AHRS, Magnetism, & Compass Deviation

October 2019

Although you may be flying with the latest glass cockpit technology for navigation, it is always a good idea to make sure your magnetic compass is giving out accurate readings. Some aircraft equipped with AHRS (attitude and heading reference systems) have an increased sensitivity to magnetism.

When an aircraft is magnetized, instead of picking up the earth’s magnetic north the aircraft’s compass finds the magnetic field coming from the airframe resulting in incorrect readings or magnetic deviation.

The most common cause for aircraft magnetism comes from improper hardware installed during maintenance. This means the hardware used on the aircraft is not the correct part number and most likely is not made of the right metal alloy. This is fixed by simply identifying and replacing the improper hardware.

Magnetism from a lightning strike, while far less common, can cause things to go awry abruptly. Not only can it cause magnetism and mess with your compass readings, it has the ability to change the properties of the alloy effectively changing the metallurgy of the component.  However, it poses very little danger to critical on-board avionics, passengers, and crews.

An aircraft can become magnetized just about anywhere.  One of the most common places to find residual magnetism from a lightning strike is in the steel hardware that attaches the parts and pieces of an aircraft.  This is because the steel used to manufacture the hardware has a high retentivity or ability to retain magnetism. 

The easiest way to find a magnetized piece is by using a simple Boy Scout compass. The needle responds near anything magnetic. Once the suspect area is found, you degauss that area and verify with a field indicator. It most cases a magnetized area can be removed by using a piece of NDT equipment called an electromagnetic yoke.

Removing magnetism with NDT

field indicator

A field indicator is used to locate the presence of a magnetic field.  The photo above shows a field measuring 5 gauss. The readings can be higher or lower depending on how much residual magnetism is in the part causing compass issues.

 

electromagnetic yoke

To remove the residual magnetism, an electromagnetic yoke is passed over the magnetized location. Sometimes multiple passes are needed, no direct contact is required.

 

field indicator at zero

 

The location is periodically checked with the field indicator after passing the yoke over the magnetized location until it's removed.  It's been our experience that even small amounts of residual magnetism can have an effect on the aircraft's magnetic compass. 

 

Brian Young NDT Team Leader Lincoln, NE (LNK)
+1 402.475.2611 ext. 1035 ext. 1035