TURNING ERRORS

Turning errors are maximum when turning through north and south, and ignoring liquid swirl zero when turning through east and west.

The basic theory of turning errors is much the same as that for linear acceleration errors. Due to the earth’s vertical component of the magnetic field, Z, the compass’s cg will be displaced from almost beneath the pivot point away from the nearer pole. In a turn, the aircraft accelerates towards the centre of the turn, and therefore an acceleration force acts through the pivot towards the centre of the turn, while the opposing centrifugal force due to inertia acts outward through the cg.

This results in the magnet assembly tending to ‘swing out’ from the turn, rotating the magnet assembly around the pivot point and producing a turning error.

Turning errors are usually more significant than acceleration errors for the following reasons:- They are inherently of greater magnitude because greater displacement of the magnet 

assembly is likely in turns.

Turns occur more often and are likely to be more prolonged than linear accelerations. 

Turning from 045° to 315° (NH).

Consider an aircraft executing a left- hand turn in the Northern hemisphere as it passes through 000°M.

The magnet’s cg is displaced from beneath the pivot point away from the north pole due to the vertical component of the earth’s magnetic field. Because of inertia the magnet assembly will be thrown out of the turn rotating the magnet assembly anti- clockwise.

If there was no turning error the magnet would remain stationary and the aircraft rotate 90° around it - resulting in the pilot seeing 90° passing beneath the compass’s lubber line.

Figure 9.10 Turning from 045° to 315° (Northern Hemisphere)

However the aircraft is turning port and the magnet assembly rotates in the same (anti- clockwise) direction.

Although the aircraft has turned 90° around the compass, the magnet has been displaced and rotated in the same direction by a number of degrees (say 20°). The pilot will therefore only see 70° pass beneath the lubber line and the compass is termed sluggish.

Whenever the magnet rotates anticlockwise it will overread .

This means that if the pilot stops the turn at 315° indicated the actual heading will be numerically smaller such as 295°- therefore the turn must be stopped early (such as 335°) to achieve the correct heading.

This can also be described as undershooting the required heading (note ‘undershoot’ is referring to turning through a smaller angle, and should not be confused with ‘under read’ which means that the numerical heading indicated is too small).

If the pilot deliberately undershoots, rolling out when the compass reads about 325°, he should observe, when the wings are levelled, the compass ‘catch up’ and settle on 315°.

PIVOT CENTRIPETAL FORCE TOWARDS CENTRE OF TURN NORTH 045° 315° INERTIA

Figure 9.10. Turning from 045° to 315° (Northern Hemisphere)

Turning from 315° to 045° (NH).

Consider an aircraft turning right through north in the Northern hemisphere as it passes through 000°M, the magnet’s cg is displaced from beneath the pivot point away from the north pole due to the vertical component of the earth’s magnetic field.

Because of inertia the magnet assembly will be thrown out of the turn rotating the magnet assembly clockwise.

Note: The aircraft and the magnet assembly are again rotating in the same direction (but that it is this time clockwise) and therefore the compass will again be sluggish.

Figure 9.11 Turning from 315° to 045° (Northern Hemisphere)

Whenever the magnet rotates clockwise it will under read . This means that if the pilot stops the turn at 045° indicated the actual heading will be numerically larger such as 065°.

Therefore the turn must be stopped early (such as 025°), or the pilot should undershoot the indication, to achieve the correct heading.

Turning from 135° to 225° (NH).

Now consider an aircraft turning right as shown in Figure 9.11 through south in the northern hemisphere as it passes through 180°M, the magnet’s cg is displaced from beneath the pivot point away from the nearer pole (the north pole) due to the vertical component of the earth’s magnetic field.

Because of inertia the magnet assembly will be thrown out of the turn rotating the magnet assembly anticlockwise. The aircraft is turning clockwise (right) but the magnet assembly is rotating anticlockwise.

Figure 9.12 Turning from 135° to 225° (Northern Hemisphere)

Therefore the aircraft and the magnet are now rotating in opposite directions. Although the aircraft has turned 90° around the compass, the magnet has been displaced and rotated in the opposite direction by a number of degrees (say 20°). The pilot will therefore see 110° pass beneath the lubber line and the compass is termed ‘lively’.

PIVOT NORTH

045° 315°

Figure 9.11. Turning from 315° to 045° (Northern Hemisphere)

PIVOT

225° _135°

NORTH

Figure 9.12. Turning from 135° to 225° (Northern Hemisphere)

Whenever the magnet rotates anticlockwise it will over read. This means that if the pilot stops the turn at 225° indicated the actual heading will be numerically smaller, such as 205°.

Therefore the turn must be stopped late (such as 245°), or the pilot should overshoot, to achieve the correct heading.

Turning from 135° to 225° (SH).

Now consider an aircraft turning right as shown in Figure 9.12 through south in the southern hemisphere as it passes through 180°M, the magnet’s cg is displaced from beneath the pivot point away from the nearer pole (the south pole). Because of inertia the magnet assembly will be thrown out of the turn rotating the magnet assembly clockwise.

The aircraft and the magnet assembly now are rotating in the same direction (clockwise) and therefore the compass will again be sluggish.

Figure 9.13 Turning from 135° to 225° (Southern Hemisphere)

Whenever the magnet rotates clockwise it will under read . This means that if the pilot stops the turn at 225° indicated the actual heading will be numerically larger such as 245°. Therefore the turn must be stopped early (such as 205°), or undershoot, to achieve the correct heading. Remember that when the wings are levelled, the compass will ‘catch up’ and settle on 225°. Turning through East or West

Consider a turning aircraft passing through the magnetic headings of 090° and 270°.

The magnets are not horizontal but their tilt is North-South, that is in the vertical plane of the magnetic meridian through the pivot.

There is no rotational couple acting round the pivot, so there is no turning error. Turning errors are zero when passing through East or West.

Other notes on turning errors:

It is easier to steer a Southerly rather than a Northerly heading, firstly because on South the compass does not indicate the wrong direction of turn as it can on North, and secondly because the ‘lively’ nature of the indications reduces the risk of over-correcting small steering errors. Magnitude of Turning Errors:

There are many factors affecting the severity of turning errors. They are worst at high latitudes where Z is strong and H is weak.

Other relevant variables include rate of turn, duration of turn, speed of the aircraft, the headings involved, and the design of the compass.

Figure 33.13. Turning from 135° to 225° (Southern Hemisphere)