bad news about Air France Crash 2009

[jackellis]

If the stall warning stops, you were in a stall.

Here lies a problem. When they pushed the nose over, the stall warning started squalling (if I remember the third party accounts, haven't read the BEA announcement yet). When they pulled up, it stopped. That will confuse the **** out of anyone.

Airplanes do, in fact, normally fly a bit nose heavy, and the main purpose of the horizontal stabilizer (aka, elevator) is to push the tail down. Forward CG is, as Loring points out, not a good thing, but aft CG is worse.

You can read more here. It seems to suggest the pilots kept the nose way up the whole way down. Very strange.

[All Thumbs]

So from your descriptions, it is possible that at a high enough altitude, you could get into a stall that you can't recover from, due to the balancing of the plane?

Could anything be done in that case, or would it just be dumb luck to get out of such a situation?

I take it these planes have GPS? Why do they rely so much on pilot tubes if GPS is available? You'd think there would be a warning system that their plane is descending too quickly based on their forward progress. Maybe there is such a warning?

It would be very interesting if Nova made an episode with a flight simulator demonstrating these conditions, and where pilots were put to the test to see if they could regain control of the plane.

[jackellis]

This is second-hand information but I believe it is reliable...and it is quite chilling.

FWIW, I received this forwarded from an NBAA member who received it in one of their list-serve mailings. It's not attributed, so who knows what the source is, but interesting anyway. Quotation is below the asterix.

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In the Airbus family, it may not be as simple as it would seem to those of us flying more conventional aircraft. Here's a note I received from a friend of mine who has flown / flies both the A330 and the comparable Boeing products.

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There are things that the Airbus is designed to do that would prevent the pilots from maintaining or regaining control if the pitots are blocked. For example if the crew tried to descend out of the icing conditions with the pitots blocked, the airspeed indication would increase. The flight control protections (speed too low or too high as in this case) could take control of the airplane away from the crew and pitch up to correct. This could further slow the airplane when in fact it was not over speeding. We are not taught to turn off any flight control computers. The conventional wisdom is that turning off computers is worst than waiting out the blocked pitots. Of course, most pilots who find themselves in an unreliable airspeed situation may be able to think through the problem, if everything else is ok. I believe that given the strong possibility that they were in moderate or greater turbulence, it was all they could do to focus on the panel, much less think through a complicated solution to the computer protections kicking in.

I talked to [a high time Airbus intructor] and he said that when he was given the problem (in the sim) as described above, he thought, "bring it on I know this airplane better than most and I can handle it". The problem came when he tried to change altitude with the pitots blocked. He said that he almost lost the airplane. He, being a check pilot and APD, was able to turn the right combination of flight control computers off and eventually regain control. He believes that if he were in turbulence or didn't teach in the program he may not have survived.

I know that you know how easy it would be to blame the pilots. I believe that Airbus will do just that. I also believe that the procedure to handle unreliable airspeed in the Fly By Wire Airbus is woefully inadequate.

The reason GPS isn't a substitute for a more direct measurement of airspeed is because GPS measures ground speed. Indicated airspeed, which is what a pitot tube measures, is critically important for avoiding stalls and overspeeds, both of which have fatal consequences.

The difference between ground speed measured by a GPS and indicated air speed measured by a pitot tube is influenced by the airplane's altitude, the outside temperature and the speed and direction of any winds. I have flown into 70 knot headwinds where my GPS registered an 80 knot groundspeed even though my indicated airspeed was 140 knots, and I have flown with tailwinds where my indicated airspeed as 140 knots and my ground speed was over 220 knots. If while flying along with that great tail wind I slowed my airplane to the point where ground speed was 100 knots, I'd be in a deep stall. On the other hand, I have had to slow the airplane down in the face of a strong headwind due to turbulence.

Flying at too high an indicated airspeed in a jet has other consequences. If the air flowing over the wings exceeds the speed of sound, it can send the jet into a dive that may not be recoverable (mach tuck). Too much speed can also cause the wings and tail feathers to flutter as the speed of the air flowing over them causes resonance (think the Tacoma Narrows Bridge disaster).

[Kristofor]

Well... As a physics problem you could probably back calculate airspeed from a combination of other sensors (ground speed, engine power output, weight, attitude, 3-axis accelerometers, etc.) but that sounds like a lot more opportunities for failure...

It would seem that an inertial/accelerometeric & gyroscopic sensor array could be built that would be able to scream "hey the ocean's getting pretty close, and we're 'flying' much like a brick does" regardless of the perceived conditions. I have no idea if such instruments exist or are used on commercial planes (presumably not based on this outcome). Perhaps again such a device would cause more confusion than it would avoid.