12.40.2. Outline the basic principle of operation of the vacuum system.

The rotors of the gyroscopes in aircraft insturments are driven by air passing through a vacuum system or elecrically
Main component is the engine-driven vacuum pump that sucks air from within the cases of the gyroscopic instruments
Replacement air is drawn through a fine mesh filter and the enters the cases of the instruments through nozzles which direct a high speed flow of air at the grooves cut into the periphery of the gyro rotators
If suctin drops below 4.5Hg the the gyro speed will be reduced and the instruments can slow down become sluggish and behave erratically or stop functioning

12.38. Magnetic Instruments

12.38.10. State the compass pre-flight serviceability checks, and the precautions whencarrying magnetic items in an aircraft. Sub Topic Syllabus Item 

Check the compass is securely installed and can be easily read
Check the liquid is free of bubbles and is not discoloured
Check the glass is not broken, cracked or discoloured
Check compass is horizontal in its mounting
Check location of the compass deviatoin card in the cockpit
Check that compass indication is apporximately correct – with the alignment of the runway and that when taxiing out turn aircraft to L and R to check operation of the turn co-ordinator
If carrying electrical or metal goods check they are located as far away as possible from the compass

12.38.8(b) . turning error. 

In a turn the plane of rotation of the magnet system and compass card become tilted with respect to the horizontal
When the aircraft turns through N or S headings a component of dip now acts in the plane of rotation
This causes the compass to turn away from its proper orientation
When turning through E or W dip acts directly toward or away from the pivot and there is no turning effect on the system

In medium bank turns turning error is significant – of the order of 30 degrees

12.38.8(a) . acceleration error; 

Aircraft accelerates on westerly heading
CG of the magnet system will lie slightly to the N of the point of suspension
During acceleration the inertia causes the CG to lag behind and it’s offest from the point of suspension sets up a turning moment which rotates the compass system
If viewed from above this rotation will be clockwise

When accelerating on magnetic N or S – no error
When accelerating on E or W headings the compass shows an apparent turn to the S
When decelerating on E or W headings compass shows an apparent turn to the N

12.38.8. State the effects of:

12.38.6(c) . define residual dip. 

A simple bar magnet suspended from its centre of gravity will align itself horizontally with the earths lines of magnetic force
Also will take up another angle in the vertical plane
This is the magnetic Dip
The Dip happens because the centres of attraction for the earths magnetism at the poles lie deep below the surfce.
The lines of magnetic force do not parallel the earths surface
Dip is greatest at or near 90 degrees above the N and S poles and zero at or near the equator

Compensated for by being constructed with its centre of gravity (CG) well below its pivot point
In S Hemisphere the CG of the compass system is always offset slightly to the N of the pivot point
Residual dip ………

12.38.6(b) . state how it is compensated for; 

A simple bar magnet suspended from its centre of gravity will align itself horizontally with the earths lines of magnetic force
Also will take up another angle in the vertical plane
This is the magnetic Dip
The Dip happens because the centres of attraction for the earths magnetism at the poles lie deep below the surfce.
The lines of magnetic force do not parallel the earths surface
Dip is greatest at or near 90 degrees above the N and S poles and zero at or near the equator

Compensated for by being constructed with its centre of gravity (CG) well below its pivot point
In S Hemisphere the CG of the compass system is always offset slightly to the N of the pivot point
Residual dip ………

12.38.6(a) . describe what it is; 

A simple bar magnet suspended from its centre of gravity will align itself horizontally with the earths lines of magnetic force
Also will take up another angle in the vertical plane
This is the magnetic Dip
The Dip happens because the centres of attraction for the earths magnetism at the poles lie deep below the surfce.
The lines of magnetic force do not parallel the earths surface
Dip is greatest at or near 90 degrees above the N and S poles and zero at or near the equator

Compensated for by being constructed with its centre of gravity (CG) well below its pivot point
In S Hemisphere the CG of the compass system is always offset slightly to the N of the pivot point
Residual dip ………

12.38.6. With respect to magnetic dip: