Magnetic Compass And RMI Flashcards
Describe the features of the earth’s magnetic field (flux).
Lines of magnetic force of the earth’s magnetic field are at right angles to the earth’s surface, and about halfway between the magnetic poles the lines are parallel to the earths surface.
They are formed from the magnetic poles located at the magnetic north and South Pole.
Explain magnetic variation.
The angular difference between the direction of true north and magnetic north at any given point on earth.
Expressed as east or west - variation east magnetic least, variation best, magnetic best.
Describe the angle of dip and components H and Z of the earth’s magnetic flux.
The angle at which the needle of a compass slants down due to it trying to align itself with the local line of magnetic force.
Angle of dip has two components H and Z. One parallel to the earths surface, component H, and the other at right angles to the earths surface, component Z.
Close to the poles the greater Z is and the smaller H is. The closer to the equator the greater H is and smaller Z is.
Describe the basic features of a typical aircraft direct-reading magnetic compass.
- a magnet with attached compass card.
- magnet system is suspended from a pivot resting in a cup supported by a holder or stem.
- magnet system is housed in a bowl filled with a suitable fluid (silicone/kerosene)
Explain the reason(s) for:
(a) pendulous suspension of the magnet system;
(b) immersing the magnet system in fluid.
(A) held in pendulous suspension to minimise the the effects of angle of dip
(B) damping - to ensure that the action of the compass system is dead bat and that after its been deflected it’ll return to equilibrium without overshooting
Buoyancy - so that the amount of weight that’s supported by the pivot is reduced
Lubrication - decreases the effect of friction, resulting in less wear.
With reference to a direct-reading compass, describe:
(a) acceleration errors;
(b) turning errors;
(c) the practical aspects of these errors.
(A) caused through residual dip and the fact that the CoG of the compass is offset to the north of the pivot axis. Greatest on compass east and west, nil on north and south. In Southern Hemisphere, acceleration produces an apparent turn to the south, while deceleration to the north.
(B) caused by the vertical component of earths magnetic field acting in the plane of rotation of the magnet system whenever that plane becomes tilted from horizontal.
greatest through the magnetic headings of north and south, reducing to nil through
east and west. In turns through northerly headings,
the compass indication leads (goes ahead of) the
actual heading. In turns through southerly headings
the compass indication lags behind the actual
heading
(C) acceleration error - South Accelerate, North Decelerate
Turning error - Overshoot north, undershoot south
Explain magnetic deviation and how it is compensated for in a direct-reading compass.
All aircraft have their own magnetic fields which affect the reading of the compass
- Magnetic components cause the compass reading to deviate from earths magnetic field resulting in differing heading indications on compass
- Compass swing compensates for most of the deviation
Explain what a compass swing is and the occasions on which it is required (7)
Checking to see if the compass is aligned with an already known heading.
Required when :
upon installation in the aircraft
• immediately after an engine change
• after major overhaul or major repair
• after installation/removal/transfer of new or modified electrical equipment
• whenever unacceptably high deviation is encountered
• after the aircraft has been stationary for a long period of time
• when required by regulatory rules (for example, regulations demand that the compass
is swung following a lightning strike)
Explain the serviceability checks for a direct-reading compass. (6)
- Pre-flight: Secure in mounting
- No leaks or bubbles in compass
- Deviation card accessible
- Static check – heading reflects aircraft approximate heading
- Taxi checks – confirm heading orientation
- Align aircraft heading with runway orientation (use airfield plate)
Outline the service limits and tolerances for a direct-reading compass.
Explain the synchro system:
Made of a transmitter and receiver. Both have stator coils 120º apart and in the centre a rotor coil. Able to turn within the stators and is connected to an AC supply.
Due to the AC supply in the rotor coil, the stator coils are induced with alternating flux which varies depending on direction of the rotor coil.
If both rotor coils are in same direction, both electromagnetic induction in the receiver and transmitter are equal and opposite so no current flow.
If rotors are at different angels, magnetic induction in the transmitter will be different to magnetic induction in the receiver.
Difference in induction will create a current between transmitter and receiver and cause receiver rotor to move back into same position as the transmitter’s rotor causing the two systems to sync.
Explain flux valve.
Flux valve sense earths magnetic flux lines which give direction information to the RMI as the flux valve knows which direction its facing based on the voltage given.
E.g when facing west, 90 volts might be felt. South 40 volts.
Operates like a synchro system. In the centre is an exciter coil which is charged with an AC current that pretty much when it is ‘minus’ it draws in the magnetic flux from the earth - the earths magnetic flux then gives a voltage to the flux valve.
Describe the relationship between the flux calve and slaving synchro to create a synchro system.
Flux valve is synched/connected by wires to a slaving synchro. The slaving synchro feels the same voltage.
The rotor inside the slaving is connected to the DGU and should always be right angles to earths magnetic flux.
If the rotor is no longer at a right angle to the magnetic flux, an error signal is created. This error signal is amplified, which then powers the slaving motor to then bring the DGU and rotor back to the correct alignment with the earths magnetic flux.
Construction of DGU What does the Directional Gyro Unit do.
Electrically powered gyro with a horizontal axis of rotation kept in the horizontal plane by torque motors and levelling switches.
Mechanically connected to the rotor in slaving synchro. Both hold their synchronised alignment in space as aircraft turns about them.
If DGU drifts off its alignment( meaning the slaving synchro’s rotor will no longer be at right angles to earths magnetic flux) error signal is created and using precession, DGU is brought back to correct alignment.
What is the relationship between the DGU, Flux valve and slaving synchro.
corrected directional information given from the flux valve and slaving synchro to the DGU, is then mechanically sent to the remote indicating compasses synchro system.
The directional information is then displayed on the instrument face for the pilot.
