compass instruments chapter 15. aim to review principles of operation of the compass instruments

Compass InstrumentsChapter 15

Aim

To review principles of operation of the compass instruments

Objectives1. Describe the principles of operation of compass

instruments

Comprises a pivoted magnet able to align itself and remain aligned with the horizontal component of the earth’s field. Allows direct reading of heading.Otherwise known as the standby compass.

Primary navigation aid for most light aircraft.

Direct Indicating Compass

Indirectly it indicates Bank angle:• Compass decreasing, left turn• Compass increasing, right turn

Susceptible to a number of errors.

1. Compass Instrument Principles

Horizontality – counter to magnetic dip by providing a high pivot point and a low C of G. Sensitivity – multiple magnets to increase field strength and liquid immersion to reduce friction.Aperiodicity – dampening of oscillation in the form of short magnets near the C of G and a wire spider type assembly to interact with the liquid. Low viscosity fluid to avoid liquid swirl during turns.

Direct Indicating CompassCompass Requirements


When we look at the compass we are reading the back of it. This means the compass in our aircraft reads backwards.

Direct Indicating CompassReading the Compass

This is best shown on a boat compass. If we are heading 300 and wanted to turn onto 270 we know the shortest turn will be to the left.

When we are looking top down on the compass we can see 270 is to the left of 300. However when we look at the front of the compass card 270 is to the right of 300.


Angular difference between true north and magnetic north.

Direct Indicating CompassVariation

The magnet in the compass aligns itself with the earths lines of magnetic flux (magnetic north).


Magnetic dip is the angle made with the horizontal by the compass needle of a vertically held compass.

Direct Indicating CompassDip


The value can be measured with an instrument typically known as a dip circle.

Direct Indicating CompassDip

Contour lines along which the dip measured at the Earth's surface is equal are referred to as isoclinic lines. The locus of the points having zero dip is called the magnetic equator or aclinic line.


Direct Indicating CompassDipAs you travel to the poles the lines of magnetic flux begin to dip, so to does the magnetic bar of the compass.

To help reduce this error the compass magnet is suspended from a pivot point.

This now means the center of gravity of the compass magnet is going to be displaced from the pivot point (apart from the magnetic equator). The C of G is always displaced to the equatorial side of the pivot.


Direct Indicating CompassChanges in the Earth’s MagnetismThe Earth’s magnetic field is subject to constant change ranging from long term field reversal, medium term secular variation and short term variation.

A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year.

Other regular changes are not of sufficient magnitude to affect normal compass application.


Direct Indicating CompassChanges in the Earth’s MagnetismMagnetic storms vary in intensity and duration and originate from solar flares. The effect is a temporary but significant change in magnetic variation. In Australia the change is unlikely to exceed 2 but can be as much as 5 nearer the poles. It can alter the directive force to a value lower than that required for navigation in polar regions.


Occurs when the compass magnet is acted upon by a magnetic field other than the Earths (i.e. the aircraft including engines, instruments and electrical circuits).

Direct Indicating CompassDeviation

The error is displayed on a compass deviation card produced by an engineer after they carry out a compass swing.


Aircraft are swung:• Compass replacement• Accuracy in doubt• As required by maintenance

schedules• Aircraft modification• Lightning strike• Change of magnetic latitude• Carriage of ferromagnetic loads

Because the center of gravity of the magnet is displaced from the pivot point, when we Bank the aircraft there is a horizontal component of gravity (centrifugal force) that acts on the compass resulting in a change of heading indicated.

Direct Indicating CompassTurning Errors

To remember which way it will rotate we use the acronym ONUS

ONUS

vershootorth

outhndershoot

This error is maximum on North and South, nil on East or West.


Magnitude of turn error depends on:• Heading change through which the

turn is made• Rate of turn• Magnetic latitudeMagnetic compass is: ‘nippy on north and sluggish on south’.

Direct Indicating CompassTurning Errors


Because the center of gravity of the magnet is displaced from the pivot point, when we accelerate or decelerate the compass will lag behind the aircraft and will rotate. This error is maximum on East or West, nil on North or South.

Direct Indicating CompassAcceleration Error

SAND

outhccelerate

ecelerateorth

In the southern hemisphere, when we accelerate the compass will swing towards the south, when we decelerate the compass will swing towards the north.

To remember which way it will rotate we use the acronym SAND


Direct Indicating CompassAcceleration Error


The size of the acceleration error depends on:• Aircraft heading• Magnitude of the acceleration• Magnetic latitudeErrors are maximum at the poles and decreasing to zero at the magnetic equator.Direction of errors opposite in opposing hemispheres.Deceleration and acceleration have opposite effects.

Direct Indicating CompassServiceability Checks


Ensure:• Glass is clean• No discolouration of the fluid• No bubbles are visible• Indicates within 5 of a precise

heading


Provides an accurate and stable magnetic heading reference.

Remote Indicating Compass

Compass Pilot HI

Direct Indicating Compass

Senses the earth’s magnetic field.

Compares compass and HI. Aligns HI to the compass.

Maintains alignment through gyroscopic property of rigidity.

Detector Slaving System Gyro Unit / Indicator


Senses the earth’s magnetic field.

Compares detector and gyro unit. Difference is synchronised.

Maintains alignment through gyroscopic property of rigidity.


The Detector UnitSenses the aircraft’s heading relative to the earth’s magnetic field.Also known as the flux valve.Uses a pendulous suspension system allowing freedom 25 in pitch and roll. Sealed and liquid dampened to prevent oscillations.



The Slaving & Transmission SystemConsists of self-synchronous transmission system (selsyn) and torque motor in the gyro unit. Receiver rotor senses misalignment and the error signal is fed to the slaving amplifier and torque motor which applies a force to the appropriate gyro gimbal.



AnnunciatorAn indicator that shows the compass system slaving correctly.During synchronisation, the error signal seeks null but is never there for more than a few milliseconds. The alignment motor activates multiple times per second. Presents oscillating dot and cross or a pointer centred within a range.



OperationThe null seeking rotor is attached to the output shaft and provided the gyro and detector remain synchronised, the changing direction of the rotor is matched to the output of the detector. It is gyroscopic rigidity and not precession the provides the change in heading information. Any gyroscopic drift error will be induced in the selsyn rotor and the gyro will be slaved back into alignment at an rate approximating 2 per minute. Some units incorporate a turn cut-out system whereby interrupting the slaving process so preventing an incorrect input from the detector and any turn error.



OperationThe gyro unit containing a horizontal gyro is slaved in azimuth. To prevent any topple (drift in the vertical plane) during normal operations, a levelling system maintains the rotor axis horizontal. Gyro is vulnerable to topple during initial spin up, violent manoeuvres or turbulence. Gimbal limits are usually +85. A fast erection/slaving system is sometimes available via a synchronising push button. The slaving rate is 60 per minute while the normal rate is 3 per minute. Continuous operation of the fast slaving system for more than 15 seconds can overheat and cause damage. If electrical power is lost, an off flap will appear informing the pilot that the heading information is no longer valid.



Two modes: • Slaved: operates automatically as discussed.• Free: reversionary mode, operates like a conventional HI.Flux valve is located in a wingtip/rear of aircraft to remove it from the aircraft magnetic fields and compensation circuits are fitted to further negate the fields.For the AC power requirement of the detector unit, an inverter is often built into the system.


Serviceability:• Good, clean condition• Manual and automatic

slaving modes functional• Agrees with standby

compassCan be presented as a HSI, RMI or as a part of the AHRS.


Advantages of the Remote Indicating CompassReduced Deviation – the remote location of the detector unit isolates the magnetic sensing element from the effects of the aircraft’s magnetic field so allowing deviation to be reduced to less than 1. Turning and Acceleration Errors – Gyroscopic rigidity provides the system with a stable reference so that it is largely unaffected by turning and acceleration errors.Improved Presentation – The display of heading can be means of a vertical card or EFIS, so eliminating the parallax errors associated with direct indicating types. Power Output of Heading – provides heading to multiple displays such as the RMI and HSI and information for autopilot and flight director functions.


Questions?

compass instruments chapter 15. aim to review principles of operation of the compass instruments

Documents

compass magnet

compass card

compass decreasing

compass needle

standby compass

boat compass

compass swing

compass instruments