Aircraft Magnetism and Compass Swing, Direct Indicating Compass System


AIRCRAFT MAGNETISM AND COMPASS SWING

Aircraft Magnetism

The permanent magnetism is mostly induced during construction by the Earth’s field. The fuselage during assembly receives a large amount of hammering, specially if much riveting takes place, and since it remains stationary in the workshop for a long time, the continual hammering will help the molecules of the metal in their tendency to align themselves with the Earth’s magnetic field. They will in fact, be slowly jolted into alignment. Thus, any metal of the “hard” variety in the fuselage will become permanently magnetized with a polarity depending on the heading of the fuselage during assembly. The half of the fuselage nearest to local Magnetic North will contain the red pole and that nearest to local Magnetic South the blue pole.

During normal flight the aircraft and the compass needle are both horizontal. Therefore, except in special circumstances, it is necessary to consider only that part of the permanent magnetism in the aircraft which is horizontal in effect.

So, to simplify correction, the aircraft’s diagonal magnetism is analysed into two component parts, one fore and aft and the other athwartships. The precise effect of these components can easily be measured in the form of compass Deviation; neutralizing magnets (usually called corrector magnets) of the required strength can then be placed fore-and-aft and athwartship as required. For convenience, the horizontal fore-and-aft component of the aircraft’s permanent magnetism is known as P, and the horizontal athwartships component Q. The vertical component only considered in certain cases, is known as R.

Vertical “soft” iron in aircraft magnetized by the Earth’s field can have the same effects upon the compass as components P and Q and will act with the latter in their respective fore-and-aft and athwartships planes.

Component P and its allied “soft” iron can be represented by a single magnet of similar strength and polarity lying fore-and-aft. In the aircraft referred to above, this magnet would lie with its red end forward and blue end aft. Its effect upon the magnetic compass needle can best be understood with reference to below:

Aircraft Magnetism

Effect of P and Q of Soft Iron

 

 

 

 

 

 

 

 

 

Effect of P and allied soft Iron         Effect of Q and allied soft iron

 

The maximum deviation will be caused when the fore an aft magnet is at right angles to the compass needle. This occurs when the aircraft is heading East or West by compass, the sign of Deviation on East being opposite to that on West. In the case under consideration, the signs will be – and + respectively. Zero Deviation will occur when the aircraft is heading North or South by compass, in which positions the fore-and-aft magnet is in line with the compass needle. Intermediate amounts of deviation occur at other headings. Deviations as shown above are is a sine curve pattern.

Desired Properties of the Magnet System

Horizontality

  • Should pivot away from CG to overcome dip
  • Magnet system should only respond to horizontal component

Sensitivity

  • High sensitivity is required
  • There should be low moment of inertia for the system
  • Iridium tipped pivots to reduce friction

Aperiodicity

  • Efficient dipping
  • Immerse magnet system in liquid
  • liquid silicon
  • Alcohol water mixture

P.S.   Dip is reduced to 2º- 3º upto 70º N/S by shifting the pivot point closer towards the nearer pole. This ensures that the magnetic compass is usable till 70º N/S.

Aircraft Magnetism

Inclination

 

 

 

 

 

 

 

 

 

 

 

 

NATURE OF PERMANENT MAGNETISM

The nature of the permanent magnetism depends upon the following factors:-

  • Magnetic heading of a/c during construction.
  • Angle of dip at the place of construction.
  • Amount of coercive of the metals used.
  • Amount of hammering, battering, vibrations and riveting.
  • Intro of electromagnetic material, radio and radar eqpt, electric current etc… although ac is ‘de-gaussed’ before leaving workshop.

 

Hard Iron. Magnetic material of the aircraft structure which has acquired permanent magnetism is described as hard iron.  This magnetism may have been acquired during manufacture, or during the flying, servicing, or structural testing of the aircraft.  Magnetic components of instruments permanently installed in the aircraft are included in the general designation hard iron.  Although permanent magnetism can change slowly with time, and rather more rapidly as the result of a lightning strike, these changes are ignored in the general consideration of compass deviation.

 

The three components of permanent ac magnetism

  •  Component ‘p’. Parameter acting along the fore and aft axis of the compass. Denoted +ive with the blue pole forward of the compass.
  • Component ‘q’. Parameter acting along the athwart ship line of the compass. Denoted +ive with the blue pole starboard of the compass.
  • Component ‘r’. Parameter acting along the vertical line of the compass. Its effects on the compass in straight and level flt may be considered negligible.

 

Parameters of temporary magnetism

Soft Iron. Magnetic material in which temporary magnetism is induced while in the presence of external fields is described as soft iron.  The temporary magnetism may be induced by the Earth’s field, the hard iron, electrical currents, and weapons or cargo.  The effects of electrical currents and payload is reduced to negligible proportions by the careful selection of the sensor position.

 

  • ‘c’. A pair of vertical soft iron rods one before and one behind the compass, each with a pole in the horizontal plane of the compass needle. In the northern hemisphere the bottom pole would be induced with a red magnetism. Assuming that the forward vertical rod has its blue pole level with the needle and the rear rod its red pole there are effective poles acting on the compass like +p.

 

  • ‘f’. Similarly considered athwart the compass.

 

Magnetic Deviation (Aircraft Magnetism)

The angular difference between Magnetic north and Compass North is called Deviation. It is annotated East or West depending upon, whether Compass North lies East or West of Magnetic North.

Components of Earth’s Magnetic Field

Aircraft Magnetism

Earth’s field low res

 

 

 

 

 

 

 

 

 

 

 

Deviational Forces

Due to Hard Iron

  • Permanent in nature
  • Acquired during manufacturing/ servicing
  • AC degaussed before leaving the factory

Due to Soft Iron

  • It is due to Earth’s magnetism
  • Temporary in nature
  • Caused by Hard Iron, Electrical circuits in ac systems, freight, Earth’s field
  • Temporary magnetism induced by hard iron parts of ac are considered along parallel

Deviation forces depend upon:

  • Location of soft iron in relation to sensor
  • Geographic location of ac (Deviation Max at Equator, H = T)
  • Heading of AC
Soft Iron Component
Components Fore and Aft Athwartship vertical
X Ax Dx Gx
Y By Cy Hy
Z Cz Fz kz

x,y,z components are components oof temporary magnetism caused by soft iron, under the influence of earth’s magnetism

Soft Iron Components

Can be considered as a magnet of variable intensity carried on board

In soft Iron field we only consider magnetism induced by earth’s magnetic field, Soft Iron magnetism is induced by x,y,z component of earth’s magnetism. x= North horizontal field component, y = East horizontal field component, z = vertical field component, positive if directed inside earth.

Aircraft Magnetism

Aircraft Magnetism

 

 

 

 

 

 

 

 

 

 

 

 

Combined effect of Soft Iron and Hard Iron

  • P and cz produce similar effect (Fore and aft)
  • These are combined and net effect is attributed to Coefficient B
  • Q and fz produce similar effect (Lateral Axis)
  • These components are combined and the net effect is attributed to Coefficient C

Coefficient

It is the maximum deviation caused by hard and soft iron components in the fore and aft axis and lateral axis of the ac, they are termed as Coefficient B and C

Coefficient B and C

Coefficients B and C are the calculated quantities of deviation due to P and Q respectively (vertical “soft” iron effects are included).

Coefficient B is the algebraic value of the deviation produced by component P and its allied vertical “soft” iron when the aircraft is heading Magnetic East. It is calculated by the formula:

Coeff B  = (Deviation on Magnetic East-Deviation on Magnetic West)/2

Example
When carrying out a compass swing on an aircraft, the following Deviations were determined:
Deviation on Magnetic East = + 3°
Deviation on Magnetic West = – 5°
Coefficient B   =  + 3° – (- 5°)/2 =  3° + 5°/2 =  4°

Coeff C = Coefficient C is the algebraic value of the deviation produced by component Q and its allied vertical “soft” iron, when the aircraft is heading Magnetic North. It is calculated by the following formula:

Coeff C = (Deviation on Magnetic North – Deviation on Magnetic South)/2 

 

Example

The following deviations were found while carrying out a compass swing on an aircraft:

 

Deviation on Magnetic North = – 2°
Deviation on Magnetic South = + 4°
Coefficient C   =   – 2° – ( +4°)/2 = – 3°

 

Coefficient A

An aircraft compass when installed heading correctly, has its lubber line (the mark which represents the aircraft’s heading and against which course is read) exactly in the fore-and-aft line passing through the compass position. If however, the lubber line is displaced, all courses read against it will have an error equal to the number of degrees by which it is out of alignment. For example, if the lubber line be displaced 3° to Starboard of the fore-and-aft line, all courses read will be 3° too great. If a course of 048° is indicated, the correct heading would be 045° and so on. The compass would, under these circumstances, have an apparent deviation, constant on all headings, of -3°.

Certain arrangements of horizontal “‘soft” iron can also produce a constant deviation indistinguishable from the mechanical kind referred to above. It is seldom of any appreciable value. Constant deviation, two causes of which are given above, is known as coefficient A. To eliminate coefficient A it is necessary merely to rotate the whole compass until the lubber line is correctly positioned. It is calculated by the following formula:

Coeff A = (Deviations on. Magnetic N + NE + E + SE + S+ SW + W +NW)/8

Example

The following deviations were found when swinging an aircraft.

 

Magnetic Heading Compass Deviation
N -1°
NE -3°
E -5°
SE -2°
S -0°
SW +1°
W +2°
NW -0°

 

 

Coefficient A   = -1° – 3° – 5° – 2° – 0 + 1° + 2° – 0°/8 =  – 8°/8  =   – 1°

 

Summary of deviations

  • Deviation due to p+c = b sin heading (m).
  • Deviation due to q+c = c cos heading (m).
  • Total deviation = a + b sin hdg + c cos hdg

 

Construction and function of a micro adjuster

      (a) Micro adjuster consists of two pairs of magnets, one pair lying fore & aft and the other athwart ships.

(b)  When neutral, the magnets in each pair are parallel, with opposite poles adjacent to each other.

Mechanics of correcting coeff ‘B’ & ‘C’

When coefficient ‘B’/ ‘C’ is to be corrected, the fore & aft  /  athwart ship  pair of magnets is opened scissor wise by inserting and turning a corrector key.  This introduces a neutralizing field.  The further the magnets are opened, the stronger the field becomes.

Compass swing procedure

The following procedure needs to be adopted in order to carry out the compass swing of the aircraft

  • Check compass for serviceability.
  • Ensure all eqpt not carried in flt removed.
  • Ensure all equipment carried in flt correctly stowed.
  • Take ac to suitable area, min 50 yds from other ac and min 100 yds from hangar.
  • Ensure all flying controls are in normal flying position, engines on, radios and electrical circuits on.
  • Place ac on headings (m), note deviation.
  • Place ac on hdg W (m), note deviation.
  • Place ac on hdg N (m), note deviation.
  • Calculate co-eff C and apply directly to compass reading and set required correct reading on grid ring.
  • Place key across the needle and turn the key until red is on red (turn anti-clockwise for + deviation).
  • Place ac on hdg E (m), note deviation.
  • Calculate co-eff B and correct as before (key placed in fore and aft position).
  • The correct swing is complete.
  • Place ac on all 8 headings and carry out a check swing, starting from SE.
  • Calculate co-eff A, loosen retaining screw on rear bracket of compass and turn instrument clockwise if A is +ive, by the quantity calculated. Retighten screw.
  • Calculate residual deviation; prepare a deviation card to be placed next to compass.

Occasions for compass swing

The occasions for carrying out the compass swing are as enumerated below

  • On acceptance of new ac from manufacturer.
  • When a new compass is fitted.
  • Every three months.
  • After a major inspection.
  • With any change of mag material in the ac.
  • If transfer to another base involving large change of latitude.
  • After a lightning strike.
  • When carrying magnetic freight.
  • Whenever specified in maintenance manual.
  • Following a heavy landing.
  • At any time when compass or recorded deviation is suspect.

 

Effect of change of latitude on deviation

Horizontal Hard Iron. Although by definition the deviating force is constant as measured, a change of the strength of the earth’s horizontal component h will cause the effect of this deviating force to vary. As h increases the effect of a constant horizontal hard iron deviating component will decrease and vice versa.

Deviation due to HHI = 1/h.

As value of h at any given place is known

Deviation at new latitude is found by

New deviation  / Old deviation   = old h / new h

Vertical Soft Iron. Deviation varies inversely as h. Additionally; the magnetism induced into vertical soft iron will vary as the dip. At poles since z is max deviation due VSI will be max and nil at the equator.

Deviation due to VSI = h/z

Since   h/z = tan dip, deviation due VSI = tan dip

New deviation/ Old deviation =  new tan dip/ old tan dip

Component r. This is vertical hard iron assumed for analysis to be situated above or below the compass. In level flt, its effects on the compass is nil, but tail down or up, the hypothetical magnet is resolved into parts, one of which into a fore & aft effect, as it were a false p.

Direct Indicating Compass System (DICS)

Aircraft Magnetism

Direct Indicating Compass

 

 

 

 

 

 

 

 

 

 

 

 

Introduction

A direct indicating compass system (DICS) consists of a freely suspended magnet system, which can align itself with the horizontal component of the Earth’s magnetic field thus defining the direction of Magnetic North. By aligning a compass card with the North-seeking (red) end of the magnet system as shown in Fig 1, the aircraft’s magnetic heading can be read off against a lubber line.

Properties

DICS must exhibit the following properties:

(a)   Horizontality.  The magnet system must remain as near horizontal as possible.

(b)    Sensitivity.  The magnet must be sensitive.

(c)  Aperiodicity.  The magnet’s behaviour must be aperiodic (i.e. without recurring oscillations).

Horizontality.

Achieved by incorporating pendulosity into the compass needle by pivoting it above the horizontal plane in which it lies. Up to 4 magnets may be used. Closely mounted together below the pivot. When tilted by earth’s vertical force (z) the cg moves out from below the pivot thereby setting up a righting force to come into play. Magnet will take up a resultant of the two equal and opposite forces. Final inclination reduced to 2-3 degs but increases with increase in z. In N hemisphere cg is S of the pivot and in the s hemisphere it is n of the pivot.

Sensitivity

Achieved by increasing pole strength of magnet to keep needle firmly fixed along magnetic meridians. Keeping pivot friction to the min by using an iridium pivot in sapphire cup. Suspending the whole system in liquid to reduce the effective wt of the sys and lubricate the pivot.

Sensitivity may be increased by the following methods:

(a)   Increasing the magnetic moment of the magnet system.

(b)   Reducing the moment of inertia of the magnet system.

( c)   Reducing the friction at the suspension point.

A compromise is reached between the magnetic moment and the moment of inertia requirements by using a number of small, light, powerful magnets as the magnetic sensing element of the compass.  Friction at the pivot is reduced by using jeweled bearings and also by suspending the magnet system in a fluid which reduces the weight acting on the pivot and lubricates the   bearing.

Properties of the Compass Liquid

The liquid in the E2B compass needs to have the following properties

(a)        Wide temperature range. + 50 to –50.

(b)        Low co-efficient of expansion. Generally about 12% over the range.

(c)        Low specific gravity.

(d)       Non corrosive.

(e)        Provision for expansion of liquid.

-expansion chamber.

-sylphon tube.

(f)        Good clearance between damping wires and the wall of the bowl to reduce swirl effect.

A practical DICS

The E2 Series

The principles of the DICS are exemplified in the E2 series of standby compasses which are widely used. The differences between the E2A, E2B and E2C are minor and mostly concern the lighting arrangements.  The compasses have a vertical card fastened to the magnet system, graduated every 10 degrees, with figures every 30 degrees.  The cardinal points are marked with the appropriate letter.  The compasses are designed to give an operational accuracy of ±10°; in good, stable flight conditions the accuracy may approach the bench accuracy of 2.5°.

The bowl of E2B is plastic with a lubber line marked on the front inside. The magnet is a steel ring to which a dome is attached.  The iridium tipped pivot screws into the centre of the dome and rests in a sapphire cup secured to the vertical stem by the cup holder.  The compass bowl is filled with a silicone fluid and a bellows at the rear of the bowl allows for a change of the volume of the liquid due to variations in temperature.  Provision is made for correction of coefficients A, B, and C.

The E-2 compass has the following features:-

  • Small, vertical reading more widely used.
  • Fitted between two pilots.
  • Correctors built in for co-eff A, B and C.
  • Transparent plastic bowl.
  • Movable luminous lubber line.
  • Magnet is domed steel circle.
  • silicone fluid in bowl
  • Sylphon tube at the rear for expansion.
  • Metal ring compass marked every 300 graduated every 100.
  • Suffers from turning and acceleration errors.

 

SERVICEABILITY CHECKS IN A COMPASS

Serviceability Checks. Before use the compass should be checked to ensure that the bowl is not cracked or damaged and is completely filled with fluid that is free from excessive discolouration, bubbles and sediment. Check liquid free from bubbles, discoloration or sediments.

  • Examine all parts for luminosity.
  • Ensure grid ring rotates freely 360 degs and locking device functions positively (in p type compass).
  • Test suspension of bowl by moving gently in all directions and that there is no metal to metal feeling.
  • Test pivot for friction by deflecting magnet 10-15 degs each way and note reading on return. Each should be within 2 degs of other.
  • Test for damping by deflecting system through 90 degs and time out its return through 85 degs. Max and min time laid down in maintenance manual, usually 6.5 to 8.5 secs.

 

In a direct reading compass, the magnet sys is made aperiodic by the use of:

  • The damping action of the compass liquid.
  • Magnets with a large magnetic moment.
  • A pendulous suspension.

 

The sensitivity of a direct reading compass varies:

  • Directly with z.
  • Inversely both with z and h.
  • Directly with h.

 

DICS – ERRORS AND LIMITATIONS

In addition to the errors caused by external magnetic fields, DICS are subject to the errors and limitations covered in the following paragraphs.

 

Turning and Acceleration Errors – Cause

If an aircraft fitted with a DICS is subjected to horizontal accelerations, the accelerating forces may cause errors in the indicated heading. The accelerations may be the result of speed changes or from the central acceleration experienced in a turn; both have similar effects on the compass system, the resultant errors being greatest when the accelerating force acts at right angles to the magnetic meridian with which the compass is aligned, i.e. when the aircraft changes speed on easterly or westerly headings, or turns through North or South.  The errors are caused by the displacement of the magnet system’s centre of gravity from the line through the pivot.  This displacement results in the formation of couples which rotate the magnet system and produce heading errors.

 

Maximum error  occurs  when  the accelerating  force  acts  at  90* to the  magnetic  meridian. this  happens while  increasing speed on  a  e/w  heading or  turning  from  north or south  on  to  e/w. No error while   accelerating on a   North / South   heading.

Displacement of c of g in the horizontal plane

Due to downward tilt of red pole in northern hemisphere, c of g shifts from the line through pivot towards blue pole.

Forces acting through the pivot & the centre of gravity

When in the northern hemisphere, ac accelerates   on   a    westerly heading or   turns through   north or south onto west. Accelerating force acts through the pivot and the reaction force acts through the c of g forming a couple which turns the magnet system anti-clockwise. The couple formed turns the magnet anticlockwise.

Effect on the vertical plane

             (a)        Because of the mechanical couple, magnet tilts out of the vertical.

(b)        ‘z’ no longer acts through pivot.  z cos q acts through pivot and z sin o at 90 deg to the pivot, q being the angle of tilt.

(c)        Component z sinq tends to pull the blue end of the magnet to the right.  this creates an equal but opposite effect at the red end forming a magnetic couple which turns the magnet                                  system anti-clockwise.

 

Causes of Acceleration Errors

If an aircraft fitted with a DICS is subjected to horizontal accelerations, the accelerating forces may cause errors in the indicated heading. The accelerations may be the result of speed changes or from the central acceleration experienced in a turn; both have similar effects on the compass system, the resultant errors being greatest when the accelerating force acts at right angles to the magnetic meridian with which the compass is aligned, i.e. when the aircraft changes speed on easterly or westerly headings, or turns through North or South.  The errors are caused by the displacement of the magnet system’s centre of gravity from the line through the pivot. Therefore the causes can be as mentioned below:-

  • Dip causing cg to be out of the line of the pivot
  • Reaction on the cg of acceleration/deceleration causing it to lag behind.
  • Counteracting effect of the cg against dip decreasing as cg, needle and the pivot are no longer in line with magnetic meridian (max effect).

 

SUMMARY OF ACCELERATION AND DECELERATION ERRORS

 

HDG SPEED NEEDLE          TURNS EFFECTS
 east increase clockwise apparent turn to N
west increase anti-clockwise apparent turn to N
east decrease anti- clockwise apparent turn to S
west decrease clockwise apparent turn to S

 

Note:

(a)        In southern hemisphere, the above errors are opposite.

(b)        No errors on northerly and southerly heading as the force acts along the needle.

(c)        They can occur in bumpy conditions.

(d)        No errors on the magnetic equator, as the pivot and the cg are coincident.

 

Causes of Turning Errors

The various causes which can attribute to the turning error are as mentioned below:-

  • Cg out of the vertical line of the pivot.
  • Centripetal of turn directed towards the centre of the pivot.
  • Centrifugal force however acts through the cg thereby pulling the cg of the needle outward in a turn.
  • Vertical force set up by the centrifugal force on the slightly tilted magnet n seeking pole. Also needle has left the magnetic meridian accentuates the fact (max effect).
  • A swirl effect adds to the effect in turn through north.

 

SUMMARY OF TURNING ERRORS

 

TURN NEEDLE EFFECT LIQUID   SWIRL   CORRN
through

north

same as

aircraft

under

indication

adds to

error

turn less than needle shows
through

south

opposite

to aircraft

over

indication

reduces

error

turn more than needle shows

 

Note:

(a)        In Southern hemisphere the above errors are opposite.

(b)        No errors in turn through e or w since forces act along needle.

( c)       Northerly turning error is greater than sly turning errors since liquid swirl is additive.

(d)       For accurate turns use directional gyro indicator.

(e)        All dynamic errors last only for the period of speed change or turns. Once constant speed or level flight is resumed compass needle finds N.

 

Similarity between turning & acceleration errors

 Error caused by effect

            (a)        Increase in speed apparent turn to north (Compass over-reads)

(b)        Turning                                    magnet turns through north  in the direction of turn, hence turn is under  indicated.

 

(c)        Turning                                   Magnet turns through south in the opposite direction to the turn, hence turn is over indicated

 

Summary of Turning & Acceleration Errors

The effects of turning and acceleration errors are summarized below:

(a)        Acceleration on westerly headings and turns to the west through ‘n/s’ cause the magnet system to rotate anti-clockwise.

(b)        Acceleration on easterly headings and turns to the east through ‘n/s’ cause the magnet system to rotate clockwise.

(c)        Acceleration in both cases causes an apparent turn to the north.

(d)       Turns through north causes the compass to under indicate the turn. turns through south causes the compass to over indicate the turn

(e)        In turns through north, liquid swirl adds to the error and in turns through south, liquid swirl reduces the error.  Hence northerly turning error is greater than southerly turning error.

 

Minor Errors

The following minor errors also occur:

(a)   Scale Error.  Scale error is caused by errors in the calibration of the compass card.

(b)   Alignment Error.  Alignment error is caused by the incorrect mounting of the compass in the aircraft, or by a displaced lubber-line.  The error is corrected by the compass swing.

(c)   Centring Error.  Centring error occurs when the compass card is not centred on the magnet system pivot.

(d)   Parallax Error.  When reading DICS care must be taken to ensure that the eye is centred on the face of the compass.  If the line of sight is offset parallax errors occur.

 

SUMMARY

Errors in Direct Indicating Compass System

  • Turning and acceleration errors
  • Scale errors
  • Alignment errors
  • Centering error
  • Parallax error
  • Errors due to ac magnetism

Turning and Acceleration Errors

  • Magnetic Dip causes the magnetic needle of compass to deviate from the horizontal by an angle of Φ. (tan Φ = Z/H).
  • The compass needle is pivoted away from C.G in order to offset the dip
  • This causes a couple to act on the magnetic needle, whenever turning/ acceleration occurs.
  • Acceleration and deceleration errors are basically false compass indication of a swing to the North or South during speed changes of ac.
  • The swing causes apparent turn towards North or South
  • This error is more pronounced when flying on Easterly or Westerly Heading and it decreases when flying on Northerly or Southerly Heading.
  • In a direct N/S heading this error does not exist.
  • In the Northern hemisphere the compass swings towards the North during Acceleration (Apparent turn to North). and towards the south During deceleration (Apparent Turn to south).
  • Implies, acceleration on easterly heading compass needle turns clockwise and shows a turn to north in northern hemisphere, on westerly heading it will turn anticlockwise. Opposite happens during deceleration. (ANDS- On easterly/westerly headings, Acceleration shows turn to north and deceleration shows turn to south).

Origin of Turning Error

  • When the ac is in a banked turn, the card also banks because of centrifugal force. In this attitude the vertical component of the earth’s magnetic field causes the compass to dip to the low side of turn.
  • This compass turning error is most apparent when turning through heading close to North/South
  • When the ac initiates a turn from Northerly heading, the compass briefly indicates a turn in opposite direction
  • When the ac initiates a turn from Southerly direction, then the compass turns in the correct direction but at a slightly faster rate.
  • So when an ac initiates a 360º turn starting at N, the compass card initially shows a wrong turn, then by easterly/westerly heading the compass card catches up with actual heading, passing through South the card starts to lead the turn considerably, as it again passes through Easterly/ westerly heading , the card more or less shows correct heading, and when ac again reaches North the compass again lags.
  • In Short..In a turn through North —-  Compass Lags
  • In a turn through South —- Compass leads
  • SONU ( Southerly — overread, Northerly — Under read)

Scale Error   Due to incorrect calibration of compass card

Alignment Error   Due to incorrect mounting of compass or due to displaced lubber line

Centering Error   When the card is not centered on pivot

Parallax Error   Due to offset line of sight while reading the compass

Liquid Swirl   Rotation of the magnet due to the swirl effect of the liquid in which it is placed.

Limitations of DICS

  • Sensitivity depends upon value of H.
  • It is insensitive and unreliable at higher Latitudes
  • Suitable only for straight and unaccelerated flights due to significant amount of turning and acceleration errors.
  • It cannot feed heading to other equipment

REMOTE INDICATING COMPASS SYSTEM

Remote Indicating Compass

Remote Indicating Compass

 

 

 

 

 

 

 

 

 

 

 

 

 

 Disadvantages of a DIC

(a)        In the cockpit the deviating effects are large.

(b)        The pendulous suspension leads to errors due to accelerations

(c)        Reduction in ‘h’ with increase in latitude renders dic unreliable in high      latitudes.

(d)       Insufficient torque for driving repeater indicators. Separate compass must             be provided for each crew member.

 

Gyroscope as a heading reference: A gyro is unaffected by changing magnetic fields (turning error) or by accelerations (acceleration error) but its heading indications may be inaccurate due to the effect of precessional forces caused by friction, incorrect balance etc.

 

Flux valve as a heading reference: The flux valve is pendulously suspended and hence is affected by accelerations.

 

Principle of gyro magnetic compass: Integrate the heading indication of magnetic compass with directional properties of a gyroscope so that   the net result is to reduce individual errors of each.

 

The working of a gyro magnetic compass An azimuth gyro is initially referenced to magnetic meridian and thereafter precessional forces are applied to the gyro based on azimuth information from the flux valve.

 

Advantages of a remote indicating compass

 The RIC has following advantages as compared to DICS.

(a)  Does not suffer from slow drift. Therefore no requirement to synchronise regularly.

(b)  Magnetic compass reliable only in straight & level flight. RIC does suffer from these requirements.

(c)  Detector can be installed in a remote corner of the ac away from magnetic influences, more accuracy. (DGI synchronisation from compass is also inaccurate).

(d) Flux valve ‘senses’ and not ‘seeks’ the magnetic meridian. Therefore no turning errors.

(e)  Compass can be detached from detector by a switch.

(f)  Can be used as a DGI when flying in the vicinity of the poles.

(g)        Can be used to monitor other equipment like auto pilot, RMI, Doppler        etc…

(h)        Enables repeaters to be provided for each crew position.

(j)         True hdg can be obtained by setting variation through variation control.

 

Dis-advantages of a remote indicating compass

(a)        Heavier than a direct reading compass.

(b)        More expensive.

(c)        Requires electrical supply to operate.

(d)       More complicated.

(e)       More expensive and difficult to maintain.

 

 

 

 

Leave a comment

Your email address will not be published. Required fields are marked *