Hi Henra, BOAC.
I've been looking at what might have caused a dynamic (Oscillatory) departure from autopilot controlled flight and I think I have found two potential paths for that bad result:
- The first is multi-pitot freezing causing erroneous airspeed input to the flight control system. This has the potential to foul up the flight control computer calculations and could lead to erroneous control loop gains being applied.
- The second is an occurrence of hydraulic supply saturation causing control rate limiting of the flight control system.
The subject can get technical quickly, but for the simple explanation of the first condition look at the results from the X-31 pitot icing caused accident. My apologies for the over stated video presentation.
Aviation Video: X-31 Crash | Patrick's Aviation A more factual report can be found here-
X-31 Crash
The subject of hydraulic supply saturation and control rate limiting is a bit technical and is related to PIO causes (but cannot be called PIO since the automatics would be in control) however suppose AF447 flew into a vigorous Cb and the control surfaces began moving briskly to keep the aircraft upright. If total hydraulic usage exceeded pumping capacity + accumulator capacity, new non-linearities would be introduced into the control surface control loops. The control loops would develop phase lags and could even drive oscillations the wrong way! The triggering event could even be as simple as autothrottles commanding a power reduction.
To help you wrap your mind around this second concept, take a look at the following two links:
Fly-By-Wire A Primer for Aviation Accident Investigators
Pilot Induced Oscillation
Besides that also the RTLU setting would mean it had to stall/spin at M0,8.
So if this type of stall entry would break the airframe, we could pretty well conclude that that the two above scenarios couldn't happen since the aircraft appears to have impacted in an essentially intact condition.
Those with flight control design experience, test flying experience and related experience are particularly invited to join in.