Originally Posted by Chris Scott
If we stick to the theory of progressively freezing pitot-probes, it is unclear whether they result in an over-reading of airspeed, or the reverse. The BEA analysis of previous events shows a large proportion of cases where a stall warning occurred. In every case, there had already been a reversion to Alternate Law. As I understand it, the stall warning in Alternate Law is based on indicated airspeed falling to a certain threshold, not the AoA. As the indications are unreliable, the stall warning might have occurred despite the airspeed being normal, or even above normal.
To be simplistic: if a pitot tube is blocked, but the static is clear, that is very likely to result in an under-reading of airspeed. (We've visited this one before...)
Chris,
To begin with the last point, if a pitot tube is blocked, but the
drain hole(s) and the static are clear, that is very likely to result in an under-reading of airspeed. That was the case in all thirteen cases of UAS studied by BEA (see Interim#2, 1.16.3)
In the same paragraph, BEA writes about Stall Warning:
Nine cases of triggering of the stall warning were observed.
Note : the manufacturer’s additional abnormal STALL warning procedure is included in appendix 5.
The stall warning triggers when the angle of attack passes a variable threshold value. All of these warnings are explicable by the fact that the airplane is in alternate law at cruise mach and in turbulent zones. Only one case of triggering was caused by clear inputs on the controls.
Note: At high altitude, the stall warning triggers in alternate law on approach to the stall. The stall manifests itself particularly through vibrations.
And in para. 1.6.11.4:
In alternate or direct law, the angle-of-attack protections are no longer available but a stall warning is triggered when the greatest of the valid angle-of-attack values exceeds a certain threshold. In clean configuration, this threshold depends, in particular, on the Mach value in such a way that it decreases when the Mach increases. It is the highest of the valid Mach values that is used to determine the threshold. If none of the three Mach values is valid, a Mach value close to zero is used. For example, it is of the order of 10° at Mach 0.3 and of 4° at Mach 0.8.
I have a few questions about this, which I hope BEA will see fit to address in their next report:
Firstly, 4° at Mach 0.8 is rather early (compare to recent discussion on a 777 incident), with the risk of becoming ineffective because pilots then consider it 'inappropriate' (see Air Caraibes memo).
Secondly, 10° at Mach 0.3 is too late, when the speed drop is due to pitot blockage while actual speed is still Mach 0.8.
Finally, the occurence of stall warning in the cases studied by BEA does not seem to match the higher threshold after loss of IAS.
Regards,
HN39