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Questions on Compass and its Errors

 
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K.Haroon
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PostPosted: Fri Jun 15, 2012 2:03 pm    Post subject: Questions on Compass and its Errors Reply with quote

Turning Errors:

When turning through the nearer pole (ie, turning through North in the northern hemisphere or through South in the Southern Hemisphere):

The compass is SLUGGISH.
The magnetic assembly and the aircraft are turning in the same direction
The compass will under-indicate the amount of turn.
The pilot should undershoot the turn/roll out early.
Liquid swirl will increase the turning error.

When turning through the further pole (ie, turning through South in the northern hemisphere or through North in the Southern Hemisphere):

The compass is LIVELY.
The magnetic assembly and the aircraft are turning in the opposite direction
The compass will over-indicate the amount of turn.
The pilot should overshoot the turn/roll out late.
Liquid swirl will decrease the turning error.

Source: (http://www.pprune.org/tech-log/161602-acceleration-turning-errors.html)

One of the errors inherent in a magnetic compass in which the compass lags behind the actual turn when turning through the north and leads when turning through the south in the Northern Hemisphere. The error is caused by the difference in the location of the pivot point and the center of gravity of the compass magnet system. The center of gravity of the magnet is placed below, but some radial distance from, the pivot. The acceleration forces in the turn—the centripetal and centrifugal forces—set up a couple in the vertical plane, which imparts a sideways tilt to the magnet system about the pivot. This is the main reason for the turning error. There are two other reasons for this error: first, the displacement of the center of gravity behind the pivot, and, second, the effect of liquid swirl. In the latter case, the liquid tends to rotate with the bowl and to drag the magnet system around with it. This increases the error in turns through the north in the Northern Hemisphere because the turning error itself causes the magnet to move in the opposite direction to the turn. This effect will be reduced in turns through the south.


Acceleration Errors:

Acceleration on E or W in either hemisphere always gives an APPARENT turn towards the nearer pole.

Source: (http://www.pprune.org/tech-log/161602-acceleration-turning-errors.html)

For Details on Comnpass Errors: http://www.theairlinepilots.com/forum/viewtopic.php?t=916


Arrow When turning right from 330o (C) to 040o (C) in the northern hemisphere, the reading of a direct reading magnetic compass will:

a) over-indicate the turn and liquid swirl will decrease the effect
b) under-indicate the turn and liquid swirl will increase the effect <-- Correct
c) under-indicate the turn and liquid swirl will decrease the effect
d) over-indicate the turn and liquid swirl will increase the effect


Arrow When accelerating on a westerly heading in the northern hemisphere, the compass card of a direct reading magnetic compass will turn:

a) clockwise giving an apparent turn towards the north
b) clockwise giving an apparent turn towards the south
c) anti-clockwise giving an apparent turn towards the north <-- Correct
d) anti-clockwise giving an apparent turn towards the south




Arrow When decelerating on a westerly heading in the Northern Hemisphere, the compass card of a direct reading magnetic compass will turn:

a) clockwise giving an apparent turn toward the south <-- Correct
b) anti-clockwise giving an apparent turn towards the south
c) clockwise giving an apparent turn towards the north
d) anti-clockwise giving an apparent turn towards the north


Arrow An aircraft is accelerating on a westerly heading in the Northern Hemisphere. The effect on a Direct Reading Magnetic Compass is:

a) Underreads North
b) Underreads South
c) Overreads North <-- Correct
d) Overreads South


Arrow You are in the Northern hemisphere, heading 135C on a Direct Reading Magnetic Compass. You turn right in a Rate 1 turn for 30 seconds. Do you roll out on an indicated heading of:

a) greater than 225 <-- Correct
b) less than 225
c) equal to 225
d) not possible to determine

Rate 1 turn is 3 deg per second. 30x3=90. 135+90=225. The compass will over-indicate.


Arrow At the magnetic equator, when accelerating after take off on heading West, a direct reading compass:

a) underreads the heading
b) overreads the heading
c) indicates the correct heading <-- Correct
d) indicates a turn to the south

The compass is pendulous and symmetrical and will sit level if there is no magnetic field or if the field is horizontal with no Z component, as on the magnetic equator. Acceleration and turning errors come into play when the card is out of horizontal with its CG no longer below the pivot point. Source: (http://www.atpforum.eu/showthread.php?t=11903)


Arrow Which of the following statements is correct concerning the effect of turning errors on a direct reading compass? "Turning errors are greatest on north/south headings, and are greatest at high latitudes" (due to high Z component)


Arrow When should a DRC be 'swung'?

a) Every 6 months
b) Following a change of magnetic latitude <-- Correct
c) For night use
d) After flying in an area where lightning is visible

DRC = direct reading compass. (d) is not valid because lightning has'nt struck.


Arrow A direct reading compass should be swung when:

1) There is a large, and permanent, change in magnetic latitude
2) After an aircraft has passed through a severe electrical storm, or has been struck by lightning
3) If the aircraft has been subjected to hammering


Arrow The purpose of compass check swing is to:

a) cancel out the horizontal component of the earth's magnetic field
b) cancel out the vertical component of the earth's magnetic field
c) measure the angle between Magnetic North and Compass North <-- Correct
d) cancel out the effects of the magnetic fields found on board the aeroplane


Arrow One purpose of a compass calibration is to reduce the difference, if any, between:

a) compass north and magnetic north <-- Correct
b) compass north and the lubber line


Arrow The direct reading magnetic compass is made aperiodic (dead beat) by:

a) using the lowest acceptable viscosity compass liquid
b) keeping the magnetic assembly mass close to the compass point and by using damping wires <-- Correct
c) using long magnets
d) pendulous suspension of the magnetic assembly


Arrow The sensitivity of a direct reading magnetic compass is

a) inversely proportional to the horizontal component of the earth's magnetic field
b) proportional to the horizontal component of the earth's magnetic field <-- Correct
c) inversely proportional to the vertical component of the earth's magnetic field
d) inversely proportional to the vertical and horizontal components of the earth's magnetic field


Arrow In a remote indicating compass system the amount of deviation caused by aircraft magnetism and electrical circuits may be minimised by:

a) positioning the master unit in the centre of the aircraft
b) the use of repeater cards
c) mounting the detector unit in the wingtip <-- Correct
d) using a vertically mounted gyroscope


Arrow Why are the detector units of slaved gyro compasses usually located in the aircraft wingtips? "To isolate the detector unit from the aircraft deviation sources".


Arrow The main reason for usually mounting the detector unit of a remote indicating compass in the wingtip of an aeroplane is to:

a) facilitate easy maintenance of the unit and increase its exposure to the Earth's magnetic field
b) reduce the amount of deviation caused by aircraft magnetism and electrical circuits <-- Correct
c) place it is a position where there is no electrical wiring to cause deviation errors
d) place it where it will not be subjected to electrical or magnetic interference from the aircraft


Arrow The main advantage of a remote indicating compass over a direct reading compass is that it:

a) is able to magnify the earth's magnetic field in order to attain greater accuracy
b) has less moving parts
c) requires less maintenance
d) senses, rather than seeks, the magnetic meridian <-- Correct


Arrow The annunciator of a remote indicating compass system is used when:

a) synchronising the magnetic and gyro compass elements <-- Correct
b) compensating for deviation
c) setting local magnetic variation
d) setting the heading pointer
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