I know the AOA sensors were iced up, and the report stated that the horizontal stabilizer remained at an angle of -11degrees (Nose up) for the remaining flight? Why coudn't the PF control the aircraft? Due to flight law degredation? Can someone explain the mechanics here?
Lift equation: lift = 0.5 * air density * TAS^2 * wing area * CL
To maintain a lift force from an aerofoil you need to increase CL as speed decreases.Therefore the main wing AOA has to increase as speed decreases, which in level flight causes the aircraft pitch attitude to increase as speed reduces.
As the speed of an aircraft reduces in level flight the horizontal stabiliser needs to generate downforce to keep the aircraft level, even though the airspeed is reducing. Therefore the CL of the horizontal stabiliser needs to increase as speed decreases, and on a conventional aircraft this is done by the elevator moving up. In the case of an aircraft with and all moving stabiliser (e.g. A320), the angle of incidence of the stabiliser with respect to the airframe will change in order to deliver the necessary downforce.
On a light aircraft the force required to hold the elevator or all moving tailplane is a deflected position is felt through conventional reversible flight controls. In the case of an aircraft with powered flight controls, if the aircraft stick or yoke is connected to an artificial feel system then the articifical feel system will provide the necessary feedback force. This force can be removed, if desired, by using the trim system; on most airliners this is done by changing the angle of incidence of the horizontal stabiliser, thus allowing the elevator to be in the neutral position.
With the A320 it is a bit different. The sidesticks provide spring feel in proportion to displacement from the centre position but they do not give any "feel" in the conventional sense and the force will not necessarily correlate to the trim position of the stabiliser. More importantly, in normal law the FCS automatically adjusts (trims) the angle of incidence of the horizontal stabiliser on a continuous basis, so that when the sidestick is in the neutral position there is no pitch rate. The pilot thus does not have to apply any stick input to maintain the selected pitch attitude because the trim system has adjusted accordingly.
In the XL888T accident the FCS normal law worked as designed, adjusting the stabiliser so that, if the pilot were to release the sidestick, the desired attitude would be maintained. Thus as speed decreased the stabiliser moved to its maximum trim setting of -11 degrees (i.e. 11 degrees down at the stabiliser leading edge), creating the necessary downforce to keep the nose up at low speed.
Had the aircraft remained in normal law, as the pilot increased speed and demanded the nose to pitch down, the horizontal stabiliser would have adjusted automatically by trimming up, thus reducing the CL of the horizontal stabiliser as speed increased. However, unfortunately for the crew the AOA sensor failure caused the FCS to degrade from normal law to alternate, thus stopping the auto-trim function. When the crew recovered from the first stall by lowering the nose and speeding up, the aircraft soon reached a speed where the stabiliser being at -11 degrees caused far too much downforce on the stabiliser, meaning that even with full down elevator commanded using the sidestick the elevator could not reduce the downforce sufficiently to stop the nose pitching up. Therefore the aircraft reached an unusually nose-high attitude, lost speed rapidly, and entered a manoeuvre often referred to as a hammerhead stall, when the nose drops rapidly.
It is not clear whether the crew failed to notice the "USE MAN PITCH TRIM" message on the ECAM screen (
Electronic centralised aircraft monitor - Wikipedia, the free encyclopedia), or failed to act on it, but in simple terms had they reduced the nose-up trim as the aircraft increased speed following the first stall, they would almost certainly have survived. Adjusting the trim would have allowed the pilot to counter the pitch up motion that occured as the speed increased during stall recovery.
This is an interesting case because it is another example of the Airbus low speed protection and FCS law mode changes occuring as designed, but the crew awareness and response being insufficient or counterproductive. AF477 was another example of this. To my mind it does beg the question as to whether the Airbus design strategy has some implicit human factors weaknesses when it comes to pitch control in low speed flight in conjunction with subtle failure modes leading to FCS law degradation. Removing conventional feel from the primary flight controls was a design decision that Airbus and the certifying authorities were and still are content with; also there are other manufacturers, particularly in military aviation, who have designed and implemented primary flying control inceptors without conventional feel systems.
However, from the accident reports it does appear that removing the influence of conventional feel from flying controls is not necessarily a helpful thing in some circumstances, because on a conventional aircraft an out of trim condition can be felt through the force one is having to apply to flight the controls, particularly in the pitch axis. Put simply, if the sidesticks had been capable of providing a level of force feedback that prompted the pilot to realise he had a large amount of nose-up trim selected, one does have to ask whether he might have adjusted the trim instinctively. But having said that, the crew should not have been doing what they did, and it is highly unfortunate that the AOA probes failed in the way they did, leading to loss of stall protection.