Discussion
Alignment tasks and the sensor used for each.
| Task | Sensor | Fails if |
|---|---|---|
| Levelling | Accelerometers | The aircraft moves |
| Magnetic heading | Magnetometer | The local field is disturbed |
| True heading by gyro compassing | Gyros | The gyros are too noisy |
| Bias estimation | All, while stationary | The aircraft moves |
What do the first and last rows have in common?
Answer: A
Correct answer: Both require the aircraft to be stationary
Levelling and bias estimation share the same precondition, as the table's failure column shows: both fail if the aircraft moves. Levelling assumes the only specific force present is the reaction to gravity, so any motion tilts the sensed vector and corrupts the initial pitch and roll. Bias estimation assumes the true rates and accelerations are zero, so that whatever the sensors report is offset - an assumption movement destroys.
This is why alignment is performed at the stand rather than while taxiing. Neither task uses the magnetometer, neither is optional in a full alignment, and neither involves satellite reception.
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