aircraft inthe general vicinity. With the aircraft facingNorth and the person in the cockpit runningthe engine(s) at 1000 rpm, a mechanic, stand-ing approximately 30 feet in front of the air-craft and facing South, “shoots” or aligns themaster compass with the aircraft center line.Using hand signals, the mechanic signals theperson in the cockpit to make additional ad-justments to align the aircraft with the master

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compass. Once aligned on the heading, theperson in the cockpit runs the engine(s) to ap-proximately 1,700 rpm to duplicate the air-craft’s magnetic field and then the person readsthe compass.

NOTE: (1) For conventional gear air-craft, the mechanic will have to posi-tion the magnetic compass in thestraight and level position or mountthe tail of the aircraft on a moveabledolly to simulate a straight and levelcruise configuration. (2) Rememberthe hairline sight compass is only in-tended to be used as a general piece oftest equipment.

(2) If the aircraft compass is not inalignment with the magnetic North of themaster compass, correct the error by makingsmall adjustments to the North-South brassadjustment screw with a nonmetallic screwdriver (made out of brass stock, or stainlesssteel welding rod). Adjust the N-S compen-sator screw until the compass reads North (0°).Turn the aircraft until it is aligned with theEast-West, pointing East. Adjust the E-Wcompensator screw until it reads 90°. Con-tinue by turning the aircraft South 180° andadjust the N-S screw to remove one-half of theSouth’s heading error. This will throw theNorth off, but the total North-South should bedivided equally between the two headings.Turn the aircraft until it is heading West 270°,and adjust the E-W screw on the compensatorto remove one-half of the West error. Thisshould divide equally the total E-W error. Theengine(s) should be running.

(3) With the aircraft heading West, startyour calibration card here and record the mag-netic heading of 270° and the compass readingwith the avionics/electrical systems on thenoff. Turn the aircraft to align with each of thelines on the compass rose and record the com

pass reading every 30°. There should be notmore than a plus or minus 10° difference be-tween any of the compass’ heading and themagnetic heading of the aircraft.

(4) If the compass cannot be adjusted tomeet the requirements, install another one.

NOTE: A common error that affectsthe compass’ accuracy is the mountingof a compass or instruments on or inthe instrument panel using steel ma-chine screws/nuts rather than brasshardware, magnetized control yoke,structural tubing, and improperlyrouted electrical wiring, which cancause unreasonable compass error.

(5) If the aircraft has an electrical sys-tem, two complete compass checks should beperformed, one with minimum electricalequipment operating and the other with allelectrical accessories on (e.g. radios, naviga-tion, radar, and lights). If the compass read-ings are not identical, the mechanic shouldmake up two separate compass correctioncards, one with all the equipment on and onewith the equipment off.

(6) When the compass is satisfactorilyswung, fill out the calibration card properlyand put it in the holder in full view for the pi-lot’s reference.

d. Standby (wet) Compass. Adjustmentand compensation of the Standby Compassmay also be accomplished by using the “com-pass swing” method.

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Par 12-38 Page 12-15

12-38. PNEUMATIC GYROS.

a. Venturi Systems. The early gyro in-struments were all operated by air flowing outof a jet over buckets cut into the periphery ofthe gyro rotor. A venturi was mounted on theoutside of the aircraft to produce a low pres-sure, or vacuum, which evacuated the instru-ment case, and air flowed into the instrumentthrough a paper filter and then through a noz-zle onto the rotor.

(1) Venturi systems have the advantageof being extremely simple and requiring nopower from the engine, nor from any of theother aircraft systems; but they do have thedisadvantage of being susceptible to ice, and

when they are most needed, they may becomeunusable.

(2) There are two sizes of venturi tubes:those which produce four inches of suction areused to drive the attitude gyros, and smallertubes, which produce two inches of suction,are used for the turn and slip indicator. Someinstallations use two of the larger venturi tubesconnected in parallel to the two attitude gyros,and the turn and slip indicator is connected toone of these instruments with a needle valvebetween them. A suction gage is temporarilyconnected to the turn and slip indicator, andthe aircraft is flown so the needle valve can beadjusted to the required suction at the instru-ment when the aircraft is operated at its cruisespeed. (See figure 12-1.)

FIGURE 12-1. Venturi system for providing airflow through gyro instruments.

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Page 12-16 Par 12-38

b. Vacuum Pump Systems. In order toovercome the major drawback of the venturitube, that is, its susceptibility to ice, aircraftwere equipped with engine driven vacuumpumps and the gyro instruments were drivenby air pulled through the instrument by thesuction produced by these pumps. A suctionrelief valve maintained the desired pressure(usually about four inches of mercury) on theattitude gyro instruments, and a needle valvebetween one of the attitude indicators and theturn and slip indicator restricted the airflow tomaintain the desired 2 inches of suction in itscase. Most of the early instruments used onlypaper filters in each of the instrument cases,but in some installations a central air filter wasused to remove contaminants from the cabinair before it entered the instrument case.

(1) The early vacuum pumps werevane-type pumps of what is called the wettype-one with a cast iron housing and steelvanes. Engine oil was metered into the pumpto provide sealing, lubrication, and cooling,and then this oil, along with the air, was blownthrough an oil separator where the oil collectedon baffles and was returned to the enginecrankcase. The air was then exhausted over-board. Aircraft equipped with rubber deicerboots used this discharge air to inflate theboots. But before it could be used, this air waspassed through a second stage of oil separationand then to the distributor valve and finally tothe boots. (See figure 12-2.)

(2) The airflow through the instrumentsis controlled by maintaining the suction in theinstrument case at the desired level with a suc-tion relief valve mounted between the pumpand the instruments. This valve has aspring-loaded poppet that offsets to allowcabin air to enter the pump and maintain thecorrect negative pressure inside the instrumentcase.

(3) The more modern vacuum pumpsare of the dry type. These pumps use carbonvanes and do not require any lubrication, as thevanes provide their own lubrication as theywear away at a carefully predetermined rate.Other than the fact that they do not require anoil separator, systems using dry air pumps arequite similar to those using a wet pump. Oneslight difference, however, is in the need forkeeping the inside of the pump perfectly clean.Any solid particles drawn into the systemthrough the suction relief valve can damageone of the carbon vanes, and this can lead todestruction of the pump, as the particles bro-ken off of one vane will damage all of theother vanes. To prevent particles entering therelief valve, its air inlet is covered with a filter,and this must be cleaned or replaced at the in-terval recommended by the aircraft manufac-turer.

c. Positive Pressure Systems. Aboveabout 18,000 feet there is not enough mass tothe air drawn through the instruments to pro-vide sufficient rotor speed, and, to remedy thisproblem, many aircraft that fly at high altitudeuse positive pressure systems to drive the gy-ros. These systems use the same type of airpump as is used for vacuum systems, but thedischarged air from the pump is filtered anddirected into the instrument case through thesame fitting that receives the filtered air whenthe vacuum system is used. A filter is installedon the inlet of the pump, and then, before theair is directed into the instrument case, it isagain filtered. A pressure regulator is locatedbetween the pump and the in-line filter to con-trol the air pressure so only the correct amountis directed into the instrument case.

System Filters. The life of an air-driven gyroinstrument is determined to a great extent bythe cleanliness of the air that flows over therotor. In vacuum systems, this air is taken

9/27/01 AC 43.13-1B CHG 1

Par 12-38 Page 12-17

FIGURE 12-2. Instrument vacuum system using a wet-type vacuum pump.

from the cabin where there is usually a gooddeal of dust and very often tobacco smoke.Unless all of the solid contaminants are re-moved from the air before it enters the instru-ment, they will accumulate, usually in the rotorbearings, and slow the rotor. This causes aninaccurate indication of the instrument andwill definitely shorten its service life. Dry air

pumps are also subject to damage from in-gested contaminants, and all of the filters inthe system must be replaced on the schedulerecommended by the aircraft manufacturer,and more often if the aircraft is operated underparticularly dusty conditions, especially if theoccupants of the aircraft regularly smoke whileflying. (See figures 12-3 and 12-4.)

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Page 12-18 Par 12-38

FIGURE 12-3. Instrument vacuum system using a dry-type air pump.

FIGURE 12-4. Instrument pressure system using a dry-type air pump.

12-39. 12-50. [RESERVED.]

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Par 12-51 Page 12-19

SECTION 4. PITOT/STATIC SYSTEMS

12-51. GENERAL. In order for the pitot-static instruments to work properly, they mustbe connected into a system that senses the im-pact air pressure with minimum distortion andpicks up undisturbed static air pressure.

Pitot pressure is ram air pressure picked up bya small open-ended tube about a ¼-inch in di-ameter that sticks directly into the air streamthat produces a pressure proportional to thespeed of the air movement. Static pressure isthe pressure of the still air used to measure thealtitude and serves as a reference in the meas-urement of airspeed.

Airspeed requires pitot, altimeter, rate ofclimb, and transponder-required static.

12-52. SYSTEM COMPONENTS. Theconventional design of the pitot system con-sists of pitot-static tubes or pitot tubes withstatic pressure parts and vents, lines, tubing,water drains and traps, selector valves, andvarious pressure-actuated indicators or controlunits such as the altimeter, airspeed and rate-of-climb indicators, and the encoding altimeterconnected to the system. (See figure 12-5.)

Figure 12-5. Pitot/static system for a small aircraft.

12-53. PITOT/STATIC TUBES ANDLINES. The pitot tube (see figure 12-6) is in-stalled at the leading edge of the wing of a sin-gle-engine aircraft, outside the propeller slip-stream or on the fuselage of a multiengine air-craft with the axis parallel to the longitudinalaxis of the aircraft, unless otherwise specifiedby the manufacturer.

12-54. STATIC PORTS AND VENTS(more modern trend) should be mounted flushwith fuselage skin. One port is located on ei-ther side of the fuselage, usually behind thecabin.

Inspect for elevation or depression of the portor vent fitting. Such elevation or depressionmay cause airflow disturbances at high speedsand result in erroneous airspeed and altitudeindications.

12-55. HEATER ELEMENTS. A heatingelement is located within the tube head to pre-vent the unit from becoming clogged duringicing conditions experienced during flight. Aswitch in the cockpit controls the heater.Some pitot-static tubes have replaceable heaterelements while others do not. Check theheater element or the entire tube for proper op-eration by noting either ammeter current orthat the tube or port is hot to the touch. (Seefigure 12-6.)

12-56. SYSTEM INSPECTION.

a. Inspect air passages in the systems forwater, paint, dirt or other foreign matter. Ifwater or obstructive material has entered thesystem, all drains should be cleaned. Probethe drains in the pitot tube with a fine wire toremove dirt or other obstructions. The bottomstatic openings act as drains for the head’sstatic chamber. Check these holes at regularintervals to preclude system malfunctioning.

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Page 12-20 Par 12-56

Figure 12-6. Pitot/Static Tube Head

b. Check to ensure the water drainsfreely. If a problem is experienced with thepitot-static system drainage or freezing at alti-tude, and the tubing diameter is less than3/8-inch, replace it with larger tubing.

c. Check the pitot tube for corrosion.

(1) The pitot probe should not haveany corrosion within ½-inch of the probe tip.

(2) Make sure there is no flakingwhich forms pits and irregularities in the sur-face of the tube.

NOTE: It is essential that the static airsystem be drained after the airplanehas been exposed to rain.

12-57. SYSTEM LEAK TEST.

a. Pitot-static leak tests should be madewith all instruments connected to assure thatno leaks occur at instrument connections.Such tests should be accomplished whenever aconnection has been loosened or an instrumentreplaced.

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Par 12-57 Page 12-21

b. After the conclusion of the leak test,return the system to its normal flying configu-ration. Remove tape from static ports and pitotdrain holes and replace the drain plug.

12-58. STATIC SYSTEM TESTS mustcomply with the static system tests required by14 CFR 91.411 and be performed by an appro-priately-rated repair station with the appropri-ate test equipment.

If the manufacturer has not issued instructionsfor testing static systems, the following may beused:

a. Connect the test equipment directlyto the static ports, if practicable. Otherwise,connect to a static system drain or tee connec-tion and seal off the static ports. If the testequipment is connected to the static system atany point other than the static port, it should bemade at a point where the connection may bereadily inspected for system integrity. Observemaintenance precautions given in paragraph12-60 of this section.

b. Do not blow air through the line to-ward the instrument panel. This may seriouslydamage the instruments. Be sure to disconnectthe instrument lines so no pressure can reachthe instruments.

c. Apply a vacuum equivalent to 1,000feet altitude, (differential pressure of approxi-mately 1.07 inches of mercury or 14.5 inchesof water) and hold.

d. After 1 minute, check to see that theleak has not exceeded the equivalent of 100feet of altitude (decrease in differential pres-sure of approximately 0.0105 inches of mer-cury or 1.43 inches of water).

12-59. TEST PITOT SYSTEM in accor-dance with the aircraft manufacturer’s instruc-tions. If the manufacturer has not issued in-

structions for testing pitot systems, the fol-lowing may be used:

a. Seal the drain holes and connect thepitot pressure openings to a tee to which asource of pressure and manometer or reliableindicator is connected.

b. Restrain hoses that can whip due toapplied pressure.

c. Apply pressure to cause the airspeedindicator to indicate 150 knots (differentialpressure 1.1 inches of mercury or 14.9 inchesof water), hold at this point and clamp off thesource of pressure. After 1 minute, the leakageshould not exceed 10 knots (decrease in differ-ential pressure of approximately 0.15 inchesof mercury or 2.04 inches of water).

CAUTION: To avoid rupturing thediaphragm of the airspeed indicator,apply pressure slowly and do notbuild up excessive pressure in the line.Release pressure slowly to avoid dam-aging the airspeed indicator.

d. If the airspeed indicator reading de-clines, check the system for leaky hoses andloose connections.

e. Inspect the hoses for signs of deterio-ration, particularly at bends and at the connec-tion points to the pitot mast and airspeed indi-cator. Replace hoses that are cracked or hard-ened with identical specification hoses. Anytime a hose is replaced, perform a pressurecheck.

Warning: Do not apply suction to pi-tot lines.

12-60. MAINTENANCE PRECAUTIONS.Observe the following precautions in all pitot-static system leak testing:

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Page 12-22 Par 12-60

a. Before any pitot/static system istested, determine that the design limits of in-struments attached to it will not be exceededduring the test. To determine this, locate andidentify all instruments attached to the system.

b. A system diagram will help to deter-mine the location of all instruments as well aslocate a leak while observing instrument indi-cations. If a diagram is not available, instru-ments can be located by tracing physical in-stallation.

c. Be certain that no leaks exist in thetest equipment.

d. Run full range tests only if you arethoroughly familiar with the aircraft instru-ment system and test equipment.

e. Make certain the pressure in the pitotsystem is always equal to, or greater than, thatin the static system. A negative differentialpressure across an airspeed indicator can dam-age the instrument.

f. The rate of change or the pressureapplied should not exceed the design limits ofany pitot or static instruments connected to thesystems.

g. When lines are attached to or re-moved from the bulkhead feed-through fittingor at a union, ensure the line attached to theopposite end is not loose, twisted, or damagedby rotation of the fitting. Such fittings nor-mally are provided with a hex flange for hold-ing the fitting.

12-61. REPLACING LINES. If necessaryto replace lines, observe the following instal-lation:

a. Attach lines at regular intervals bymeans of suitable clamps.

b. Do not clamp lines at end fittings.

c. Maintain the slope of lines towarddrains to ensure proper drainage.

d. Check the lines for leaks.

12-62. RELOCATON OF PITOT TUBE. Ifpitot tube relocation is necessary, perform therelocation in accordance with the manufac-turer’s recommendations and consider the fol-lowing:

a. Freedom of aerodynamic distur-bances caused by the aircraft.

b. Location protected from accidentaldamage.

c. Alignment with the longitudinal axisof the aircraft when in cruising flight.

12-63. TROUBLESHOOTING THE PI-TOT/STATIC PRESSURE SYSTEM.

a. If instruments are inoperative or er-ratic operation occurs, take the following ac-tion:

Table 12-1. Color codes for pitot-static systems.CODEABBR. DEFINITION COLOR

PPPITOT

PRESSURE NATURAL

SP

STATICPRESSURE

(PILOT) REDSTATIC

PRESSURE(CO-PILOT)

GREEN

STATICPRESSURE

(CABIN)YELLOW

STATICPRESSURE(STANDBY) BLUE

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Par 12-57 Pages 12-23 (and 12-24)

(1) Check for clogged lines. Drainlines at the valves (especially after aircraft hasbeen exposed to rain). Disconnect lines at theinstruments and blow them out withlow-pressure air.

(2) Check lines for leaks or loosenessat all connections. Repair as required.

b. If the pitot heating element(s) areoperative, check the following:

(1) Are circuit breaker(s) tripped?

(2) Reset the circuit breaker to deter-mine if:

(a) The system is OK, or

(b) The circuit breaker trips again,if so:

1. Check the wiring continu-ity to the ground. If the switch(s) is defective,repair as necessary.

2. Check the heating element;replace it if it is defective.

12-64. 12-69. [RESERVED.]

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Par 12-70 Page 12-25 (and 12-26)

SECTION 5. AVIONICS TEST EQUIPMENT

12-70 GENERAL. Certificated indi-viduals who maintain airborne avionicsequipment must have test equipment suitableto perform that maintenance.

12-71 TEST EQUIPMENT CALIBRA-TION STANDARDS.

a. The test equipment calibration stan-dards must be derived from and traceable toone of the following:

(1) The National Institute of Standardsand Technology.

(2) Standards established by the testequipment manufacturer.

(3) If foreign-manufactured test equip-ment, the standards of the country, where itwas manufactured, if approved by the Admin-istrator.

b. The technician must make sure that thetest equipment used for such maintenance isthe equipment called for by the manufactureror equivalent.

(1) Before acceptance, a comparisonshould be made between the specifications ofthe test equipment recommended by the manu-facturer and those proposed by the repair facil-ity.

(2) The test equipment must be capableof performing all normal tests and checking allparameters of the equipment under test. Thelevel of accuracy should be equal to or betterthan that recommended by the manufacturer.

(3) For a description of avionics testequipment used for troubleshooting, refer tothe equipment or aircraft manufacturing in-struction manual.

12-72 TEST EQUIPMENT CALIBRA-TION. Test equipment such as meters, torquewrenches, static, and transponder test equip-ment should be checked at least once a year.

c. National Institute of Standards andTechnology traceability can be verified by re-viewing test equipment calibration records forreferences to National Institute of Standardsand Technology test report numbers. Thesenumbers certify traceability of the equipmentused in calibration.

d. If the repair station uses a standardfor performing calibration, that calibrationstandard cannot be used to perform mainte-nance.

e. The calibration intervals for testequipment will vary with the type of equip-ment, environment, and use. The accepted in-dustry practice for calibration intervals is usu-ally one year. Considerations for acceptanceof the intervals include the following:

(1) Manufacturer’s recommendation forthe type of equipment.

(2) Repair facility’s past calibrationhistory, as applicable.

f. If the manufacturer’s manual doesnot describe a test procedure, the repair stationmust coordinate with the manufacturer to de-velop the necessary procedures, prior to anyuse of the equipment.

12-73 – 12-83. [RESERVED.]

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Par 13-1 Page 13-1 (and 13-2)

CHAPTER 13. HUMAN FACTORS

13-1. HUMAN FACTORS INFLUENCEON MECHANIC’S PERFORMANCE. Toaccomplish any task in aviation maintenance atleast three things must be in evidence. A me-chanic must have the tools, data, and technicalskill to perform maintenance. Only recentlyhas the aviation industry addressed the me-chanic job functions, pressures, and stress, byidentifying those human factors (HF) that im-pact the mechanics performance.

13-2. THE FAA AVIATION SAFETYPROGRAM has condensed these HF reports

into a personal minimums checklist, whichasks the mechanic to answer 10 ‘yes or no’questions before the maintenance task is begunand 10 ‘yes or no’ questions after the task iscompleted. If the mechanic answers “NO” toany of the 20 questions, the aircraft should notbe returned to service. We have provided thechecklist in figure 13-1 for your evaluation andreview. A color copy of the checklist is avail-able from any Flight Standards District Office.Just ask for the Airworthiness Safety Programmanager.

DO I HAVE THE KNOWLEDGE TO PER-FORM THE TASK?

DID I PERFORM THE JOB TASK TO THEBEST OF MY ABILITIES?

DO I HAVE THE TECHNICAL DATA TOPERFORM THE TASK?

WAS THE JOB TASK PERFORMED TO BEEQUAL TO THE ORIGINAL?

HAVE I PERFORMED THE TASK PREVI-OUSLY?

WAS THE JOB TASK PERFORMED IN AC-CORDANCE WITH APPROPRIATE DATA?

DO I HAVE THE PROPER TOOLS ANDEQUIPMENT TO PERFORM THE TASK?

DID I USE ALL THE METHODS, TECH-NIQUES, AND PRACTICES ACCEPTABLETO INDUSTRY?

HAVE I HAD THE PROPER TRAINING TOSUPPORT THE JOB TASK?

DID I PERFORM THE JOB TASK WITHOUTPRESSURES, STRESS, AND DISTRAC-TIONS?

AM I MENTALLY PREPARED TO PER-FORM THE JOB TASK?

DID I REINSPECT MY WORK OR HAVESOMEONE INSPECT MY WORK BEFORERETURNING TO SERVICE?

AM I PHYSICALLY PREPARED TO PER-FORM THE TASK?

DID I MAKE THE PROPER RECORD EN-TRIES FOR THE WORK PERFORMED?

HAVE I TAKEN THE PROPER SAFETYPRECAUTIONS TO PERFORM THE TASK?

DID I PERFORM THE OPERATIONALCHECKS AFTER THE WORK WAS COM-PLETED?

DO I HAVE THE RESOURCES AVAILABLETO PERFORM THE TASK?

AM I WILLING TO SIGN ON THE BOTTOMLINE FOR THE WORK PERFORMED?

HAVE I RESEARCHED THE FAR’S TO EN-SURE COMPLIANCE?

AM I WILLING TO FLY IN THE AIRCRAFTONCE IT IS APPROVED FOR THE RETURNTO SERVICE?

FIGURE 13-1. Personal Minimum’s Checklist