COMPASS DEVIATION AND CORRECTION

IntroductionIntroduction

1. An aircraft contains both hard iron and soft iron giving

rise to permanent and temporary magnetism.

2. The nature of this permanent magnetism depends on:

3. Magnetic heading during construction.

4. The Angle of Dip at the place of construction.

5. Amount of coercive force of the metals used. And

6. Amount of metalwork ( hammering, riveting etc.).

Some permanent magnetism can also be set up by radio and radar equipment and other EM materials.

IntroductionIntroduction

• Though permanent magnetism can be measured and corrected for, The ‘Induced’ or temporary magnetism in soft iron components cannot be easily corrected.

• The reason is the changing nature of the magnetic field components (H and Z) of the Earth.

Practically, a pilot needs to know the correction to be applied to the magnetic course to steer with the magnetic compass.

This is called deviation correction.

Compass Deviation

The difference between the direction of the horizontal component(H), of the Earth’s field, & the direction of the horizontal component of the resultant of all the fields at the sensor position, is the DEVIATION ANGLEIt is named easterly(+) or westerly (-), depending whether the resultant field direction is to the East or West of the Earth’s field.The deviation angle varies depending on :• The position of the sensor in the aircraft.• Aircraft Heading.• Geographical position of the aircraft.• The passage of time.

Magnetic componentsAn aircraft structure consists of a number of components capable of being magnetized. Their effects on the magnetic compass can be due to 1. Hard Iron Magnetism: Resulting from permanent and sub-permanentmagnetic fields.2. Soft iron Magnetism: Resulting from transient magnetic fields.

Hard Iron Components: Along the longitudinal axis of the aircraft, ‘P’ is a component which is +ve when BLUE POLE IS FORWARD.Along the lateral axis of the aircraft, ‘Q’ is a component which is +ve when BLUE POLE IS STARBOARD.Along the vertical axis of the aircraft ‘R’ is a component which is +vewhen BLUE POLE IS BELOW.

Magnetic componentsSoft Iron Magnetism: The vertical soft iron components are consideredwhere these effects are allied with those of hard iron components P and Q.the fore and aft component is ‘c’, and lateral component is ‘f ‘.The combination of Hard Iron with Soft Iron for correction leads us to the Coefficients A, B, and C.Coeff. ‘B’ is caused by ‘P’ and ‘c’ which cause magnetic force P+c. Sine Coeff B = P + c

H ( Deviations vary as the sine of the headings).

Coeff ‘C’ is caused by ‘Q’ and ‘f ‘ which cause magnetic force Q+f. Cosine Coeff C = Q + f

H ( Deviations vary as the cosine of the headings).

Magnetic components

Coefficient ‘A’ is made of two parts:1. Real ‘A’ due to HORIZONTAL SOFT IRON which causes a constant deviation on all headings and2. Apparent ‘A’ due to mechanical misalignment of the lubber line. Both these effects are compensated by rotating the compass bowl to realign the lubber line.

Total Deviating Forces

AXIS of Deviating Force aircraft Hard iron component Soft iron component

Fore & aft

Athwartship

Vertical

P

Q

R

aX, bY & cZ

dX, eY & fZ

gx, hY & kZ

Magnetism & Compasses

Graphs of devn. Components & total deviation

Total Deviation on a Hdg.

On any compass heading ° Total devn= A+ B Sin ° +C Cos ° +D SIN 2 ° +E Cos2 °.

Considering only the hard iron effects, the deviation on ANY HDG is:A+ B Sin Hdg +C Cos Hdg.

Deviation Coefficients

Relationship between aircraft magnetism and deviation coefficients

Magnetism & Compasses

Coeff C = Dev N – Dev S2

Coeff B = Dev E – dev W2

Misalignment due to Apparent A is corrected by turning the base ofthe compass: clockwise for +A.

Compass Swing-1

Correcting Swing: L/C E2 Devn Corrected Hdg

S 184 183 +1 180

W 272 268 +4 270

N 360 353 +7 356

E 088 088 0 086

Coeff C = +7 – (1) / 2 = +3 . Coeff B = 0 – (4) / 2 = - 2 .

The corrected readings are at the end of the column.

Turn key anti-clockwise for + deviation.

Compass Swing-2

Check Swing: L/C E2 Devn Coeff A Residual Devn

134 129 +5 +4 +1

181 177 +4 “ 0

225 222 +3 “ -1

272 270 +2 “ -2

314 308 +6 “ +2

357 353 +4 “ 0

048 044 +4 “ 0

094 091 +3 “ -1

+31

Coeff A = +31 / 8 = +4

Earth Rate

Gyro CorrectionsEarth Rate CorrectionThe effect of apparent drift caused by Earth Rate can be corrected for byinducing real drift in the opposite direction. This is done by using alatitude nut which will correct for Earth Rate at one particular latitude,usually that of the aircraft's home base.A nut is fitted to one side of the inner gimbal on a spindle as shown

Gyro Corrections

Transport Wander

Transporting the gyro over the surface of the Earth also induces apparent drift known as Transport Wander. Like Earth Rate, it happens because of the convergency of the meridians and its magnitude is proportional to the East/West component of groundspeed,and latitude .With an easterly component, transport wander is in the same sense as Earth Rate in the northern hemisphere. That is, in the northern hemisphere it causes the gyro to underread. Westerly movement has the opposite effect.Sense of Apparent Drift You may find the following table helpful in remembering the sense of the apparent drift and the latitude nut correction.

Notice that if you can keep in your head the fact that Earth Rate in the northern hemisphere caused an underread (-) then all the other signs can be filled in byreversing plus and minus.

ExampleA perfectly balanced (ie no random drift) DI has its latitude nut set for 50N.Would the indicator reading increase, decrease or remain the same when:(a) Flying on a track of 090° (T) in latitude 70° N(b) Flying on a track of 270° (T) in latitude 20° N(c) Flying on a track of 090°(T) in latitude 40° S Solution to (a)Based on the diagram below, a flight to the North of the set latitude causesa decrease in reading, as does a track of 090° (T) in the northern hemisphere,therefore the DI underreads and the readings will decrease.

Example Solution to (b)When South of the set latitude the latitude nut will cause an overreading.Flight on a track of 270° (T) in the Northern Hemisphere also causes the gyro to overread. The overall effect is therefore to cause the DI readings to increase.

ExampleSolution to (c)Flight to the South of the set latitude causes overreading, flight on a track of090° (T) in the southern hemisphere will also cause an overreading. The DI readings will increase.