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In the AF447 disaster, one of the elements that contributed to the crash was an "excessive" nose-up input shortly after the disconnection of the auto-pilot.

This caused the aeroplane to climb and lose airspeed, precipitating the ultimately fatal stall.

  • Why did the pilot flying pull back (excessively) on his stick? Did the investigation find probable causes?

He was at the same time correcting for turbulence-induced roll. Would a nose-up input in such circumstances be appropriate?

  • If so, how easy is it for a pilot to make an excessive input of that kind without realizing it, and what would normally prevent that from happening?

  • If not, is there any explanation for why he might have made it?

Most of the crew's actions in the incident are comprehensible, even if they were incorrect responses. They make sense in the context of the situation, and it's possible to have a fairly clear picture why they made the judgements and decisions they did.

Is there a good explanation for that initial nose-up input, that makes sense of it too in the context?

Daniele Procida
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    I'm voting to close this question as off-topic because accident speculation is explicitly called out as off-topic in Help/On-topic – Dan Pichelman Sep 20 '17 at 21:24
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    We have a few questions about AF447 already; this one might have some relevant info. And the official report is available, that has all the objective data. – Pondlife Sep 20 '17 at 21:45
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    He didn't recognize the stall, the captain did but by the time he realized that the control inputs were wrong it was too late. This accident resulted in more stall training and hand flying for pilots when automation fails. – Ron Beyer Sep 20 '17 at 21:45
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    Daniele, have you read the report issued by the French accident investigators? It's pretty thorough (BEA). If that body after an exhaustive investigation, could not come up with the why, who do you think can? The "why" being nailed down requires something like mind reading. If you have not read the report, I suggest you do that. Once you've read it, consider editing your question to reflect what you learn. – KorvinStarmast Sep 20 '17 at 22:11
  • @KorvinStarmast it addresses why he held that attitude for an extended period of time but doesn't really say why he pulled back at the beginning. the aircraft didn't stall until later after airspeed bled off and the nose was held up. – Ron Beyer Sep 20 '17 at 22:14
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    How are questions like Would a nose-up input in such circumstances be appropriate?, how easy is it for a pilot to make an excessive input of that kind without realising it, and what would normally prevent that from happening? or is there any explanation for xxx? "opinion-based"? I am not interested in guesses as to the contents of his mind; I am asking about piloting responses to conditions. Anyone who imagines those are a matter of opinion doesn't understand aviation. Or perhaps, questions. If the answer is that his initial action remains unexplained, that's also a possible answer. – Daniele Procida Sep 21 '17 at 07:52
  • @RonBeyer Please read the question. I'm not asking about how the crew failed to recognise the stall, or anything else about the incident. I am asking about why Bonin made the initial nose-up input as a response to the conditions (turbulence, reduced speed, roll and over-correction). Was that an appropriate piloting response? Is it one that is easy for a pilot to misjudge? I can repeat the questions in this comment, but they were in the question to begin with... – Daniele Procida Sep 21 '17 at 07:58
  • @DanieleProcida you have to consider that the title influences the way the post is read and received. I will try to adjust the title, but please review my edit afterwards. Also, asking "why person did X" is opinion based, we are not in their brains. – Federico Sep 21 '17 at 08:23
  • Closed as opinion based? There is an accident report which also addresses this issue, hardly opinion based I reckon. It's a different matter if there is no accident investigation report yet, as the help centre (which needs to be re-written anyway to reflect what the site morphed into!) clearly indicates. Have voted to re-open. – Koyovis Sep 21 '17 at 09:14
  • @Federico "Why" can address objective causes and reasons (as well as personal motivations, states of mind, etc). – Daniele Procida Sep 21 '17 at 09:53
  • @DanieleProcida Have you read the BEA report or not? – KorvinStarmast Sep 21 '17 at 17:50
  • @KorvinStarmast I do not understand the challenging tone of your repeated question. – Daniele Procida Sep 22 '17 at 13:37
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    Daniele, I believe that you will ask a better question if you read that (very well written) report. When you edited the question back to "why" it sent a signal that you are not taking these comments on board. I will again strongly recommend that you read the report. It is well worth the time. There isn't a simple answer. – KorvinStarmast Sep 22 '17 at 13:51
  • @KorvinStarmast The reason I removed the change to "probably causes" from the title is because it was simply wrong. None of my question was about causal factors, other edits to it notwithstanding. If there were a simple answer, I'd expect it to be generally understood by now. You're making rather a lot of assumptions. – Daniele Procida Sep 22 '17 at 16:18
  • Daniele please read pages 43-45 of the link I provided. – KorvinStarmast Sep 22 '17 at 17:31
  • Daniele, those are references to the pdf file pages, the report pages as numbered in the report are 41-44. – KorvinStarmast Sep 22 '17 at 17:48

2 Answers2

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It's not accident speculation when it has been investigated.

The full accident report is available online with a very adequate summary on Wikipedia.

There are many reasons, but from the human factors study:

The final BEA report points to the Human Computer Interface (HCI) of the Airbus as a possible factor contributing to the crash. It provides an explanation for most of the pitch-up inputs by the pilot flying (PF), namely that the Flight Director (FD) display was misleading.

The pitch-up input at the beginning of the fatal sequence of events appears to be the consequence of an altimeter error.

The crew present in the cockpit at the time of the pitot blockage lacked experience/training, and were bombarded with conflicting warnings and indications.

As I've been reminded in a comment, they were also in darkness over an ocean with no external visual references.

A note on how human factors studies can relate cause and effect. There is a certain reaction time that we are unable to exceed, roughly it's 200 milliseconds for a trained person anticipating an event. It's longer otherwise and there are many studies on that. When a control input is put up against the various indications, and taking into account the reaction times to the different aural and visual warnings, then scientifically the probable cause can be known within reason.

From page 42 in the BEA report:

The drop in Mach also impacts the SAT and thus the true air speed and the wind speed.

In the following table, the case an A330-200 flying at FL 350 at Mach 0.8 in standard atmosphere with a 30 kt head wind is given as an example to illustrate the consequences of pitot icing that would result in a drop in Mach from 0.8 to 0.3.

enter image description here

During Pitot probe de-icing, the same variations occur in the opposite direction.

From the table, you can see how the BEA may have assessed that the likely reason for the initial nose up reaction by the pilot flying was to correct a ~300' altitude error.

They also say (p. 157):

The report specified that the stall was caused by inappropriate crew reactions to erroneous speed and Mach displays that resulted from blockage of the Pitot probes through atmospheric icing. The report stated that contrary to standard operating procedures, the crew had not switched on the Pitot probe heating.

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First Officer Bonin's actions are almost a textbook example of the startle effect.

Simply put, he was shocked by the sudden disconnect of the autopilot that he mentally froze and couldn't think straight. He appears to already have been in a state of anxiety due to the St Elmos fire immediately beforehand, and the thunderstorm in the vicinity. It is a very interesting but sad human factors discussion.

A good explanation can be found here.

Ben
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  • @KorvinStarmast Edited to make it clear that I was referring to Bonin. Physically he did not freeze, but mentally he was stuck on his initial reflex, and could not take a step back to figure out what was happening. – Ben Sep 21 '17 at 00:29
  • Yeah, I see the point you are making. Also fits better with your link. – KorvinStarmast Sep 21 '17 at 02:14
  • @Ben I don't believe this is entirely correct. Although the startle effect is certainly implicated in the case, far from freezing, Bonin appeared to make a number of immediate corrections. Some were appropriate in kind if not degree (e.g. he overcorrected for roll) but the initial nose-up input of those first few seconds is harder to fathom. – Daniele Procida Sep 22 '17 at 16:29