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Old 9th Oct 2018, 08:34
  #1149 (permalink)  
FGD135
 
Join Date: Apr 2008
Location: Australia
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Here is a plausible theory as to how the rudder trim came to be in the full NL position. This is based on an experience I had whilst being on the King Air 200, many years ago:

The rudder trim position indicator is not a true indicator of the tab position. It is merely an indicator of where the wheel has been rotated to. The indicator is part of the wheel. It is all one assembly which connects to the drive shaft via a single (?) grub screw. Loosen the screw slightly and you can then remove the wheel/indicator assembly from the shaft.

Then, you can rotate the wheel, with the indicator moving in reduced proportion - despite now being completely separated from the shaft! As can be seen, the indicator is mechanically geared to the wheel. The indicator thus shows the rotational position of the wheel - not the position of the trim tab!

And when putting the wheel back onto the shaft, it is possible to put it on at any rotational position, thus rendering the indication completely invalid! So, for example, you could wind the trim to full NR, then remove the wheel/indicator assembly, then put it back on with the indicator centred. To a pilot, the rudder trim would then look to be set at neutral, when in reality the trim tab is fully to one side.

I experienced a situation where exactly this scenario may have played out. Myself and the other pilots noticed that the rudder trim position indicator was well to one side during the cruise when everything was perfectly trimmed out. This went on for several days. Incredibly, some pilots decided to manage the situation by centering the trim, then setting differential power in order to get the ball centered and wings level (!).

When I next flew the aircraft, and was manipulating the trim wheel, the assembly fell off the shaft and onto the floor. I simply picked it up and put it back on the shaft. The mystery was immediately revealed.

So, in the case of ZCR, could the wheel assembly have been dislodged, due the loose screw, then been put back on, but with the trim indicator now showing full NR trim? Did this operator use the same cleaners of the office to also give the aircraft interiors a vacuum and wipe?

The next pilot of the aircraft, whilst conducting his pretakeoff checks would have found the rudder trim to be indicating full NR. He would have been momentarily puzzled about this, but then wound the trim to centre the indicator. The indicator was then centered, but the trim tab was set to full nose-left.
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