Air Navigation Revision Booklet
Chapter 4 Compasses
The directional force that makes a compass needle point north and south is
weaker at the poles due to the magnetic force operating vertically at the poles.
This means that at angles of 75 degrees and above away from the equator
the magnetic force is so weak in the horizontal plane that magnetic
compasses are useless.
The Direct Indicating Compass (DIC) is the simplest compass, it is a ball that
floats in liquid to dampen its movements.
The limitations of the DIC are, they affect the accuracy of the DIC:
- Correct reading only given in straight and level unaccelerated flight
- It is affected by the magnetic fields or the metal/electrical equipment
contained within the aircraft.
- The DIC does not have enough power to repeat magnetic bearings to
elsewhere in the aircraft
- The DIC only indicates magnetic headings
- The DIC is unreliable at 70 degrees North and South and above
due to the weak magnetic force, the compass will dip toward the poles giving
an inaccurate reading
Advantages of the DIC are:
- Simple and reliable
- Cheap and lightweight
- Does not require power so will work in a power failure
To overcome the limitations of the DIC the Gyro Magnetic Compass was
invented (GMC). It is made up of
- A magnetic detector (flux valve) which electronically senses the earth’s
magnetic field.
- A turn/accelerator cut out switch
- A gyroscope which continues to point to one location regardless of what the
aircraft does
- Various compass repeaters
The GMC works by using a flux valve to point to magnetic north in straight
and level flight. In a turn the flux valve is unreliable so the gyroscope takes
over keeping the GMC pointing towards magnetic north. A GMC has more
torque so it can be used to repeat the heading to other displays.
A GMC can have real errors (manufacture errors) or apparent errors.
Apparent errors are when the gyro appears to be in error due to the rotation of
the earth.
Over time a gyro can become inaccurate this is called gyro wander
Inertial navigation system (INS) removes the problems with magnetic variation
(the difference between magnetic north and true north). The INS uses
accelerometers to detect change in position obtaining speed from acceleration
and distance from speed. So long as the start point is accurately known your
location at any position will be known.
Chapter 4 Compasses - Revision questions
1. Which of the following statements is true concerning the direct indicating
compass (DIC).
a. The DIC needs only a small power supply
b. The DIC is not affected by turns and accelerations
c. The DIC only reads magnetic headings
d. The DIC gives a reading of aircraft true heading
2. The RAF aircraft are equipped with a Direct Indicating Compass (DIC).
Why is this
a. The DIC gives a reading of true heading
b. The DIC is the most accurate compass system available
c. The DIC is not affected by turns and accelerations
d. The DIC is reliable and needs no power supply
3. Which of the following statements is true about the gyro-magnetic compass
is true:
a. The gyro-magnetic compass is less accurate that the Direct Indicating
Compass
b. The flux-valve controls the speed of the gyroscope
c. When the aircraft climbs or descends , the flus valve takes over from the
gyroscope
d. The gyroscope takes over from the flux valve whenever the aircraft turns
4. When would a Direct Indicating Compass be the most accurate?
a. In a steady descent
b. In unaccelerated flight
c. In a turn
d. In a steady climb
5. Which of the following is no a component within a gyro-magnetic system
a. A gyroscope
b. A flux valve magnetic detector
c. A suspended magnet
d. A turn/acceleration cut out switch
6. A gyroscope cannot be perfect and so over a period of time it becomes
inaccurate this is called
a. Gyro rigidity
b. Gyro wander
c. Turn/acceleration error
d. Variation
7. Which of the following is one advantage of a gyro-magnetic compass over
a direct indicating compass
a. A gyro-magnetic compass is cheaper
b. A gyro-magnetic compass does not work during turns and accelerations
c. A gyro-magnetic compass requires no electricity
d. A gyro-magnetic compass can feed compass repeaters around the aircraft
8. Where are variation values at their greatest
a. At the equator
b. In the polar regions
c. In the southern hemisphere
d. In the northern hemisphere
9. As a compass nears the magnetic north pole, the compass detector will try
to point at magnetic material inside the earth. This titling is called
a. Dip
b. Drap
c. Wander
d. Variation
10. Within an inertial navigation system the movement of the aircraft is
measured by sensors called
a. Inertials
b. Accelerators
c. Axis
d. Accelerometers
11. What principle does and inertial navigation system use, to calculate the
position of the aircraft.
a. The navigator must update the inertial navigation system all the time
b. it uses compass heading and doppler values to compute aircraft position
c. It is set accurately on the ground and then measures the accelerations in
the fore, aft and lateral
d. A gyroscope feeds position to the computer