Quote:
Originally Posted by
Machinbird
There the aircraft pitches up into a high speed stall, freezing the RTL unit at the pre-existing correct value, since the high AOA triggers ADR disagree results similar to the Perpignan A-320.
Takata
I'm really unable to follow you here: any "high speed stall" won't freeze the RTLU, only wrong air data will.
Also, if the aircraft would wrongly pitch up, the end risk would be a low speed stall if both wrong airspeed retained were in reality lower than the system would think... not a high speed one.
Next, if your pitots are really clogged for inducing the system in error, you'll need to clean them again before they would record some wrong values due to any "stalled/spin" attitude (and now trigger unreliable airspeed, freezing RTLU at Mach .80, as mm43 quoted above).
Moreover, Perpignan is a really bad case of reference as this whole sequence was way too short as to even close a correlation window and to valid or reject any airspeed discrepancies [meaning, you can have many intermittent UAS alerts popping up in the cockpit without having a real/solid UAS event being triggered at the maintenance computer level]
Hi Takata, sorry if I went over the concept too quickly.
A dynamic loss of control can occur due to rate limiting in the control system. This can be recognized from the actual control position traces by control surfaces moving in one direction and then immediately reversing direction before achieving the initial commanded position. The resultant position trace does not have a normal rounded "top", but instead forms a tri-angular top (or bottom). This can be caused by a number of possibilities however hydraulic demand rate exceeding supply is one potential cause. Occasional "peaking" of the trace does not prove rate limiting and rate limiting need not cause loss of control. If only small control motions are required to maintain control (low gain behavior), then generally nothing happens. But if large control motions are being commanded, and if a phase lag results that is large enough, then the resulting control motion can actually drive increasing oscillation. That is the theory in layman's terms. The actual theory uses transfer functions and complex mathematics and involves math that I haven't used in almost 50 years. I have posted links to papers on related subjects in the not too distant past and can again if requested.
What a dynamic loss of control means is that a beautifully flying aircraft can put on its Mr.Hyde face and act very nastily essentially with little warning.
Flight conditions can be a trigger for these oscillations. With hydraulic rate limiting, a power reduction could be a triggering event. If the aircraft begins pitching, the flight control laws system cannot react quickly enough to damp or limit the motion of the aircraft. The hydraulic system is not defective and likely would not set a warning (depending on its design). But the aircraft could tromp all over its normal limits. If the aircraft then stalled, it would not be surprising to lose the 3 ADR channels.
From page 86 of BEA accident report on A-320, D-AXLA
The Captain controlled a left roll movement, caused by the stall. The aeroplane’s high angle of attack and the roll movements generated asymmetry, and a speed variation between ADR 1 and 2 appeared. This increasing divergence caused a rejection of the three ADRs by the FAC then the ELAC
If this can happen in an A320, why cannot the same thing occur in the A330?
As soon as the 3 ADRs are rejected, the RTL will be left at last value. This could likely occur in just a few seconds after the beginning of loss of control. Note: no freezing of pitot tubes is required to generate this situation!
BTW, I am not trying to discredit Airbus. I have flown on A330 aircraft several times as SLF since the loss of AF447, and am not particularly concerned. The situation to create this type problem would not be common.
At this point in my life, I probably am a bit of an aviation dinosaur, but I am not opposed to the new technology, nor do I wish for the "good old days" of aviation. I merely think that Airbus may have been a bit presumptuous when they didn't give the pilots the ready ability to outvote the computers.
