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Old 7th Jul 2009, 16:58
  #3199 (permalink)  
PJ2
 
Join Date: Mar 2003
Location: BC
Age: 76
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NARVAL;

Lowering the gear would certainly place the aircraft in Direct Law but to what end? The thesis is, the pilot has "more control" to push/pull the aircraft out of an abnormal attitude - the "kind of control" we find on more "accessible" designs such as those that don't have control laws to "interfere" with what the pilot would do with the controls. I submit that such thinking is faulty and not supportable in fact.

I understand the long-posited argument that when circumstances are so dire that access to all possible control (direct law) with the attendant possibly of a) high-speed-stalling of the aircraft or b) breaking the aircraft, is preferable to the certainty of a crash. But this argument presumes that there is more performance available from the aircraft, under the 'g'-loads and/or attitudes/speeds contemplated, than the control laws are permitting access to. In the absence of clear cases to the contrary, the question then becomes, is there a very slight hint of romanticism in assuming that "the airplane ain't read the manual" and that the pilots would, in all cases, be better off with immediate and full authority over all flight controls, setting aside the engineering data regarding structural and aerodynamics issues?

The argument then takes both forks in the automation road: Design software and hardware that will prevent such excursions in all cases, thereby removing the pilot altogether, or hand full authority over the aircraft in all cases, thereby placing recovery of the aircraft in the pilots' hands. Control laws in fbw aircraft occupy the design-space in between these two forks in the road.

I can't think of a single accident where this kind of reversion (instant access to Direct law via the "big red button" notion) would have made a difference in the survival of the aircraft, bearing in mind that such laws may already be exercising that very fine balance between loss of control (high-speed stall through pulling 'g') and breaking the airframe and that the aircraft has no more to give, (Perpignan case?) On the contrary, we have at least one clear example where Airbus and 777-type laws would have saved the aircraft, (AMS B737).

At the same time I recognize the complexities of the issue and would not hold out such a view if it could be demonstrated otherwise but if that were the case, (and perhaps this lies in "greatest hope" territory), likely Airbus and probably Boeing would be far ahead of any of us in responding with a different view of computerized flight and its reversion laws.
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