Compass System
Introduction
A compass is an instrument designed to indicate direction on the surface of the earth, relative to some known datum. The magnetic compass works on the principle that a freely suspended magnet will align itself with the earth's magnetic field such that one end will point toward the north magnetic pole.
The purpose of a magnetic ‘steering’ compass in an aircraft is to indicate heading, the direction in which the aircraft is pointing. Magnetic influences - iron/steel components, electric currents - distort the earth’s field so that the compass magnet assembly deviates from the magnetic meridian. This is called compass deviation.
It is mandatory that all civil aircraft must carry such a compass and in all but light general aviation aircraft it serves as a standby compass.​
Compass Requirements
The direct reading magnetic compass contains a pivoted magnet which must be able to align itself, and remain aligned, with the horizontal component of the earth’s magnetic field. To be effective a magnetic compass must meet three basic requirements.
​
Horizontality
The compass card to indicate the correct indication must lie horizontal, to the earth's surface. This is not always possible since, earth's magnetic field dips toward the poles and is only parallel to the surface near the equator.
For simplicity, the magnetic force is resolved into its horizontal and vertical components, H and Z respectively.
Thus, the nearer one is to either terrestrial magnetic pole, the greater the Z force and the weaker the H force. This causes the compass magnet to dip toward the earth's surface and, clearly, the greater the angle of dip, the less accurate the compass becomes.
The amount by which the compass magnet system dips can be reduced significantly by suspending it so that the centre of gravity of the magnet system is well below the pivot point of the circular plate to which the magnets and the compass card are attached.
​
Sensitivity
It is essential that the magnet system of the compass shall point firmly along the magnetic meridian toward the north magnetic pole, and this is achieved by using magnets of sufficient pole strength. To ensure that they continue so to do when the aircraft heading changes, friction at the pivot point is minimised by using a jewelled bearing with an iridium pivot. Furthermore, the compass system is suspended in a clear liquid, which lubricates the
bearing.
​
Aperiodicity
When a compass is deflected away from its north seeking direction, it is desirable that it will return to that direction as quickly as possible. If it tends to oscillate about north for a significant period of time before once again coming to rest it is said to be periodic. Ideally, it should come to rest with no oscillations, when it would be said to be aperiodic.
Aperiodicity is achieved in aircraft magnetic compasses by three measures:
-
The interior of the compass case, known as the bowl, is filled with a clear liquid, typically silicone or methyl alcohol. The compass card has filaments attached to it and these provide a damping effect in the liquid to minimise oscillation.
-
Instead of a single powerful magnet, several small powerful magnets are positioned close to the pivot point and this reduces the moment of inertia of the magnet system.
-
The suspension of the magnet system in fluid helps support the apparent weight of the magnet system, further reducing the moment of inertia.
​
Provision is made for the expansion and contraction of the compass liquid by fitting a bellows, or in some cases a flexible diaphragm, inside the compass bowl.
Magnetic Variation
The earth's true and magnetic north poles are not coincident and there is, in fact, some considerable geographic distance between the two. At most locations there will be an angular difference between the magnetic north and true north, and this difference is known as magnetic variation.
On the shortest distance line joining the true and magnetic poles, variation at any point is theoretically 180º, whereas elsewhere on a line joining the two, known as an agonic line, variation is zero.
​
At locations where magnetic north lies to the east of true north, variation is said to be easterly. At points where magnetic north lies to the west of true north, variation is said to be westerly.
​
Magnetic variation at any point on the earth's surface can be plotted and is shown on charts as a series of lines joining points of equal variation, known as isogonals.
MAGNETIC VARIATION
Magentic Deviation
A compass needle will indicate the direction of magnetic north, provided that it is only influenced by the earth's magnetic field. In an aircraft there are many influencing local magnetic fields caused by hard or soft iron or electrical circuits. These will deflect the compass needle away from the direction of magnetic north. The angular difference between compass north and magnetic north is referred to as deviation.
​
The strength of the aircraft's magnetic field is usually reasonably constant and its effect on the compass reading can be determined during a procedure known as compass swinging. The aircraft is placed on a number of known magnetic headings and the compass reading is compared with the aircraft heading, to find the angular difference between compass north and magnetic north. The compass reading can be adjusted by means of a compensation device within the instrument until it agrees as nearly as possible with the aircraft heading. It is usually impossible to adjust the compass so that it exactly agrees on all headings and the remaining small angular difference is known as residual compass deviation.
​
Upon completion of the compass swing the residual compass deviation is recorded on a compass correction card, in either tabular or graphic form, which is mounted in the cockpit. The correction card shows the deviation as positive (+) or negative (7), indicating how it must be applied to the compass reading to obtain the correct magnetic heading.
​
When compass north lies to the east of magnetic north, deviation is said to be easterly; when it lies to the west of magnetic north, deviation is said to be westerly. Deviation easterly is positive and deviation westerly is negative.
Thus, if the correction card states that the compass has -1º residual deviation on a heading of 180º (M), then the pilot must steer 181º (C) for the aircraft to actually be on a heading of 180º (M). Alternatively, when the compass reads 180º (C) the aircraft's magnetic heading will be 179º (M).
MAGNETIC DEVIATION
Magnetic Dip
Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward.
​
Magnetic dip results from the tendency of a magnet to align itself with lines of magnetic field. As the Earth's magnetic field lines are not parallel to the surface, the north end of a compass needle will point downward in the northern hemisphere (positive dip) or upward in the southern hemisphere (negative dip).
MAGNETIC DIP
The dip angle is in principle the angle made by the needle of a vertically held compass, though in practice ordinary compass needles may be weighted against dip or may be unable to move freely in the correct plane.
ANGLE OF DIP
The aircraft compass magnet is pivoted to keep it horizontal at all times hence due to magnetic dip the centre of gravity of the magnetic will tend to shift, which will depends on the hemisphere in which the compass is.
​
At equator the centre of gravity (CG) of the magnet will be in the centre a the magnet is horizontal plane. When the magnet is placed in the northern hemisphere the north end (red end) of the magnet will dip forcing the centre of gravity (CG) to shift to the sound end (blue end) due to it pivot. Similarly in the southern hemisphere the south end (blue end) of the magnet will dip and shift the centre of gravity (CG) to the north end (red end).
EFFECT OF MAGNETIC DIP OF CENTRE OF GRAVITY (CG) OF A MAGNET
Acceleration Errors
During linear acceleration or deceleration, direct reading compasses are subject to large errors , due to the centre of gravity (CG) of the magnet assembly to moving away from its normal position producing an error.
​
Northern Hemisphere Easterly Heading
In the northern hemisphere the magnet dips towards the north pole, so the north end (red end) dips, thus the centre of gravity (CG) shifts towards the south end (blue end).
​
When the aircraft is on an easterly heading and it accelerates the centre of gravity (CG) resists moving forward due to its inertia, which rotates the magnet clockwise, thus indicating an apparent turn to the north.
While if the aircraft decelerates on an easterly heading, the centre of gravity resist the backward motion or in other wards continues its forwards motion due to its inertia, which rotates the magnet anticlockwise (counter clockwise), thus indicating an apparent turn to the south.
ACCELERATION ERROR IN NORTHERN HEMISPHERE ON EASTERLY HEADING
Northern Hemisphere Westerly Heading
In the northern hemisphere the magnet dips towards the north pole, so the north end (red end) dips, thus the centre of gravity (CG) shifts towards the south end (blue end).
​
When the aircraft is on an westerly heading and it accelerates the centre of gravity (CG) resists moving forward due to its inertia, which rotates the magnet anticlockwise (counter clockwise), thus indicating an apparent turn to the north.
While if the aircraft decelerates on an westerly heading, the centre of gravity resist the backward motion or in other wards continues its forwards motion due to its inertia, which rotates the magnet clockwise, thus indicating an apparent turn to the south.
ACCELERATION ERROR IN NORTHERN HEMISPHERE ON WESTERLY HEADING
Southern Hemisphere Easterly Heading
In the southern hemisphere the magnet dips towards the south pole, so the south end (blue end) dips, thus the centre of gravity (CG) shifts towards the north end (red end).
​
When the aircraft is on an easterly heading and it accelerates the centre of gravity (CG) resists moving forward due to its inertia, which rotates the magnet anticlockwise(counter clockwise), thus indicating an apparent turn to the south.
While if the aircraft decelerates on an easterly heading, the centre of gravity resist the backward motion or in other wards continues its forwards motion due to its inertia, which rotates the magnet clockwise, thus indicating an apparent turn to the north.
ACCELERATION ERROR IN SOUTHERN HEMISPHERE ON EASTERLY HEADING
Southern Hemisphere Westerly Heading
In the southern hemisphere the magnet dips towards the south pole, so the south end (blue end) dips, thus the centre of gravity (CG) shifts towards the north end (red end).
​
When the aircraft is on an westerly heading and it accelerates the centre of gravity (CG) resists moving forward due to its inertia, which rotates the magnet clockwise, thus indicating an apparent turn to the south.
While if the aircraft decelerates on an westerly heading, the centre of gravity resist the backward motion or in other wards continues its forwards motion due to its inertia, which rotates the magnet anticlockwise (counter clockwise), thus indicating an apparent turn to the north.
ACCELERATION ERROR IN SOUTHERN HEMISPHERE ON WESTERLY HEADING
Acceleration or deceleration on a northerly or southerly heading will not cause false indications, since the CG and pivot point are aligned.
​
Summarising, when accelerating regardless of the hemisphere, the compass with show an apparent turn to the nearest pole regardless of easterly or westerly heading.
In other words in the northern hemisphere acceleration on an easterly or westerly heading will produce a false indication of a turn toward north; deceleration on those headings will produce a false indication of a turn toward south and in the southern hemisphere acceleration on an easterly or westerly heading will produce a false indication of a turn toward south; deceleration on those headings will produce a false indication of a turn toward north.
​
Another way to remember it is by using the following acronyms
NADS (some use ANDS) used for northern hemisphere
-
Apparent turn North when Accelerate
-
Apparent turn South when Decelerate
​
SAND used for southern hemisphere
-
Apparent turn South when Accelerate
-
Apparent turn North when Decelerate
NANS & SAND
Turning Error
Turning errors due to the displacement of the CG are maximum on a northerly or southerly heading and are significant up to 35º on either side of those headings.
​
When talking about turning errors it is important to understand the display dial or indication that the pilot receives. The magnet when suspended freely will always point in north south direction, however in an aircraft compass it is enclosed in bowl and covered with an indicating dial.
​
If the aircraft's heading is to the north the magnet inside the compass would be pointing north south direction, however the south end of the magnet would be closer to the pilot since he is reading it from the front. So the north Indicating dial would be fitted on the south end of the magnet inside the compass, in order to indicate or display North to the pilot.
​
Similarly the north end of the compass magnet would be fitted with south indications, the est end of the compass with west indications and the west end of the compass with east indication.
​
So when seen from the pilots eye the indications show are correct.
AIRCRAFT COMPASS INDICATIONS
During a turning a compass can indicates two kinds of errors either the compass with under read or it would over read.
​
If the compass is under reading it implies that the compass is turning slower (lagging) than the aircraft, thus an undershoot is required with respect to the compass indications.
​
While if the compass is over reading it implies that the compass is turning faster (leading) than the aircraft, thus an overshoot is required with respect to the compass indications.
​
Northern Hemisphere Northerly Heading
When an aircraft in the northern hemisphere and turning through a northerly heading, i.e when a turn toward east or west is initiated.
​
As the aircraft turns, the centrifugal force acting upon the centre of gravity (CG) of the magnet system creates a turning force on the compass card in the same direction as that of the turn.
​
Since the compass card should always remain aligned with the magnetic meridian. The effect of the turning card is to cause the compass to under read during the turn.
TURNING ERROR ON NORTHERN HEMISPHERE ON NORTHERLY HEADING
Northern Hemisphere Southerly Heading
When an aircraft in the northern hemisphere and turning through a southerly heading, i.e when a turn toward east or west is initiated.
​
As the aircraft turns, the centrifugal force acting upon the centre of gravity (CG) of the magnet system creates a turning force on the compass card in the opposite direction as that of the turn.
​
Since the compass card should always remain aligned with the magnetic meridian. The effect of the turning card is to cause the compass to over read during the turn.
TURNING ERROR ON NORTHERN HEMISPHERE ON SOUTHERLY HEADING
Southern Hemisphere Northerly Heading
When an aircraft in the southern hemisphere and turning through a northerly heading, i.e when a turn toward east or west is initiated.
​
As the aircraft turns, the centrifugal force acting upon the centre of gravity (CG) of the magnet system creates a turning force on the compass card in the opposite direction as that of the turn.
​
Since the compass card should always remain aligned with the magnetic meridian. The effect of the turning card is to cause the compass to over read during the turn.
TURNING ERROR ON SOUTHERN HEMISPHERE ON NORTHERLY HEADING
Southern Hemisphere Southerly Heading
When an aircraft in the southern hemisphere and turning through a southerly heading, i.e when a turn toward east or west is initiated.
​
As the aircraft turns, the centrifugal force acting upon the centre of gravity (CG) of the magnet system creates a turning force on the compass card in the same direction as that of the turn.
​
Since the compass card should always remain aligned with the magnetic meridian. The effect of the turning card is to cause the compass to under read during the turn.
TURNING ERROR ON SOUTHERN HEMISPHERE ON SOUTHERLY HEADING
When turning through east or west there is no turning error, since the CG and pivot point are in alignment.
​
Summarizing that when turning with the magnetic compass as reference it is necessary to roll out of the turn early (i.e. before the new desired heading is reached) when it is under reading and late when it is overreading.
​
UNOS used for northern hemisphere
-
Undershoot heading through North.
-
Apparent turn South when Decelerate
​
ONUS used for southern hemisphere
-
Apparent turn South when Accelerate
-
Apparent turn North when Decelerate
​
UNOS & ONUS
Remote Reading Compass / Slaved gyro compass
The main disadvantage of direct reading compass are acceleration and turning errors, also since it has to be located in the cockpit it is susceptible to aircraft magnetism and suffers from deviation.
​
The other heading indication instrument the directional gyro is subject to apparent drift, and needs to be adjusted constantly.
​
The slaved gyro compass utilises the earth's magnetic field to sense magnetic north and this magnetic flux is used to provide the constant correction required by the gyroscopic element of the compass system to maintain a magnetic north reference.
The gyroscopic stabilisation reduces the turning and acceleration errors inherent in the magnetic sensing element and the device has the further advantage of being capable of operating any number of remote compass indicators.
​
The slaved gyro compass, also known as a remote indicating compass or gyro-magnetic compass, and combines the best features of the directional gyro and the direct reading compass.
Flux Valve
The magnetic detecting element of the slaved gyro compass is a device known as a flux valve. This senses the direction of the earth's magnetic field relative to the aircraft heading and converts this into an electrical signal.
A simple flux valve comprises two bars of highly permeable magnetic material that are laid parallel to each other, each surrounded by a coil of wire, called a primary coil, supplied with alternating current and connected in series. Surrounding these is a secondary, pick-up coil.
SIMPLE FLUX VALVE
When a current is passed through a coil of wire a magnetic field is set up around the coil and, if the current is alternating current, the field will be of continuously varying strength and polarity. Because the primary coils are wound in opposite directions, the fields surrounding them are of opposite polarity and the permeable flux valve bars are magnetised with opposite polarity by the primary fields. The alternating current is of sufficient strength that, at its peak value, both bars are magnetically saturated.
​
A variable magnetic flux field cutting through the windings of the secondary coil would normally induce a current flow in them, but because the primary fields are of opposite polarity and therefore self-cancelling, no current is induced in the secondary coil.
​
However, if the flux valve is placed parallel to the earth's surface it will be subjected not only to the electromagnetic field due to the primary current flow, but also to the horizontal (H) component of the earth's magnetic field. This flux due to the earth's magnetic field will saturate the bars of the flux valve before the alternating current supply to the primary coils reaches peak value and will cause a varying strength of flux around the primary coils.
​
As this varying flux cuts through the windings of the secondary coil, it will induce a current flow in the secondary windings. The strength of this current is directly proportional to the strength of flux in the flux valve bars due to earth magnetism. This will depend upon the direction of the bars relative to the earth's magnetic field, which will be greatest when the bars lie parallel to the earth's (H) field and weakest when they are at right angles to it. The output of the secondary coil can be used to drive a remote indicating compass or, in the case of the slaved gyro compass, to apply corrections to the gyro unit.
Detector Unit
The detector unit contains the flux valve that senses the direction of the earth's magnetic field relative to the aircraft heading. However, a simple flux valve of the type described above would be inadequate, since it is prone to ambiguity in that it will produce a signal of identical strength and polarity on different headings. In order to overcome this anomaly an aircraft sensing unit consists of three simple flux valves connected at a central point and spaced at 120º intervals.
Each flux valve has a collector horn attached to its outer end to improve collection of the relatively weak earth flux. The assembly is pendulously suspended so that it will remain horizontal regardless of aircraft attitude and is mounted where it will be least affected by aircraft magnetism, typically at the wingtip or near the top of the fin (vertical stabiliser).
DETECTOR UNIT
Remote Compass Indication
The detector unit is mounted such that it remains essentially earth horizontal, in order to best sense the H component of earth magnetism, and fixed so that one `spoke' of the detector is permanently aligned with the aircraft's longitudinal axis.
Each spoke has its primary and secondary windings, and the secondary windings are connected to the stator windings of a device called a signal selsyn. The signal selsyn comprises three stator windings, mounted at 120º to each other, surrounding a rotor upon which is wound a further coil.
​
The following is a description of the principle of operation of a remote indicating compass system. Let us suppose that the aircraft is on a heading of 000ºM. Coil A of the detector unit is aligned with the earth H field and therefore maximum current is induced in it. Coils B and C have weaker strength current induced in them and, because of the direction of their windings, it is of opposite polarity. These currents are supplied to coils A, B and C of the signal selsyn and the combination of their electro-magnetic fields reproduces a flux field identical to that sensed by the detector unit.
​
Assume for the moment that the rotor of the signal selsyn is aligned with this reproduced field. The lines of flux cutting through the windings of the rotor coil will induce maximum voltage and current flow within the coil.
​
This current flow creates an electromagnetic field, which will seek to align with the reproduced `earth' magnetic, causing the rotor to rotate until it is at right angles to the reproduced field. When this is reached there will be no voltage induced in the rotor coil and the motor will cease turning, with the compass pointer indicating the heading as sensed by the detector unit. This is known as the null point of the signal selsyn rotor. Attached to the rotor is a compass pointer, indicating the aircraft magnetic heading.
​
Because the flux valves of the detector unit are at 120º to each other their combined secondary coil outputs are unique on every heading. This ensures that the null-seeking rotor will always be at right angles to the selsyn reproduced `earth' magnetic field.
​
As aircraft heading changes, the relationship of the detector unit flux valves to the earth's magnetic field will change with it. This field will be reproduced in the signal selsyn and the rotor will turn to maintain itself at right angles to the field, moving the compass pointer so that the heading indication changes with the heading change.
​
In the case of the slaved gyro compass a signal from the null-seeking rotor is used to precess a gyroscope to the correct magnetic heading reference.
REMOTE READING COMPASS INDICATIONS
Gyro compass operation
The gyroscope of the slaved gyro compass is basically similar to that of the directional gyro. Its horizontal spin axis is mounted within an inner gimbal, which is in turn pivoted to an outer gimbal that has freedom of movement about the vertical axis. Attached to the inner gimbal is a permanent magnet.
​
A precession coil is wound around the permanent magnet and is supplied with direct current (d.c.) from the null-seeking rotor coil. When current flows through the precession coil a magnetic field will be produced, the polarity of which will depend upon the direction of the current flow. This magnetic field will react with that of the permanent magnet, applying a force to it that tends to tilt the spin axis of the gyroscope.
​
However, the gyroscope will precess this force through 90º in the direction of rotation, which will cause the gyro to rotate about its vertical axis. It will continue to do this until the current flow to the precession coil ceases, which will happen when the null-seeking rotor is at right angles to the selsyn reproduced magnetic field. Thus, aircraft heading changes will generate a current flow from the null-seeking rotor, precessing the gyro to align itself to the new heading.
​
The gyro is mechanically connected through bevel gears to the compass display pointer and to the null-seeking rotor. This latter ensures that any tendency of the gyro to drift will move the rotor from its null position, causing a current to be generated in the rotor coil. This will be transmitted to the gyro precession coil, precessing the gyro to keep it aligned with the aircraft magnetic heading. Apparent drift due to earth rotation or transport wander is thereby eliminated.
​
As with the directional gyro, the inner gimbal of the gyro compass gyro must be maintained horizontal and to achieve this an erection system is required. In this case the erection system consists of a torque motor mounted on the outer gimbal, activated by a levelling switch mounted on the inner gimbal.
REMOTE READING COMPASS OPERATION
Gyro compass errors
Since the detector unit of the slaved gyro compass is pendulously mounted, it follows that during aircraft accelerations the unit will tilt and sense the vertical (Z) component of earth magnetism. In order to minimise the errors that would otherwise arise during turning and acceleration, the precession rate of the gyroscope is deliberately kept slow. Thus, during an aircraft acceleration the heading reference is maintained by the gyro, although the rotor coil may be transmitting a false signal to the precession coil.
Errors due to detector unit tilt during a turn only become significant if a slow rate of turn is maintained for a significant period of time. During a normal turn the error is very small due to the slow precession rate of the gyro. Deviation errors are small in the slaved gyro compass because the detector unit is mounted as far as possible from deviating magnetic influences. Compensation for those small remaining influences may be made by means of a compensator unit that produces small electro-magnetic fields to oppose those causing deviation.
Gyro compass outputs
Output from the slaved gyro compass may be used to supply magnetic heading information to the radio magnetic indicator (RMI), the horizontal situation indicator (HSI) of a flight director system, the autopilot system and navigation systems such as INS and Doppler.