AF 447 Thread No. 12

[Concours77]

Hi KayPam,

"However, I have no idea what the THS did during the incident.
The THS position is a truly important flight parameter and I do believe airbus should display it more clearly."

What it did is critically instructive. From a position of 2? Degrees Nose Up it cycled, "...smoothly and consistently Full Nose Up, and remained in that position until impact."
The THS trims automatically, and cannot be manipulated effectively (it can be "held" manually, but when released it keeps moving). Only in Direct Law is it a legitimate Flight control, in fact, it is the only way to control Pitch "Manual trim only".

The BEA report relies almost exclusively on one foundational "fact". From the instant of the abnormal, the pilot flying held ".....mostly Nose Up stick...."

So, the Trim is full Nose Up, the aircraft is Stalled, and descending quickly. The pilot's have tried everything, "try climb (Captain)" "I have, many times....." Pilot flying.....

Aircraft attitude, Stalled, in rapid descent.... In the Stall, the aircraft is in "established" descent, though no pilot would call it that, look at the ROD!! But the computer, with NCD AoA, no reliable airspeed, and VS also wild, might call it "stable", no?

Is there an "airstream audio data channel" for the computer to sense!? Is the computer's best solution to lock its commanded Pitch where it is, not to be overridden by pilots? The pilots could hear the noise, and arguably knew the descent was certainly fatal, not to be locked in, but to be corrected!!?

Back to the stick. What is this control, really? It can "command" attitude, but not directly, it can only send a request to the FCM, to be analyzed, and "normed", then modified, and applied.

The stick, actually, is nothing more than a "suggestion box" with a handle.

So WHY, when Robert was pestering Bonin to "you climb, so go down...." Did Bonin not descend? It made no difference, the aircraft was already climbing, and nothing Bonin could do would arrest this climb. What is this feature? Not Overspeed Protection. Not a mistake either, the THS would continue this auto climb when the elevators lost authority, as we see in the THS trace, and the BEA statement of fact.

Why would this happen? This aircraft's flight control system is programmed to not let the Nose drop. The Airframe is designed to exhibit neutral stability in Pitch approaching the Stall......it is "longitudinally stable". It makes no difference that there is poor annunciation of the THS travel. And as I say, this NOSE UP is not negotiable. This aircraft does not drop its nose at Stall. By design. I think by the time the crew decided to get serious and boot Direct Law (".......PRIM/SEC.....fault") the Tail was stalled, the elevators were completely ineffective, and there was no time to get the Nose to drop via Direct Law; it is a laborious process.