Aircraft Magnetism

Wg Cdr Rajagopal

Aircraft Magnetism

Introduction to Aircraft Magnetism

Compass north is the north indicated by an aircraft’s compass needle
Compass north deviates from magnetic north due to deviation
Aircraft magnetism induces deviation error in the compass
Ferrous metals and electrical fields cause aircraft magnetism
Deviation is the angular difference between compass and magnetic north
Compass swing procedure enables us to correct deviation in our compass

Deviation Compass Swing

Compass swing is used to calculate, correct and record residual error
Aircraft’s compass is compared with datum compass outside the aircraft
Deviation on cardinal headings is used to find total deviation
Coefficient ‘b’ is the effect of aircraft magnetism on the longitudinal axis
Coefficient ‘b’ varies as the sine of heading
Coefficient ‘c’ is the effect of aircraft magnetism on the lateral axis
Coefficient ‘c’ varies as the cosine of heading
Coefficient ‘a’ is due to misalignment of aircraft or compass lubber line
Coefficient ‘a’ is common in all directions

Total Deviation Effect

Resultant deviation on any heading =
Addition of coefficients A, B x Sin of heading and C x Cos of heading
Positive sign means the value has to be added to the indicated heading
Negative sign means the value has to be subtracted to indicated heading

Correction for Deviation

Coefficient A is reduced by aligning lubber line with longitudinal axis of aircraft
Coefficient B and C are reduced by creating local magnetic fields
Local magnetic fields are equal and opposite to aircraft’s magnetic field
Coefficient B is removed on easterly and westerly headings
Coefficient C is removed on northerly and southerly headings
Residual deviation is recorded on compass card
Accuracy of a direct reading compass is limited to 10 degrees
Accuracy of remote indicating compass is limited to 1 degree

Latitude Effect due to Hard Iron

Deviation at different latitudes will change with latitude
Horizontal component of earths magnetic field reduces with latitude
Relative strength of aircraft magnetism due hard iron increase with latitude
Deviation error increases at higher latitudes as we fly closer to poles

Latitude Effect due to Soft Iron

Deviation at different latitudes will change with latitude
Aircraft magnetism due to soft iron increases with vertical component
Deviation error increases at higher latitudes as we fly closer to poles

Occasions for Compass Swing

On installation of compass components
When compass accuracy in doubt
After maintenance inspection
After modification, repair, replacement of parts
When carrying Ferro-magnetic payloads
When compass subjected to shock
On change of magnetic latitude
After storage in a single heading
After lightning strike
After radio or electrical system modifications

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