Previously, in
Air France 447 — Summarizing the Summary, I outlined the mishap sequence, and related how a combination of unknowingly inadequate pitot probe design and testing allowed the possibility of complete loss of airspeed sensing due to icing.
All of which leaves completely untouched a fundamental question. How does the complete loss of airspeed indications cripple an aircraft?
Short answer: it doesn't.
(Note: the following was extremely difficult to write, because the conclusions I have drawn are extremely unpleasant. If my tone seems unduly harsh, particularly considering I am speaking of the dead, that is unfortunate, but unavoidable.)Immediately after AF447's pitot probes iced over, the autopilot shut itself off, the fly-by-wire flight control system (FBW FCS) degraded to alternate law.
(Alternate law is the first level of A3xx FCS degradation. The main differences between Normal and Alternate laws are the latter's lack of flight envelope protection -- which means the airplane can be flown into a stall -- flight management system (FMS), autopilot and auto throttles.) Which means that the pilots were left hand flying an airplane with fully functional engines, flight controls, attitude indicators, altimeters, and vertical speed indicators, but without any
direct way of knowing that thing responsible for creating distance between dirt and plane; namely, how fast it is going through the air.
The Pilot Flying's (PF)
(In the big airplane world, the two pilots have sharply delineated duties. The PF, as the term indicates, is responsible for maintaining aircraft control, directing configuration changes, and calling for checklists. The other pilot is referred to as the Pilot Monitoring (PM), and is responsible for communications, flight plan, executing checklists, FMS inputs other than during cruise, and monitoring aircraft performance. While ultimate decision authority always rests with the Captain, typically the Capt and First Officer will alternate PF and PM on each leg during a trip.) response was to increase pitch attitude to at least twice that possible for sustained flight at FL350, resulting in a climb rate far exceeding available excess power.
(In the airplane I fly, the first two steps for loss of airspeed are turn off the autoflight system then stabilize pitch and power at normal cruise values.)In other words, the PF was trading airspeed for altitude, until the airplane no longer had any speed to give. At this point, the airplane is at the stall angle of attack (AOA), which is the angle between the wing and relative wind that produces maximum lift. Cruise AOA is roughly 2-3 degrees; stall AOA is about 22 degrees. Approaching stall AOA, drag dramatically increases.
At high altitude, there is only one way out of this: down. Because the airplane was so far into the region of reverse command (AKA being behind the power curve), the PF needed to set the pitch attitude at 5 - 10 degrees below the horizon, select max power, and sacrifice altitude in order to regain airspeed. Instead, the PF drove the elevator to the maximum nose up position and flew the airplane into an aft stick stall, characterized by very low forward speed and extremely high rate of descent.
Then did nothing about it.
All this time, the PM did not note the wildly excessive pitch attitude, unsustainable climb rate, or the gross altitude deviation. Even passing through 10,000 feet, after having lost four miles of altitude, and having mentioned that salient fact, the pilots completely failed to apply any control or power inputs to break the glaringly apparent stall.
The available evidence points in one direction: pilot error of such magnitude as to defy explanation.
There are some contributory factors.
FBW FCSs have their advantages, but one thing they do not provide is aerodynamic feedback. An Old School FCS (OS FCS) is "speed stable", which means the flight controls, all other things being equal, will get very heavy in the nose down direction with a significant loss of airspeed. By the time AF447 reached stall AOA, an OS FCS would have had something like 70 pounds of nose down control forces. In contrast, with a FBW FCS
there is no feedback whatsoever of changing airspeed into the flight controls. A FBW FCS is, all things considered, better than an OS FCS when everything is working. However, should multiple system failures put the FCS into a degraded mode, a FBW FCS has no inherent self correction.
[IMHO] Adding to the debit side of the ledger is the "pilot out of the loop" problem that has come along with the "glass-cockpit" territory, regardless of FCS type. Prior to roughly the mid-1980s, aircraft cockpits had "steam gauges", round dial electromechanical indicators, and did not have flight management systems worthy of the name. Steam gauge instrument flying, done well, requires a high-rate observe / orient / decide / act (OODA) loop: observe airspeed, heading, and vertical speed / orient those observations with respect to desired parameters / decide what changes in power and attitude are required to eliminate the difference between observed and desired parameters / move the flight controls and power levers as required.
Glass cockpits come with very capable FMSs, which led many flight departments (including, until a year or so ago, the one at the company for which I work) to essentially require use of the flight director except in very abnormal circumstances. Relying on a flight director makes the OODA loop a pointless exercise. No need to decide on attitude and power, just center the pitch and bank steering bars; it is a task even a modestly gifted monkey can manage.
But wait, there's more. Transport category aircraft completely exclude the most fundamental parameter of them all: wing AOA. Yes, the ATC system is very much like a conductor and a symphony orchestra, where airspeed stands in for rhythm. Since, for a given airspeed, AOA varies based upon weight and configuration, having a bunch of airplanes flying around at their individual optimum AOAs won't work. But, for pete's sake, if the FMS is smart enough to know airspeed is unreliable, the least it can do is replace it with an alternate means of determining speed. One which, BTW, is far less prone to icing, and is mechanically far simpler, than air pressure sensing.
So. FBW airplanes are perfectly happy to run out of airspeed right when knowing airspeed is impossible. Glass cockpits can turn piloting into monkey business. Airliner design and pilot training, no matter how much ingenuity was brought to bear, could more thoroughly ignore an alternate means of determining airspeed.
Does all that suffice to explain putting an otherwise completely flyable airplane into a deep stall, then riding it in?
No.
Using Sherlockian reasoning -- having eliminated the impossible, whatever remains, however improbable, must contain the explanation -- the only alternative left is sheer incompetence. A flight deck with three ostensibly fully qualified pilots were incapable of maintaining basic aircraft control in a situation that,
had they done nothing more than stare with gobsmacked amazement we would never have heard about.
[/IMHO]
To an even greater extent than the sea, the sky is incredibly unforgiving of any human carelessness, incapacity, or neglect.
-- unknown
