Old 1st Jan 2019, 21:04
  #51 (permalink)  
Machinbird
 
Join Date: Jul 2009
Location: Not far from a big Lake
Age: 77
Posts: 1,457
Identifying the fluctuation in the AoA signal as electrical noise does not make sense to me.

Being only present on the ground it is much more likely it is some form of imbalance situation in the AoA vane.
Normally you want the AoA vane combined with the internal rotating parts of the sensor to be balanced so it doesn't react too much to bumps in the runway or turbulence in the air.
So i find a mechanical problem with the AoA vane like a broken off part combined with a bend much more likely.

Also no one has explained so far why electrical noise would never be visible during flight.
Wiedehopf, I gather that you have not had to troubleshoot intermittent electrical problems before-fair enough.
Consider an out of balance AOA sensor as a potential cause. This would create a constant torque on the sensor in the increasing AOA direction (assuming vane heavy). As we go faster in the air, the restoring force is increasing and the error would decrease, but this is not what we are observing-we are seeing a constant error between sides.
With regard to a bent vane causing the error, we would be assuming gross incompetence on the part of a number of people, both maintainers and whomever in the aircrew preflighted the aircraft for the last two flights. A bent vane with a 20 degree offset should be as obvious as the nose on your face.
Why would we be seeing Left AOA noise on the ground but not in the air? Consider the difference in vibration environments. The SMYD boxes are mounted in the nose avionics bay, correct? The front end of the airplane experiences much more jiggling on the ground than does the area of the aircraft closer to the CG and main gear. (Watch any Youtube video of a transport crew making al landing, and there should be no doubt in your mind when the aircraft has touched down just from the bouncing around the crew gets when they touch down.)
Finally, consider the intermittency of the problem. Different presentations (or lack of presentation in 1 case) of the problem over the course of 5 flights. During maintenance, the problem would often fail to present itself, and only leave its calling cards. This level of complexity shouts intermittent electrical problem, loud and clear.
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