An initiating item in this accident appears to be the blockage of all pitot systems (weather related), which resulted in ADCs degrading / shutting down.
A significant, but not catastrophic problem for the crew is the loss of all airspeed indications. There is a crew drill and flight procedure for this, and based on other incidents it has been demonstrated that the aircraft can be flown with relative safety until the conditions improve – based on weather research, circa 5nm average.
However, it is not know if flight without airspeed in this instance was that easy, either because of the environmental conditions (severity / extent), crew capability / ability, or lack of other flight instrument displays.
The flight procedure relies on engine power setting and attitude information.
- Power:- Other incidents had identified engine problems in similar conditions. However, these events were generally subsequent to any pitot (TAT) problems (ice shedding vs ice forming) and were cleared quite quickly. Even if the engines were damaged the aircraft could have glided for some time, the engines relit, and systems reinstated; thus the lack of engine power / setting information due to engine problems might be discounted.
- Attitude:- There has been speculation about ADC - IR interaction leading to the loss of attitude information, but even with multiple ADIRS failure the ISIS should have provided an independent attitude display. However, airspeed – attitude interaction within ISIS should not be discounted.
Question – of the previous airspeed related incidents, how many aircraft has ISIS vs conventional stand by instruments?
Another possibility is an erroneous switch selection which resulted in the loss of all attitude displays.
Other airspeed incidents reported instrument indications and alerts which might lead the crew to mishandling the aircraft – over-speed / stall, and it is the result of these which could have developed the accident.
- Over speed:- This condition might be detected by vibration – buffet or external noise, and up to a point, depending on trim state, the aircraft might self recover.
- Stall:- In the event of slowing down without ‘protections’ (ALTN law) the aircraft could stall. As I understand there would be some AOA input (stall warning / stall annunciation?) yet at the same time an over-speed alert could be given due to the faulty ADC.
If the aircraft were flown into the stall then the (over) trimmed condition might further delay the recovery – a possible connection with the recent A320 accident, 737 accident (AMS), and GA incident (the latter two required forward trim to enable stall recovery).
Spinning is discounted as there is no supporting evidence; a spin requires stall and roll conditions to initiate it, also a combination of aerodynamic and inertia aspects which might be difficult to achieve/sustain (ALTN law has a strong roll centring force).
A stall could account for the subsequent high cabin rate and most aspects the impact description.
However, even in a ‘stable’, long duration stall – high rate of descent, it might be expected that the pitot functions and some instrument displays could be recovered as the ice melted. If ice crystals were the problem, then the aircraft only had to exit the conditions enabling the pitot heaters to regain their effectiveness - not a descent below the freezing level.
Thus the focus of attention might be on attitude displays, abnormal procedures, and the trimmed condition when flying without airspeed.