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As far as I can tell, the following factors all contributed to this crash:

  • Pitot tubes susceptible to ice crystal blockage
  • Crew not aware that they can stall the airplane due to alternate law
  • No return to normal law after reliable speed information was available to the flight computers
  • "Unlinked" sidesticks which allow the pilots to command different inputs, unaware of what the other pilot is commanding
  • Stall warning goes quiet when the airspeed is too low

Of these, the first factor (pitot tubes) was already in the process of being mitigated. What about the other factors though? Were the onboard systems modified in any way to fix these shortfalls?

I'm no expert but it seems like at the very least that the following would help:

  • If the stall warning didn't stop, but instead changed into an even more ominous "you're now tumbling down at less than 60 KIAS" warning.
  • Also, given how extremely rare alternate law appears to be, it might not be out of order for the airplane to announce that it is now very much possible to stall the plane by pulling up (in actual words).
  • Finally, it sounds like the unlinked nature of the sidesticks was a big part of the problem; are there good reasons to leave them unlinked in future designs?

Some context:

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    $\begingroup$ I don't have a reference, but I remember hearing that they did fix the inhibition of the stall warning below 60KIAS. They also now require high altitude stall awareness training for all flight crews, which should help as well. As you pointed out, the pitot tubes were already in the process of being fixed, and they accelerated the schedule of replacements, and I believe that they have all been replaced. $\endgroup$
    – Lnafziger
    Commented Feb 22, 2015 at 21:03
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    $\begingroup$ Airbus should definitely implement the "You're now tumbling down at less than 60 KIAS" warning. In exactly those words. $\endgroup$
    – reirab
    Commented Apr 2, 2015 at 22:42

2 Answers 2

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What improvements did Airbus make as a result of the Air France 447 accident?

The official report has a section headed

5 - CHANGES MADE FOLLOWING THE ACCIDENT

...

5.2 Airbus

Review of the “Unreliable speed indication” procedure

Flight Operations Telex (FOT) of 9 September 2009 recommending, at the next recurrent training course, a session on the simulator at high altitude in normal and alternate law including:

  • Manual aeroplane handling,
  • Carrying out the UNRELIABLE SPEED INDICATION / ADR CHECK PROC procedure.

Most of the changes were carried out by other parties, Air France, EASA, etc.

If the stall warning didn't stop, but instead changed into an even more ominous "you're now tumbling down at less than 60 KIAS" warning.

Opinion: If the computers have decided they don't know what the heck is happening, it's probably a bad idea for them to make something up at random to shout at the crew. If the computers always know best, they should just probably shut-up, activate the cockpit wrist-restraints and fly the plane.

it might not be out of order for the airplane to announce that it is now very much possible to stall the plane by pulling up (in actual words).

Wild guess: Maybe this would help but not when the PF is already ignoring stall warnings and determinedly pulling up almost continuously. Generally less is more when the crew are likely to be confused and overwhelmed. There's probably a hundred things you can more easily accidentally do when you are flying in alternate law and when you are in alternate law you may not have time to patiently wait and listen whilst the computers enumerate them all to you.

the unlinked nature of the sidesticks was a big part of the problem; are there good reasons to leave them unlinked in future designs?

So that the pilots can engage in wrestling matches? Yes, it seems there needs to be an even clearer indication of conflicting inputs but I'm not sure if mechanical feedback is necessarily the best method. Pilots are already supposed to be trained and tested on clear handover of control.

Addendum:

Linking controls can also cause problems. This was experienced in a Boeing 777 in an incident in Air France flight 011 in April 2022. The pilots thought they had flight control malfunctions but were simply pushing against one another's inputs. Dual input events occur as often in Boeing aircraft as in Airbus aircraft despite linked controls. The AF 011 icident shows that linkage doesn't always make pilots realise that they are making opposing inputs.

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    $\begingroup$ Right, improved flight crew training is the answer! $\endgroup$
    – Peter Kämpf
    Commented Feb 22, 2015 at 22:27
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    $\begingroup$ Good reference, I think that answers my question exactly. Though, between "engage in wrestling matches" and "have no idea that the other pilot is pulling up", option #1 doesn't sound that silly. Gulfstream decided to implement the wrestling option, sounds like it's a matter of time now. As for the stall warning I agree if the computer decided it doesn't know, but once all the pitot tubes are back in agreement, it doesn't seem too much out of order. $\endgroup$
    – Roman
    Commented Feb 22, 2015 at 23:10
  • $\begingroup$ @romkyns: You make some good points. I'm sure many of us mulled over similar thoughts when reading the reports of what happened on AF447. I wonder if the emphasis with the automation is that the computers should voluntarily give up control to pilots but shouldn't snatch it back unless and until the pilots agree circumstances are right for this. Maybe re-application of flight protections also comes under this principle? $\endgroup$
    – RedGrittyBrick
    Commented Feb 22, 2015 at 23:22
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    $\begingroup$ In this incident, the stall warning didn't stop because the computers didn't know what was happening. Indeed, by the time that the airplane stalled, the systems were all working perfectly, with three reliable and matching airspeed indications. The stall warning stopped only because it was inhibited (programmed) to stop below 60 KIAS. This was done with the intention of preventing erroneous stall warning indications while on the ground (and surely if the airplane is below 60 KIAS it is on the ground). This stopping is part of what caused the confusion in the cockpit. $\endgroup$
    – Lnafziger
    Commented Feb 23, 2015 at 1:24
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    $\begingroup$ @curious cat: It is indicated on the PFD and in ECAM messages. There is an audio warning. $\endgroup$
    – RedGrittyBrick
    Commented Jan 31, 2016 at 18:15
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We had one of the first A320 simulators with cockpit 1.7 standard in 2011. That had a direct Angle Of Attack tape that could be displayed on the captains' left side, as I understood that was an improvement made due to AF 447.

Update

It turns out to be the Back Up Speed Scale (BUSS), which shows red, amber and green zones, and is based on Angle-Of-Attack information. The needle needs to be kept in the green zone by providing pitch stick inputs. Flight control law is direct law.

Source: Airbus FCTM.

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  • $\begingroup$ can you please provide sources? otherwise you shoiuld have left a comment. $\endgroup$
    – Federico
    Commented Jun 3, 2017 at 16:37
  • $\begingroup$ Sources of what? The Airbus head of training was at hte facility and asked to check if the direct AoA tape was installed, and it was. $\endgroup$
    – Koyovis
    Commented Jun 3, 2017 at 20:58
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    $\begingroup$ sources of "I understood that was..." so far this is just a personal anecdote. $\endgroup$
    – Federico
    Commented Jun 4, 2017 at 6:00

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