Ask An Engineer - Calibrating and Servicing an Aircraft Compass
When Should a Compass Calibration be Performed Each installed aircraft compass should have a calibration check when the following events occur:
• Prior to the issue of a Certificate of Airworthiness (CofA).
• At least every 24 months when installed in an aircraft issued with an CofA, unless the approved maintenance programme prescribes a different period.
• Air transport aircraft, at the period prescribed in the operator’s System of Maintenance.
Additionally each compass should also have a calibration check if an unusual event occurs such as:
• When a compass is initially installed or reinstalled in an aircraft.
• After an engine change, unless the manufacturer prescribes otherwise.
• Whenever a magnetic sensing element has been changed or relocated.
• After modification of an electrical or avionic installation in the aircraft, unless the certifying engineer is satisfied that the modification will not affect the compass.
• After a lightning strike, unless at least two heading checks 90º apart shows that no change of deviation has occurred. • After any maintenance involving the addition, removal, or relocation of magnetic materials likely to influence compass deviation. Note: Maintenance manuals may indicate the components that, if changed, would require the compass to be swung • Following any operational occurrence, such as an accident, or heavy landing, that is likely to affect compass deviation.
• After long-term storage of the aircraft.
• Whenever there is reason to suspect that a change of deviation may have occurred .
When Should a Compass Service Be Performed?
A compass service should be performed if the compass has the following;
•Low fluid levels
•Discolouration making it difficult to read
•Or any suspect operation
This compass pictured on the right, was difficult to read, the liquid was discoloured and there were bubbles in the top of the compass.
The compass was removed from the aircraft and a full serviced conducted.
Once the seals were replaced and components cleaned it was re-installed in the aircraft and a full compass calibration was carried out.
A compass service involves the removal of the compass from the aircraft, replacement of compass seals and liquid and the re-calibration of the compass once it has been re-installed.
This operation can only be performed by a qualified LAME.