Harmon Technologies, Inc.

Magnetic Compass

The magnetic compass displays the magnetic heading being flow by the airplane.  This compass is a floating, spirit damped compass much like the sort found on boats.  While compasses don't depend on any power or external support, they do have errors during acceleration and turning and also swing back and forth making it difficult to read accurately.  For this reason, pilots use a gyro stabilized compass to actually fly the airplane and simply check the gyro's reading against the magnetic compass from time to time.

N4907J has a HSI to provide gyro stabilized compass headings.  Since the HSI on N4907J is slaved to an external compass sensor, the pilot does not have to manually set the gyro, it happens automatically. 

Should a gyro fail, the magnetic compass can be used as a substitute.  This offers one more layer of backup.

All airplanes including airliners have magnetic compasses.  It is required equipment on all certified aircraft.

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