Hi Keith,
This is quite a long post, but I hope it helps. I've done this of the top of my head without the manuals, so if i've left something out or got something wrong can someone please correct.
Nigel
The process is called "Swinging the Compass"
1. Find a flat, hard surface area, large enough to place the Chipmunk and rotate it through 360°. The area must be well away from steel objects such as hangers and lampposts. Beware also of underground steel pipes. Some airfields have compass swinging platforms installed to do this.
2. Using a compass that you know is correct mark a line North-South across the middle of the area. Magnetic direction of course.
3. Mark another line East-West at 90 ° near the middle of the first line.
4. Position the Chipmunk with the nose pointing North and the fuselage centre line aligned with your North-South line. Raise it to the line of flight position and chock it.
5. Assuming you have P11 compasses, set the compass bezel with North aligned to the Aircraft centre line and lock it.
6. There is a small plate in the aircraft which says the compasses should be swung with the radio switched on and the flaps down, so switch on the radio and lower the flaps. The old 28 pound valve radio probably had more magnetic impact than modern radios but do as it says nevertheless. Switch on the transponder as well if fitted. GPS?
7. Allow the compasses to settle and record the direction in degrees indicated by the needles.
8. Rotate the aeroplane 90° and point the nose to the East. Realign the fuselage centre line with the East-West line. Maintain or restore the line of flight position and chock the aeroplane.
9. Allow the compasses to settle and record the direction in degrees indicated by the needles.
10. Rotate the aeroplane 90° and point the nose to the South. Realign the fuselage centre line with the South–North line. Maintain or restore the line of flight position and chock the aeroplane.
11. Allow the compasses to settle and record the direction in degrees indicated by the needles.
12. Rotate the aeroplane 90° and point the nose to the West. Realign the fuselage centre line with the West-East line. Maintain or restore the line of flight position and chock the aeroplane.
13. Allow the compasses to settle and record the direction in degrees indicated by the needles.
14. Rotate the aeroplane 90° and point the nose to the North once again. Realign the fuselage centre line with the North-South line. Maintain or restore the line of flight position and chock the aeroplane.
15. Allow the compasses to settle and record the direction in degrees indicated by the needles.
16. Compare with your first set of numbers to see if you are accurate.
17. You should now have a table of numbers something like this:
N E S W N mean
Degrees 0 90 180 270 0
Front cockpit 15 103 194 281 14
Difference +15 +13 +14 +11 +14 +13.4
Rear Cockpit 355 84 176 263 355
Difference -5 -6 -4 -7 -5 -5.4
The difference is the number that is know as the deviation.
Under each compass on the compass mounting platform is a compass adjuster box. This requires a special key to rotate the adjusting magnets inside. If you do not have P11 compasses, or similar, such as the P8, you will need to read the compass documentation to find the deviation adjustment method.
Rotate each adjuster to minimise the deviation. For example, for the above numbers try by setting North to +2 degrees in the front cockpit and +1 degree in the rear cockpit.
Repeat actions 4 to 17 and create a new table.
If you are satisfied stop there and record the deviations on new deviation cards and place them in the front a rear cockpits. Mark the date, aircraft serial, the compass serial and front or rear on the card. The back of the card will do.
If you think you can do better readjust and repeat 4 to 17.
Note you are unlikely to have a constant difference at each point of the compass, just look at the correction cards in different aircraft. You may also get slight differences creeping in as you adjust so that the change in deviation is more in some directions than in others. The object is to get the mean as low as possible and the deviations evenly spread around, nor to get North to Zero and forget the rest!
This procedure should be repeated if either of the compasses is changed or the radio is changed or if there is any major work that might change the magnetic field generated by the aircraft.
