mm43 has objected to the Vmo/Mmo pitchup scenario as follows:
The crux of my argument is that when changing from Normal to ALT Law, the RTLU is locked at the maximum allowable travel associated with the aircraft's current level and CAS, and the BEA reported -
Quote:
1.12.3.5.5 Examination of the Rudder Travel Limiter Unit (RTLU)
The RTLU was found in its place in the fin and disassembled. An examination was performed at the manufacturer’s and showed that it would allow travel of the rudder measured as 7.9° +/- 0.1°. As an example, at FL350, this travel is obtained for Mach 0.8 +/- 0.004, corresponding to a CAS of 272 +/- 2 kt.
This means that when the proverbial hit the fan, the a/c was proceeding in light chop (cobble-stoning) and the airspeed had dropped slightly from M0.82 to M0.80. Just prior to the AP disconnect the RTLU was last adjusted to represent the valid computated CAS, and if this was the apparently real Vmo M0.86, the RTLU would have been set at 4.7° per the following graphic.
I too have had some misgivings about the concept for the very same reason when I posted the following in post 1653:
The only other thing that might contra-indicate such a pitch up is the rudder limiter position which probably would have locked at the polled airspeed indication existing when Normal Law was abandoned.
But if an aircraft were pitching up and then the blockages began to let go, which is what would have triggered the switch to Alt Law, then the last acceptable polled airspeed prior to the switch could have been almost anything including M.79. The trigger would be the 30 knot drop in 1 second. Yes it would be highly coincidental but not impossible.
Now here is something else to consider. Is there any limit on how far the Vmo/Mmo protection is allowed to take the nose in the air other than (presumably) the 30 degree attitude limit? Is that much attitude necessary or sensible? Wouldn't it make sense to limit such protection pitch attitudes to where it would force the aircraft to slow gradually even at maximum available thrust? The key to safe recovery is not the rapidity with which you decelerate the aircraft but the fact that speed is trending in the correct direction. With defective airspeed indications, such a protection that largely overrides the crew's input is hazardous.
As I have said earlier, my primary intent is to determine if a prompt entry into a stall is possible (even if somewhat unlikely). If so, it could strongly affect where to search for AF447.