PPRuNe Forums - View Single Post - AF 447 Thread no. 4
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Old 1st Jul 2011, 13:58
  #611 (permalink)  
Chris Scott
Join Date: Jan 2008
Location: Blighty (Nth. Downs)
Age: 73
Posts: 2,091
I presume this discussion on phugoid oscillations relates to the A340 AIRPROX? I used to do a nice one myself occasionally in the climb on aeroplanes like VC10 and B707 unintentionally, of course, and usually due to coarse pitch selections using the AP pitch wheel.

AF447, on the other hand, seems to have shown no sign of one probably because of the G-control provided by Pitch-Alternate Law. Unfortunately, that same law, coupled with a lack of any sustained pitch-down commands on the sidestick, seems to have led to the super-stall.

So we can now see only too clearly that the flight-path stability provided by C* Law, with its normal low-speed protections invalidated due to the UAS, necessitates firm action by the pilot if for whatever reason the aircraft enters a climb which cannot be sustained by its performance capability. This action seems to have been largely absent in AF447, and seems to have come late in the day on the A340 AIRPROX incident.

This leads me to wonder if, in the event of the systems diagnosing UAS, it would be better for the EFCS to downgrade to Pitch-Direct instead of Pitch-Alternate. It also occurs to me, however, that the PF needs some warning, if at all possible, that he/she is about to lose the longitudinal stability of C*. The AP has already disconnected, but a short period of mental preparation for stick-to-surface control might be useful.

Stick-to-surface hand flying at high altitude, particularly in turbulence, requires a fair amount of concentration. I once shared the task with my skipper on a B707 for the last 6 or 7 hours of a CCS-LGW flight, including step-climbs up to FL410. 20 minutes at a time was enough, and we had the benefit of reliable airspeed indications.
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