Sunday, December 2, 2018

Response to AeroTime New Hub article

The following is my response to an AeorTime New Hub interview between two Air France pilots; a B777 captain and an A320 captain.

Interview: Would Air France F447 have happened with Boeing?


Of course, I covered this subject in Understanding Air France 447, (Could this have happened in a Boeing? - my conclusion was: it already has!)

Most of the comments  I agree with. Especially the emphasis on being able to fly the airplane, and not just rely on computers. Those pilot skills must be there and be able to be called upon at the most inopportune moments. Maintenance of pilot skills, like any skill, requires practice. That means hand flying the airplane when practical. Otherwise, it's like being called upon to give a surprise concert for a large audience and you haven't practiced for a year.

The comment section on the page could not contain my full remarks, which are below:

Air France 777 captain appears to misunderstand how the Airbus works. His statement "how Airbus auto trim works. As the co-pilot pulled back on the stick, the pitch trim helped him going backward to trim the aircraft because the computer acts as if Bonin* wanted the speed as low as possible... The pilots were not aware of the fact that the pitch trim was trimming all the way back, because on an Airbus you don’t have an artificial feeling, no stick shakers."

Autotrim only serves to align the elevators with the stabilizer. In Normal and Alternate laws it has NO EFFECT on how the airplane feels or behaves. In the flight control law where AF447 found itself (ALT2) and the sidestick demands g-load. As a result, it's basically a point and PROT) the stick changes from g-load demand to full back law the hard protection is replaced (due to various failures) with a "stability", which is the airplane's natural aerodynamic stability to pitch down when very slow.

However, the transition from g-load to stability mode is based on indicated airspeed. In the case of AF447, the airspeeds were rejected by the flight control computers because it had become unreliable due to the pitot tube issues. This left the airplane in full-time g-load demand.

When at least 2 of 3 airspeed sources were deemed unreliable, the autopilot disconnected. This dropped the job of hand flying the airplane into the laps of the unsuspecting and very-soon-to-by confused pilots, when they least expected it, and probably least prepared to handle it. "Startle factor" is the term used here that describes their diminished mental capacity to perform.

He said "They should have read the pitch trim indicator, which is something you forget to do on an Airbus, because it’s always in trim! They also should have had a visual warning telling them to use the pitch trim manually (Man pitch trim)."  This is not correct. At least not for the fist full minute with the stall warning going off.  All they had to do initially was to point the airplane down. It did not require any trim action. Now later on, when the angle of attack was extremely high (>45°) and the trim was full nose up, my own tests indicate that manually reducing the nose-up trim may have been required for a successful recovery from the 20,000 ft/min descent they put themselves in.

But instead of nose-down input, which would have worked, they instituted the only stall recovery action they apparently remembered: full power. Except at cruise altitude, they were pretty much already af full power and the extremely minor amount of power still left would produce no results.

Unfortunately, Bonin's initial actions on the stick (be then intentional or as a consequence of something else) were to pitch the airplane up. But the airplane's neutral stability allowed the nose to just stay there.
 It did not require trim to hold it there. That was its job. If he had pulled it back and then just let go completely, the result would have been much the same. The lack of the low-speed stability meant that as the airplane approached the stall AOA, there was nothing to pitch it down except the pilot - who was probably trying to follow the ever-reliable flight director which was telling him to continue the climb.

As the airplane began to stall and lose lift, the airplane started to sink, and thus the actual g-load was reduced. The airplane responded to this less-than-demanded g load with all it had at its disposal to increase the g load: Up Elevator. This obviously just made things worse and within a few seconds, the angle of attack increased to 45° and more (though the pitch attitude remained at 15° or less).
What this required from the start was for the pilot flying to FLY THE AIRPLANE. That is, command a pitch and power setting commensurate with level flight (2.5° & 83% N1). Any pitch and power setting remotely close would have kept the crew out of trouble for a long time. It only took about 40 seconds for all of the pitot tube issues to resolve themselves, and they could have been on their merry way like the dozens of other crews that employed that strategy in similar circumstances. However, by the time the pitot tubes were clear, the flight was in a far worse position, having climbed 3000 feet and lost over 100 knots of indicated airspeed. A recovery at this point required far greater skill than the minimal skill required to fly straight and level just 60 seconds earlier that they failed to exhibit.

All this and more, including what recovery steps were required at each step of the way are detailed in my book "Understanding Air France 447".

It is further interesting to note that Airbus seems to have taken this accident to heart in the design of the A350. It has extremely robust airspeed backups (even if all pitot tubes become unusable), and Alternate law maintains all of the same hard protections as Normal law (assuming the physical flight control surfaces are not damaged beyond carrying them out). Further, the autopilot remains engaged in an extended envelope (right up to the abnormal attitude parameters (120° bank, 50° pitch, etc) So, in this case, the AP/FD would not have disengaged at all.

A320/330/350 Captain/instructor/check airman, also B777 type rated