by WoodSTokk » Tue Sep 22, 2020 12:21 pm
The HSI (also the DG) on most GAs works with a unslaved gyro. If you turn on battery and avionic, the gyro should start to spin.
If the gyro is spinning, the compassrose should rotate if the aircraft rotate. The gyro has no meaning of the magnetic field!
The pilot it self must set the compassrose to align it with the magnetic compass.
Also, the gyro maybe drift away over time. Therefor the pilot should periodically (around all 30min) check the DG/HSI against the magnetic compass and reset it accordingly.
Bigger aircrafts have a magnetic flux sensor that slave the gyro automatically (gyro slaving).
But even than, the pilot should check periodically if the DG/HSI is correct. It can be that the magnetic flux sensor is broken.
If you find some misalignment, switch the slaving from AUTOMATIC to MANUALLY and set it by hand.