The Final Report of the Air Asia 8501 has been released. This was a loss of control accident that does contain some lessons that are worthwhile sharing.
There had been an ongoing maintenance item that resulted the ECAM message of a AUTO FLT RUD TRV LIM SYS. While it later turned out to be faulty wiring, their maintenance appeared to be treating it each time as a “one off” type of event rather than a repetitive maintenance item. The maintenance procedures for a repetitive issue is often different than it would be for a single event. Those of us who fly “electric jets” know that having a temporary “nuisance” type of alert is not rare, we call them “stray electrons” and are often the result of a slightly delayed power transfer or similar. As a result they can often be fixed by just waiting a minute or, if that doesn’t work, the system can be rebooted. These are issues common to any computer, whether it is your iPhone or an airplane. Of course, in the case of our airplanes there are many systems that are inter-related and so a momentary glitch in one system can lead a second system to not start up correctly, etc.
The point here is that if it is a repetitive issue then it is likely a real fault of some sort and so when we write up a problem it is important to note that it is repetitive. Now most company computer systems will also be tracking these but by putting the words “repeat item” in the maintenance logbook we can reduce the chance that it will be overlooked and proper procedures applied.
The next issue worthwhile looking at was what happened next. Apparently the Captain on this flight had seen this maintenance item before and was watching while a mechanic “fixed” the alert by pulling some circuit breakers for the Flight Augmentation Computers (FAC). During the accident flight the alert appeared several times. Finally, after the fourth time, the Captain decided to pull those breakers inflight. This is not a book procedure and resulted in the flight control system reverting to alternate law and the autopilot disengaging. Procedures published for system problems are carefully thought out and absent an extreme emergency there is no reason to deviate from the published procedures.
The result of the alternate law resulted in the roll control on the control stick going from a rate command (you make an input and it sets a desired rate) to direct command (similar to a non-FBW airplane), while the pitch command remains in a mode pretty much similar to normal law without protections. This means that in pitch the command on the stick is a “g” command, in which a neutral stick is commanding 1 g which will mean it will not change its vertical velocity, but unlike normal law it will not prevent the aircraft from stalling.
For reasons suspected to be distraction the first officer, who was flying, did not immediately notice that after autopilot disconnect the aircraft started to roll. When he did recognize it he started to recover with both a roll and an aft stick movement and rolled from a 54 degree left bank to 9 degrees in under 2 seconds. This might have created some other effects such as a vestibular illusion. Following this, like AF 447 it appears that the first officer had a difficult time keeping the wings level. This might seem surprising but remember that at FL 320 the aircraft was in a regime in terms of low air density that the first officer had likely never flown the aircraft, and, like AF 447, this was coupled with flying in a control law that was relatively unfamiliar. In multiple previous events of this nature it resulted in some challenge for the pilot to control the roll.
The pull back is not explained but it may just be the nature of a rapid mechanical input where the hand moves to the right and back at the same time. In any event, it resulted in the aircraft to rapidly pitch up. From the reactions of the crew it appears that they were completely startled. One wonders if they thought they had some secondary flight control issue going on, as it is clear that there was some confusion. In any event, the aircraft progressed into a stall and aft stick position was maintained as the angle of attack increased to extreme values. The communication between them was in English, which was neither of the pilots first language, and that might have contributed to the confusion. For reasons unknown the Captain did not authoritively take command of the airplane and it appears that neither pilot recognized the stall. For discussion on that, see my previous article.
It should also be noted that the aircraft descent rate became extreme and so was going to be much less than 1 g, let alone the higher g-demand that was being commanded by the aft stick pull. The aircraft elevators will work to try to maintain the g-demand so even with the stick neutral if the aircraft is experiencing less than 1-g then the elevator will try to pitch to hold that 1-g. Also, it should be noted that pushing forward on controls when at half a g or less requires overcoming a lot of natural human response.
One more note is that the Captain of this flight had considerable aerobatic and hands on flying as a military pilot so those that believe that more of that type of flying would prevent such accidents need to reconsider. Clearly training is necessary and it is also clear that the industry has not addressed these issues satisfactorily at this time.