Magnetic Compasses

The Direct Reading Magnetic Compass (DRC) is based on a simple magnetic needle, and points towards the northern end of the earth's magnetic field. It is also installed in an instrument of dimensions and weight that makes it suitable for use in aeroplanes. It is a mandatory requirement that all modern civil transport aeroplanes carry a direct reading non-stabilised magnetic compass as a standby direction indicator. The most commonly found direct reading compass is the "E" type, which is illustrated below.

Properties of a Direct Reading Compass

For a direct reading compass to function efficiently, the magnetic element must possess the following properties:

Horizontality

This ensures that the magnet system remains as near horizontal as possible, thereby sensing only the horizontal or directive component of the earth's magnetic field. This is achieved by making the magnet system pendulous, by mounting the magnet, below the needle pivot, as shown in the diagram above.

The magnet system when freely suspended in the earth's magnetic field will tend to align itself with the direction of that field, ie. align itself in the direction of the total field (T), where T is the resultant of the earth's horizontal (H), and vertical (Z) fields.

If the system is tilted the C of G will move out from beneath the pivot, and will introduce a righting force upon the magnet system, which will tend to oppose and reduce the overall ‘Z' component. The compass will thus take up a position along the resultant of the two forces, ‘H' and the reduced effect of ‘Z', thus minimising the effect of dip.

In temperate latitudes the final inclination of the needle will be approximately 2° to 3° to the horizontal, but this inclination will increase when flying nearer the poles, such that, by about 70° north or south, the compass is virtually useless. The displacement of the C of G is purely a function of the system's pendulosity, and is not a mechanical adjustment, so it will work in either hemisphere, without further adjustment.

Sensitivity

This ensures that the DRC is capable of operating effectively down to low ‘H' values, and is achieved by increasing the pole strengths of the magnet being used, so that it remains firmly aligned with the local magnetic meridian. Sensitivity is also aided by keeping pivot friction to a minimum by using an iridium-tipped pivot, which is free to move in a sapphire jewelled cup. The compass bowl is additionally filled with a liquid, which reduces the overall effective weight of the magnet system, and also helps to lubricate the pivot.

Aperiodicity

This ensures that the oscillation of the sensitive element about a new heading, following a turn, is minimised, i.e. a ‘Deadbeat Return' characteristic. If a suspended magnet is deflected from its position of rest and then released, it will tend to oscillate around the correct direction for some time before stabilising.

This is obviously undesirable, as it could, at worst, lead to the pilot chasing the needle. The compass needle should thus come to rest with minimal oscillation, which is achieved by:

• Filling the bowl with methyl alcohol or a silicon fluid, and fitting damping filaments to the magnet system.
• Keeping the lever arm of the magnet system as short as possible, but keeping its strength high. This has the effect of maximising its directional force, whilst reducing its moment of inertia.
• Using the fluid to reduce the apparent weight of the system.
• Concentrating the weight as close to the pivot point as possible, to further reduce the turning moment.

Pre-flight Checks

Prior to flight the flight crew should carry out the following checks:

• Check the security of the compass.
• Carry out a visual check for signs of any external damage.
• Check that the liquid is free from bubbles, discoloration and sediment.
• Check that the compass illumination system is serviceable.
• Test for pivot friction by deflecting the magnet system through 10-15° each way, and note the readings on return, which should be within 2° of each other.

Principle of a Pendulum

Consider a plain pendulum that is freely suspended in the aeroplane fuselage. If the aeroplane maintains a constant direction and speed, the pendulum will remain at rest, but if the aeroplane turns, accelerates or decelerates the pendulum will be displaced from its true vertical position.

This will occur because the inertia of the pendulum will cause the centre of gravity to lag behind the pendulum pivot, thus deflecting it away from its normal vertical position, directly beneath its point of suspension.

The magnet system (in the compass) is pendulous, so any acceleration or deceleration in flight will similarly result in a displacement of the C of G away from its normal position. This will result in a torque being established about the vertical axis of the compass, and unless the compass is on the magnetic equator, where the earth's field vertical component ‘Z' is zero, it will be subject to dip.

Acceleration Errors

The force applied by an aeroplane when accelerating or decelerating on a fixed heading is applied to the magnet system at the pivot, which is the magnet's only connection with the rest of the instrument. The reaction to this force will be equal and opposite, and will act through the C of G of the magnet system, which is below and offset from the pivot (except at the magnetic equator), as shown below.

The two forces will thus constitute a couple which, dependent on the aeroplane's heading, will cause the magnet system to alter its angle of dip, i.e. attempt to restore the magnet to its horizontal position, or to rotate it in azimuth.

The diagram above shows how the forces affect a magnet system when an aircraft is accelerating on a southerly heading. The resulting acceleration force is similarly applied to the magnet system at the pivot, whilst an equal and opposite reaction ‘R' will act through the C of G, which is below, but offset from the pivot. The resultant couple will cause the southern end of the magnet system to dip further, thus increasing the angle of dip without any rotation in azimuth.

This will occur because the pivot ‘P', and C of G, are both in the plane of the local magnetic meridian. Conversely, if the aircraft decelerates when flying in a southerly direction, the resultant couple will tilt the magnet system down at its northern end.

The opposite will be observed when accelerating/decelerating in a southerly direction along the magnetic meridian in the Northern Hemisphere. If the aircraft is flying in either hemisphere, any changes in speed on headings other than northerly or southerly, will also result in azimuth rotation of the magnet system, and will produce errors in the heading indication, as shown below.

Acceleration errors are also caused by the vertical component of the earth's magnetic field, which occurs because of the magnet systems pendulous mounting, and causes the compass card to tilt during changes of speed. This deflection will cause a further error, which will be most apparent on easterly and westerly headings.

When an aircraft is operated in the Southern Hemisphere and accelerates on either of these headings, the resulting error will cause the magnet system to rotate, and the compass to indicate a turn to the south.

Conversely if an aeroplane decelerates on either of these headings, the resulting error will cause the magnet system to rotate, and the compass to indicate a turn to the north.

These indications will however be reversed in the Northern Hemisphere. If the aircraft decelerates when flying in a westerly direction, the action and reaction of ‘P' and ‘R' respectively, will have the opposite effect, and will cause the assembly to turn in the opposite direction, with all of the forces again turning in the same direction.

The errors due to acceleration and deceleration are summarised in the following table:

• In the Northern Hemisphere, the errors are in the opposite sense.
• Similar errors can occur in turbulent flight conditions.
• No errors occur at the magnetic equator, as the value of "Z" is zero and hence the pivot point and C of G will be co-incident with each other.

Turning Errors

During a turn, the compass pivot is carried along the same curved path as the aircraft. The centre of gravity (of the magnet system), being offset from the pivot, which is used to counter the effect of ‘Z', is thus subject to centrifugal acceleration. Furthermore, in a correctly banked turn the magnet system will tend to maintain a position parallel to the athwartships (wingtip to wingtip) axis of an aeroplane, and will thus be tilted in relation to the earth's magnetic field.

This will place the pivot and C of G out of alignment with the local magnetic meridian. The magnet system will thus be subject to a component of ‘Z', and this will cause it, when turning through South in the Southern Hemisphere, to rotate in the same direction as the turn. This will further increase the turning error, and will cause the compass to under-indicate, as shown below.

The magnitude and direction of the turning error is thus dependent on the aircraft's heading, its angle of bank (the degree of tilt of the magnet system), and the local value of ‘Z' (dip). The turning error will be a maximum value on northerly/southerly headings, and will be particularly significant within 35° of these headings.

If an aircraft turns from south to east, as soon as the turn is commenced, the magnet system's C of G will be subject to a centrifugal acceleration, and will cause the system to rotate in the same direction as the turn. This will in turn tilt the magnet system, and will allow the earth's vertical component ‘Z' to exert a pull on the southern end, which will cause further rotation of the system. The same effect will occur if the heading change is from south to west in the Southern Hemisphere.

The speed of rotation of the system is a function of the aircraft's bank angle and rate of turn. As a result of these factors, the following indications may be registered by the compass:

• A turn in the correct sense, but smaller than that carried out when the magnet system turns at a slower rate than the aircraft.
• No turn when the magnet system turns at the same rate as the aircraft.
• A turn in the opposite sense because the magnet system turns at a faster rate than the aircraft.

When turning from a southerly heading in the Sourthern Hemisphere onto an easterly or westerly heading, the rotation of the system and indications registered by the compass will be the same as when turning from north, except that the compass will under-indicate the turn.

The effects of turning through North and South in the Southern Hemisphere are summarised in the following table:

The liquid in the bowl not only provides damping, but it also tends to turn with, and in the same direction as the turn. This is referred to as ‘Liquid Swirl', and its motion will either add to, or subtract from, the overall needle error, which is dependent on its relative movement.

In the Southern Hemisphere the south magnetic pole will dominate and, in counter-acting its downward pull on the compass magnet system, the C of G will move to the northern side of the pivot. The errors will thus be in the opposite sense of the northern hemisphere characteristic.

If an aeroplane turns from a northerly heading onto an easterly heading, the centrifugal acceleration acting on the C of G will cause the needle to rotate more rapidly in the opposite direction to the turn, thus indicating a turn in the correct sense but of greater magnitude than that actually carried out, initial overshooting the desired heading before settling back to the desired bearing.

Turning from a southerly heading onto an easterly or westerly heading in the Southern Hemisphere will, because of its C of G which is still north of the compass pivot, result in an initial undershoot and a slow compass movement to the desired bearing.

• In the Northern Hemisphere, the errors are opposite to those occurring in the Southern Hemisphere.
• The Southerly turning error is greater than northerly, as liquid swirl is additive to the compass magnet system movement.