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Presented by

Claude LELAIE

Experimental test pilotSenior Vice President Product Safety

A380 wake encounters flight tests28th June 2010

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Airbus follow-on methodology : actual wake encounter flight-testing

Encounter test consists of physically flying an aircraft through the wake of another to measure specific parameters, thus determining the actual separation to apply.

The follower aircraft flies encounters alternatively behind A380 and a suitable reference aircraft, with both wake generators flying side by side.

Encounter test principle (1)

Generator

Follower

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Many parameters recorded with focus on the following flight parameters:

Encounter test principle (2)

Altitude loss

Vertical acceleration

Roll rate

Roll acceleration

Flight mechanic equations show that the roll acceleration is representative of the effect of the encounter on the aircraft.

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Defining encounter technique: how to visualize the wake

In cruise, wake can be visualized within the contrail.At lower altitude, visualization requires smoke generation.Two solutions envisaged

Generator pods under both wings (tested on A340-300)

Acceptable smoke but limited smoke timeRanges from 4 to 7 nm

Oil spray system (used on A340-600 and A380, preferred solution)

Excellent smoke visualizationSufficient production timeRanges from 4 to 8 nm (representative distance for 75t Medium follower)

A343 with generator pods

A380 with oil spray system

Cruise test with contrails

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Cruise test techniques (1)

Numerous preparation flights were necessary to tune the flight test techniques.First question: normal law or direct law?

Initial tests showed no difference as the motions on all axis were rather abrupt. Therefore normal law was used.The question came whether an encounter in normal law with the A318 could be compared to the same situation with a B737. The answer is yes due to the fact that in the worst cases, the roll rate is well above what is commanded by the A318 flight controls computers.Few tests with autopilot.

Stick free or not?No obvious answer to qualify the strength of the vortex. An equal number of each have been performed.For stick free tests, stick was released just prior entering the wake and taken back when roll motion decreased.For piloted tests, the pilot tried to minimize the effect of the encounter. It is NOT a realistic technique for Airline Pilots, but it is a mean to compare 2 aircraft.

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Cruise test techniques (2)

Observation of the rate of descent of vortex of both aircraft was an important issue due to RVSM.Tests were performed from 5 NM, standard separation for the B747 up to 15 NM, distance approved for the A380 at the time of the tests.All results were confirmed by Lidar observations.

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Flight test procedureComparison flight tests in cruise

Figure referenced to wake generating aircraft

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Cruise: Overview of flight tests performed

Flight tests to develop the flight test techniques:

C1: 11 JAN 2006: A380, A318 for wake encounters

C2: 23 FEB 2006: A380 and A346 side-by-side,A318 for wake encounters

C3: 07 MAR 2006: A380 and A346 side-by-side, DLR Lidar on-board Falcon, A346 encounters in trail of A380

C4: 09 MAR 2007: A346, DLR Lidar on-board Falcon

Comparison evaluation flight tests:

C5: 23 JUN 2006: A380 and A346, A318 for encounters,simultaneously DLR Lidaron-board Falcon

C6: 25 JUN 2006: A380 and B744, A318 for encounters,simultaneously DLR Lidaron-board Falcon

Follower aircraft

Generator aircraft

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Example:A318 11.4 NM behind A380(FT C5)

Example results (cruise)Video recording

A318msn1599fl783leg3vtp_082700b.mpg

Video

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Example results (cruise)A318 – stick-free encounters

Example FT C6 (A318 behind A380 side-by-side with B744)

Vertical load factor

Altitude deviation

Roll acceleration

Roll rate

B747-400A380

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Cruise test results

Rate of descent of vortex of both aircraft is identical. It reach –1000 ft at a distance between 12 and 15 NM.In some cases, rough encounters were obtained.For some parameters, the decrease with distance is rather slow.Looking at all the parameters, it was found that the A380 and the B747 are similar. The pilots impression was that no difference could be felt.All that was confirmed by the results of the on-board Lidar of the Falcon 20.

In cruise, same separation for the A380 than for the other Heavies has been approved.

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Cruise: applicable separations for all aircraft

Any aircraft following a

Heavy747: 5 nm

1,000 ft

1,000 ft

Separation apply to all categories aircraft

ICAO State letter Nov 05

A380: 15 nm

+ 10 nm 1,000 ft with precautions

1,000 ft with precautions

ICAO State letter Oct 06

1,000 ft

1,000 ftA380: 5 nm

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Approach: applicable separations for Heavy aircraft, based on Lidar tests

Heavy following a Heavy 747: 4 nm

ICAO State letter Nov 05

A380: 10 nm

+ 6 nm

ICAO State letter Oct 06

A380: 6 nm

+ 2 nm

Potential reduction pending current Work Group analysis of the Airbus wake encounter flight test data

ICAO State Letter July 08 A380: 6 nm

+ 2 nm

Future

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Approach: applicable separations for Medium aircraft, based on Lidar tests

Medium following a Heavy 747: 5 nm

ICAO State letter Nov 05

A380: 10 nm

+ 5 nm

ICAO State letter Oct 06

A380: 8 nm

+ 3 nm

A380: 7 nmICAO State letter

July 08

+ 2 nm

Potential reduction pending current Work Group analysis of the Airbus wake encounter flight test data Future

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A380 Wake Encounter Safety Assessment

When analyzed with a Lidar, wake encounter hazard can be described by the measured circulation Γ5-15.It is an accepted methodology, but it still rely on a model.The measurement area does not include the important core part of the vortex.

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Limitations when using Lidar data for separation standards

Vortex circulation obtained with Lidar was used to provide an indication of the severity of a wake encounter.

But, what are the effects of the vortices considering:Wake velocity profile

Aircraft weight

Roll inertia, wingspan of the follower?

=> This can only be determined by actual encounter testing

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Overview of encounter tests during approach

Flight tests to develop the flight test techniques:

L1: 20 NOV 2006: A320 behind A343

L2: 20 DEC 2006: A320 behind A343

L3: 21 DEC 2006: A380 behind A343

L4: 18 APR 2007: A320 behind A380

L5: 6 JUN 2007: A300B2 behind A380

L6: 8 JUN 2007: A300B2 behind A380

L7: 11 JUN 2007: A320 behind A346

L8: 13 JUN 2007: A380 behind A380

Comparison evaluation flight tests:

L9: 18 JUL 2007: A320 behind A380 side-to-side with A346

L10: 16 OCT 2007: A300B2 behind A380 side-to-side with A346

L11: 12 DEC 2007: A320 behind A380 side-to-side with A346

Followeraircraft

Generatoraircraft

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Flight test procedureComparison flight tests

Figure referenced to wake generating aircraft

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Approach: encounter flight test techniques (1)

Altitude: between 5000 and 12 000 ft (for safety reasons)Numerous parameters to be consideredWhich flight control law?

It appeared that the encounters were so “smooth” that the normal law was hiding most of the attitude variations.Deviations in pitch (altitude and acceleration) are rather small and are not a basis for comparison.Roll direct provides a good assessment of the strength of the vortex (roll acceleration).For the A320 the final choice has been alternate law with yaw damper (normal law in pitch, roll direct plus yaw damper).Few encounters in normal law and with autopilot.

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Approach: encounter flight test techniques (2)

Which flying technique?As for cruise, tests have been performed partly stick free, partly piloted.

Best angle to enter in the wakeComputation and models have shown that the worst case is a 10convergence. It is what has been targeted in average.If lower, it might be impossible to penetrate the wake.If higher, the upset is too short.

Descent or level flight?Level flight has been preferred, as it avoid changing the atmospheric conditions with altitude.Some tests performed on a 3 descent slope, but no difference identified.

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Approach: encounter flight test techniques (3)

Which follower aircraft?For the Heavy category, the choice was to get the smallest possible twin (less inertia in roll) in this category. We choose the A300B2, around 130 t MTOW.For the Medium, we took a fully instrumented A320. It is not the “worst” aircraft in this category because it is not the smallest, but it must be considered that the purpose was only to make back to back tests and not to have an absolute judgement on safety.

Data acquisition:Time histories of all flight mechanical parameters.Videos.Pilots assessments.

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Stick-free encounters:

A300B2 4.3 NM behind A380 (#67)

A320 4.6 NM behind A380 (#49)

Encounter test resultsVideo recordings and Vimosac3Danimations

67.mp2

A320_SBS2_Enc49_sf_380.avi

Video

sf_380_300_V1238_no67_track.avi

A320_SBS2_ap_sf_49_vimo_2.avi

Vimosac3D

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Comments on the videos

The videos are coming from tests behind A340-600 or A380, and it is not possible to make the differences between those aircraft.

As the A320 is flown in “roll direct”, in order to facilitate the measurements, the roll motion is amplified. When in “normal law”, flight by wire is fully active and the roll angle becomes very small.

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Encounter test results A300B2 – stick-free encounters

Example FT L10 (A300B2 behind A380 side-by-side with A346)

Vertical load factor

Altitude deviation

Roll acceleration

Roll rate

A340-600A380

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Encounter test resultsA320 – stick-free encounters

Example FTs L9 and L11 (A320 behind A380 side-by-side with A346)

Altitude deviation

Roll accelerationVertical load factor

Roll rate

A340-600A380

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Approach: A380 as follower

The large roll inertia and roll control capability of the A380 implies a very low sensitivity to wake induced effects.

This has been demonstrated by Airbus flight tests, with the A380 performing encounters in the wake of another A380 at a distance in trail below 1 nm whilst being fully controllable.

The Working Group and Steering Group have recommended to apply Minimum Radar Separations to A380 as follower (ICAO State Letter from July 2008 for approach and take-off).

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Conclusions for approach

The Working Group is presently assessing all these results with various methodologies, including a new one proposed by Airbus. It is the DWIM: Direct Wake Impact Methodology.

Some more flight tests are planned by the end of the year with Authorities on board. The analysis on these flights and the preparation of a Safety Assessment Report will probably not allow to conclude before Autumn 2011.

For the time being, all separations are based on Lidar validations.

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Comparison: circulation (Lidar) / roll accelerations (encounters) in approach

LIDAR is NOT the right tool to measure the effect of a wake vortex on a follower aircraft

Lidar measured circulation of A380 wake is stronger

The roll acceleration which represents the effect on the follower aircraft is identical behind A380 and A340-600

A340-600A380

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Lidar / encounters: conservative or not ?

If we use Lidar to compute the A380 separation in comparison with the B747, it is obviously conservative versus encounters and therefore safe.

If we imagine that the A380 is the “old” aircraft and the B747 the “new” one, using the Lidar data would lead to a decrease of B747 separation compared to A380. Therefore, we could have stronger encounters behind a B747 than behind the A380.

Lidar is not conservative. In some cases it could lead to a reduction of safety margins.

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RECAT – Consequences for the A380

RECAT is necessary to optimize the traffic flow and safety.

If RECAT is using Lidar and models to validate separations, there is a strong disadvantage for the A380 as shown on the Lidar curves.

However, we have demonstrated, thanks to the encounters, that the Lidar is NOT the right tool for the assessment of the consequences of an encounter.

Based on the results of our tests, the A380 and the B747 has to be put in the same category.

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Conclusions

More than 300 flight hours have been performed to study the effect of wake wortex.

More than 1000 encounters have been done, in cruise and in approach. This experience is unique.

It has allowed to show that A380 and B747 vortex have the same effect on follower aircraft in cruise and in approach.

All these tests have also demonstrated that the circulation obtained by Lidar measurements is not always the optimum tool to qualify the effect of a vortex. Validation of separations must also take into account results of encounter campaigns.