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Old 4th Nov 2018, 01:07
  #523 (permalink)  
Join Date: Nov 2018
Location: Brisbane
Posts: 20
Originally Posted by D Bru View Post
As concerns the cause, it would seem to me relatively rare for several pitot systems to fail simultaneously. To note that the ias/alt pitot system is separate from the elevator pitot system, the first being located at the front of the A/C, the second on each side of the front base of the V/S. (Repeated) Simultaneous failure of both systems would IMO indicate something more structural then the pitots themselves.

As to the effect, in particular the "FEEL DIFF PRESS LT ILL" could be of particular relevance. The elevator feel computer provides simulated aerodynamic forces using airspeed (from the said elevator pitot system) and stabilizer position. Feel is transmitted to the control columns by the elevator feel and centering unit. To operate the feel system the elevator feel computer uses either hydraulic system A or B pressure, whichever is higher. When either hydraulic system or elevator feel pitot system fails, excessive differential hydraulic pressure is sensed in the elevator feel computer and the FEEL DIFF PRESS light illuminates.

I stand to be corrected, but if (barring hydraulic problems) the elevator feel and centering unit enters into a condition that triggers the FEEL DIFF PRESS annunciation (for whatever reason, either pitots or a more systemic failure, since in this case also the "main" pitot system could have been affected simultaneously), it would mean that particularly in manual flight the elevator inputs through the control columns could have a much different (greater) effect than normal. It would be interesting to know whether such inputs with a "dysfunctional" feel and centering unit could lead to upsets with an ultimate loss of control, which at relatively low altitude would be difficult to recover from in time.
Thanks Bru, am also operating on the assumption that the maintenance logs are genuine and the faults on the previous flight point are likely factors on the accident flight.

The FEEL DIFF PRESS light from the elevator feel system is very puzzling because (as you said) the pitot system is separate, and to my knowledge totally isolated from the ports/ADMs/ADIRUs at the front of the aircraft. I am assuming an actual large hydraulic pressure difference between systems A and B would lead to more fault indications than just the FEEL DIFF PRESS. One thing I can't find out is the static source for the elevator feel pitots, but this system should be in an unpressurised part of the plane and hence could just sample it's ambient surrounds to pair with it's pitot. So how could these problems be related?

On thing to consider is whether the STS (if operating on bad airspeed data) could effectively contaminate the feel system operation via the changes it makes to the stab trim. Stab trim position is an input to the elevator feel system along with airspeed from it's dedicated pitot system (which should have been working even with issues with the nose airspeed sensing systems). In this case the operation of the STS with bad airspeed is the major issue, with likely alteration of elevator feel a secondary (but likely very unhelpful) issue.
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