744 What if we lose an AOA vane

[JammedStab]

"The Cargolux Airlines S.A. Boeing 747-400, LX-FCL, was operating as flight CLX775 from Calgary Intl, AB (CYYC) to Luxembourg-Findel Intl, Luxembourg (ELLX). During takeoff, the pilots heard a loud bang followed by the stick shaker being activated. The airspeed indicator was still functioning, however the stall margin indicator was lost. The crew continued with the climb and assessed the situation. The crew suspected a nose wheel tire failure and elected to return to CYYC. An emergency was declared and the crew dumped fuel to get the aircraft within landing weight limits. The aircraft landed safely at CYYC with ARFF in attendance.

No damage was found to the nose wheel tire. Subsequent inspection by the operator’s maintenance revealed that the right AOA vane was missing, along with evidence of a bird strike. The AOA vane was replaced and a bird strike inspection was carried out. No other damage was found and the aircraft was returned to service."

[NSEU]

The airspeed indicator was still functioning, however the stall margin indicator was lost.

Do they mean the yellow band (hockey stick) or the red bricks (barber pole) or both?

The red bricks on the Captain's and F/O's are sourced separately (e.g. the F/O's indications are from the Right Stall Warning Computer which uses the AOA data from the F/O's selected ADC). Note that the Centre ADC has an input from both AOA vanes. Each AOA has two position transmitters. Unfortunately, I don't know how the data from both transmitters is used.

With flaps not up, the red brick data is driven by the respective stall warning computer. However, either stall warning computer can cause both sticks to shake. If the F/O's AOA is showing a greater angle than the other, the stick will shake because of that.

It's difficult to say what damage a bird will do to the AOA vanes. They can bend them, or in this case break them off completely, so the results may be completely different. If the vane is broken off, the position transmitter will most likely stay in its last position. I've noticed that the mechanism is slightly resistive to movement. This is probably why the AOA vane at the gate can be in different positions (not just completely drooped).

I've seen bent AOA vanes, but not missing ones. Accrording to the experts at Boeing, errors due to faulty 744 AOA sensors can be significant. Depending on flap position, 1 degree AOA error can cause 5~10kts error in VSSNG.

Cheers.

[piratepete]

This is an interesting problem.On one flight after MTCE the 767 I was flying had its Capts AOA replaced before flight but this was not told to us.The guy who did the work dropped the AOA unit damaging it but said nothing.On liftoff the stick shaker went bananas due to the "stuck" vane, inducing the PF to start to lower the nose near the ground.......a quick cross cockpit scan revealed the different angles of the PLIs and this allowed us to ignore the stick shaker and fly on IAS.......normal landing, not much fun at the time though.

[CCA]

Pulled the onside AOA and onside alternate CBs after takeoff in the SIM.

The onside air data, airspeed and altimeter failed with flags on the onside PFD.

Couldn't simulate a AOA failure in the SIM through the instructor panel.

[NSEU]

Thanks, CCA.
I guess pulling the CBs is only simulating a failure of the AOA sensor internals which would be much less likely to occur due to a bird strike. The vane would most likely have to be ripped from the shell of the AOA sensor to generate airspeed flags (or the bird go through the skin and affect the electrical wiring).

I wonder if the shape of the vane has any influence on impact resistance. There are older wedge-shaped vanes and newer wing-type vanes on 744s.

http://www.iinet.net.au/[email protected]/PPRuNe/ProbesLH1.jpg

[underfire]

NSEU, havent ever seen the old type vanes, the component AOA seems like it would have been an issue with influence from the arm.

back to the original issue, arent there at least 2 AOA vanes per ac, assuming redundancy, so why one damaged cause such issues?

Perhaps someone can explain why this is a vane outside the aircraft, rather than measured with the gyro, or some other level instrument inside?

[RealUlli]

(Interested SLF, not a pro pilot)


Perhaps someone can explain why this is a vane outside the aircraft, rather than measured with the gyro, or some other level instrument inside?


That's because AoA doesn't stay the same. It depends on static and dynamic loads, airspeed (indicated!) and air density. It doesn't depend on the position or direction the aircraft is pointing in relation to anything fixed (e.g. ground).

Static load is the mass of the aircraft (doesn't stay the same, either - fuel gets burned off during the flight)

Dynamic load is the acceleration in any direction (usually vertical relative to the cabin floor e.g. what a passenger feels when he gets pressed into his seat during takeoff, turns, etc.).

All of these (and possibly some others I forgot) combine to give you an angle of attack.

Usually, in a small aircraft, you fly your maneuvres with an eye on the airspeed indicator to keep it within a speed range that was determined to be safe. However, that is only necessary because the small plane lacks an AoA indicator. (Actually, the stall warning horn is a very rudimentary AoA indicator)

For a given wing profile, the stall always occurs at the same AoA (AFAIK, for the wing on a Cessna 172 that is about 17 degrees). The stall speed is just the speed at which the critical angle usually is reached. (AFAIR, the 172 has a rated stall speed of 55kn, however, there have been accidents where pilots did steep turns at (let's say) 80kn, stalled and lost control because of the higher dynamic load in the turn.

So, having a vane outside that gets turned by the airflow is a very simple but very effective way to find out the exact angle of the air flow relative to the aircraft.

(I hope I explained this reasonable accurately.. :-)

[NSEU]

NSEU, havent ever seen the old type vanes,

I've been out of the industry for a year or two, so I don't know if the newer vanes were retro-fitted to older aircraft (or perhaps you simply don't have any older aircraft in your fleet?).

back to the original issue, arent there at least 2 AOA vanes per ac, assuming redundancy, so why one damaged cause such issues?

Yes, there are only two vanes. Therefore, if one vane is physically broken off, only one vane can provide valid data. This good vane can feed the onside PFD and, if the other pilot selects the Centre ADC, then it seems that the other pilot will get his instrumentation back (or some, at least). "CCA" didn't mention selecting the Centre ADC (after he pulled the CBs in the sim).

Unfortunately, I think in the Cargolux incident, rather than no data being sent by the broken AOA, it was putting out false data. It's then up to the pilots to determine which data is correct. I didn't read the Cargolux report, so I don't know if they selected the Centre ADC (or if this helped with the stall warnings). According to my books, switching to the Centre ADC will allow the Centre ADC to provide AOA information to the Stall Warning Card giving out the false warning, but since the C ADC receives information from both AOA vanes, the data may still be corrupt (Pure speculation, but you might be able to get rid of the false warning by pulling the CB's mentioned above and switching ADCs, but I certainly wouldn't recommend doing this unless you knew exactly what was wrong with the AOA sensor).

Perhaps someone can explain why this is a vane outside the aircraft, rather than measured with the gyro, or some other level instrument inside?

The AOA vane is measuring the angle of airflow relative to the fuselage. You can't assume that the airflow is perfectly horizontal (especially during windshear). You have to have something physically outside in the airstream to measure AOA (just as pitot tubes have to be outside the aircraft to measure airspeed). Note that the pitot probes in the photo above are the newer type which put the tubes further away from the fuselage (to reduce the influence of the fuselage on airflow into the tubes).

Hope this makes sense.
Cheers

[msbbarratt]

Unfortunately, I think in the Cargolux incident, rather than no data being sent by the broken AOA, it was putting out false data. It's then up to the pilots to determine which data is correct.

It would be possible for design an AOA sensor that detected the jolt of a bird strike and then stopped sending data as a result.

Trouble is that this then becomes yet another sensor with it's own unique failure modes which have to be second guessed by pilots working in stressful conditions, etc etc.

This is why we still have pilots in cockpits. They adapt far more easily than a computer can, once they know what the problem is.

I've often thought that airliners under-use video cameras. It really wouldn't be hard these days to fit a few small cameras so that the pilots can actually put their own Mk1 eyeballs on the problem. A quick glance at a picture of a snapped off AOA sensor or a perforated engine case with flames coming out the back end would allow a pilot to get a quick (quicker?) understanding of a problem when all their instruments are showing crazy data.

With today's vision processing it would be easy to have a view come up automatically when something has changed appearance.

Any thoughts?

[underfire]

Hope this makes sense.

Thank you very much for the explanation, and yes, this does make sense.

In boarding ac, (A320) I have noticed that because of the shape, when on the ground, the weight of the wedge shape causes the vane to point straight up. (I was going to take a picture, but who knows who I would get to meet if I was noticed doing that!)
Thinking about this, I was wondering what sort of airflow it takes to overcome gravity and provide a reliable reading.

similar to this image
http://www.b-domke.de/AviationImages/A400M/Images/A400M_EC-402_34333.jpg


I have also noticed quite a few pairs of AoA and pitot tubes mounted on the same side of the aircraft
Looking at windshear or crosswinds, if these are both on the inside of the crab (either pitot or AoA) the airflow would be far less than the outside angle of the crab being shielded by the fuselage?....

http://www.journal-aviation.com/actualites/images/JAV20121220165426.jpg http://bp2.blogger.com/_oYg3CTTFzTQ/RllSBcheqOI/AAAAAAAAADw/Akbx_jBf0ts/s320/60+(1).jpg

I've often thought that airliners under-use video cameras.
AFAIK, it is currently not legal for pilots to use the cameras that are mounted outside the aircraft. I would have thought the nose wheel cam (that the passengers can see), would be very helpful at the gate...not sure why this is....

[underfire]

Boeing 777. the AoA pointing up.

http://i.imgur.com/1BOEHyU.jpg