Nigel_
 Second Officer
Flight distance : 388642 ft
United Kingdom
Offline
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"My question is this: Why does the AC move from its position because of a compass error? I would expect the GPS would hold it in place and the barometer would hold its altitude."
Generally the barometer does hold altitude, unless the aircraft falls over.
Also, it generally will hold it's position, for a while at least, as long as you don't go too high or start to move sideways.
The problem comes when you gain a bit of altitude and get into the wind, the wind then pushes it out of position, to get back into position without VPS it knows which way to move because of GPS, but to go in that direction it has to tilt in the correct direction and for that it needs the compass, GPS can't tell it which way is north, only where it is. If it has a compass error then when it tilts to fly back to the GPS position it tilts the wrong direction and so flies in the wrong direction, it then has to correct that error so changes the tilt angle repetitively and thus ends up flying in a curve.
While holding position using VPS instead of GPS the VPS tells it what angle to tilt for the correction so it doesn't need the compass, thus compass errors don't normally cause crashes while you are still near the ground and haven't started to fly away. |
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