As some technical "niceties" about the auto-trim system starts to emerge, it's beginning to sound as if the vagaries of the auto-trim system in respect of Airbus Law Change philosophy were instrumental in the inability of the AF447 crew to recover from the type of stall that they'd entered. i.e. they may have been victims of the circumstances of their stall entry scenario. To explain:
a. During the zoom climb to FL380 (in Alternate Law) the auto-trim transitioned from 3 deg N/up to 13 degs n/up (near to, if not max nose-up THS trim for the type).
b. Upon entering the stall ballistically and starting its "deep"-stalled descent, the aircraft would have been (and remained) in Abnormal Law, with the auto-trim inop and the trimmable horizontal stabilizer (THS) therefore locked at 13 degs nose-up..... unless it was to be manually changed by the pilot (and we now know that it was not).
c. The engines were at TOGA at this stage, so we have the pitch-up effect of TOGA power plus the THS "stuck" (i.e. de-automated) at 13 degs nose up.... both tending to pitch-up and support a stall continuance..... no matter what the pilots did (unless they were to change the acft configuration somehow - aerodynamically or CofG wise).
d. Airline pilot exposure to stalling tends to concentrate upon the approach to the stall and recovery (i.e. avoidance) with minimum altitude loss. In that stock standard scenario, one of the primary and persistent preliminaries in the incipient stall experience is the airframe buffeting and stick-shaker progression. This is de rigeur in a standardised level 1kt/sec deceleration towards a level 1g stall and is the cue for the trainee to take recovery action. However, if one enters a stall ballistically at high altitude, will there be any buffeting? The AoA system will issue a stall warning but is this against a distracting background medley of other aural alarms? Stall recognition and realisation then becomes an obscurity of the first order.
So the question now becomes:
"What would be required to un-stall this aircraft (now in Abnormal Law) if there is insufficient elevator authority at 13 degs nose-up THS to get the nose down (and thus air over and across the wings and tail), during a stall locked in at 40 degs AoA (as predicated by their entry config)?" One answer might be:
"Idle the power" (and this was done soon after the captain re-entered the flight-deck, possibly because he'd just misinterpreted their predicament as a L.o.C., based upon a quick scan). Another might be:
"Manually run the trim" (i.e. not something that comes naturally to an Airbus pilot - particularly not if he's unclued and quite unaware that the 13 deg nose-up THS is now his basic problem). A pilot's normal cueing to adjust trim is that the airplane is "out-of-trim" and tending to deviate from a chosen flight-path i.e. not a player in this deep-stall scenario.
Three questions: 1.
"Will the pilot be aware that he's in abnormal Law? (and the portents of that)" The answer might well be:
"Probably not" (there's nothing to promote awareness of this being the case i.e. no aural annunciation - and thus we arrive at: what now needs to be done that's essential for recovery?)
2.
"Does the elevator alone have sufficient authority to unstall the wings at max power or at idle?" The answer is probably not, at least not while the superior trim authority of the THS at 13 degs n/up holds sway.... and particularly not whilst at TOGA power.
3. "Why doesn't the elevator have sufficient authority to unstall?" The whole design premise of the THS is to reduce trim drag and allow the elevator to become more of an active trim and less of a primary flight control. This ideation works well 99.99% of the time and it's used in all models of airliners to some degree. They need the capability of coping with large CofG ranges to accommodate loading, fuel burn-off and configuration changes. Some aircraft augment this capability with tail-located fuel trim-tanks. However this
minimalistic elevator design feature in the A330 apparently won't "work" in the progression of events that AF447 underwent.
So are Airbus elevator throws and areas (i.e. authority) an under-design or does the THS have undue authority? That becomes the question here as we switch our attention to not blaming the pilots but agonizing over possible Airbus design deficiencies. But before we look into that, we need to pose the question:
"Were the AF447 pilots aware that they were locked in an aerodynamic stall?" I'd suggest that they were NOT..... mainly because of the circumstances of their entry being wholly unfamiliar..... and that medley of aural alarms mentioned earlier that had overloaded their capacity to assimilate transient info. It's called cognitive disequilibrium, an idiosyncratic need to ignore or de-prioritize - a close relative of cognitive dissonance (a tendency to rigorously deny or disbelieve), two behavioral paradigms to which professional pilots are prone. One final thought, centering upon human factors. When the captain entered the cockpit and tried to take it all in at a glance one of the things he would've missed, because of its positioning, is what the pilot's input was on his sidestick. With a yoke (thinking of Egyptair MS990 here) it's visually apparent that what's afoot is directly related to pilot action. Food for thought.
Are there any precedents in the Airbus incident and accident annals? There's the Air New Zealand A320 test-flight crash and the Tarom Airbus near accident on ILS finals at CDG..... that spring to mind.
It's worth citing an extract from the Tarom incident:
"Under the aerodynamic effect of this THS deflection (of 13 degs) and under the mechanical effect of thrust, the aircraft was thus subjected to a nose-up force that could not be controlled by elevators. It rapidly assumed an extreme pitch attitude and angle of attack.
There may be others. The 1994 crash on go-round of a (non-FBW) A300 at Nagoya? (
link). Para 4.2.b of
this link lists 6 Airbus incidents and one B747 involving pitch-trim anomalies. An extract from that Nagoya narrative might help emphasize the sometimes inordinate power of a THS in some circumstances: After the PF inadvertently pressed TOGA on finals,
"The autopilot automatically went into GA mode, and this would have shown on the primary flight display (a very vague alert really). The aircraft was flying 18 degrees nose-up, normal for a go-around, but the FO was pushing heavily on the yoke to get the nose down. He was meeting heavy resistance, a design indication on almost all airplanes that that his manual commands were in conflict with the autopilot. For nearly 20 seconds, as he applied down-elevator, the autopilot moved the trimmable horizontal stabilizer (THS) in the opposite direction to keep the nose up. At T+30, THS reached maximum nose-up; at T+42, the autopilots were disengaged. Pilot C asked for autothrottles engaged and took control, increasing down elevator to full deflection as the aircraft began climbing. Alpha-floor (an Airbus automatic protection mode) triggered at T+50 from excessive AOA. Alpha-floor triggered maximum thrust for climb-out, but that added thrust in fact increased the nose-up attitude to 52.6 degrees (one may surmise that the thrust centerline is below the rotational center of the airplane, and at low speed there is not much aerodynamic force to maintain resistance to this rotation). (52.6 degrees is very steep. A high-friction granite rock face of this angle would nevertheless be considered a technical rock climb.) C disengaged alpha-floor by retarding thrust and tried to get the nose down again with trim. Airspeed had dropped to 78 kt., the aircraft stalled at 1,800 ft, and control was not regained before it hit the ground."
If the BEA Inquiry into AF447 heads down this well-trammeled path towards THS excess authority and control law anomalies in stalls, Airbus is going to have to do the old quickstep that they've always done so well - in order to avoid changing physical design. No doubt it will take the form of a circuitous software patch to the control laws and/or yet another aural alarm and cautionary bulletin.
Edited to add:
However, despite all of the above, there is no denying that “no clear-cut stall recognition and persistent warning” is the apparent deficiency that put AF447 into the Atlantic. Recognizing that the overwhelmed pilot can become clueless in a time of great stress, perhaps aural alarms should become aural admonitions: as in
“Stall Warning. Reduce angle-of-attack and trim nose-down for recovery”