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Anonymous early Spitfire Mk. I on compass swing, location unknown. This view shows to advantage the dinstinctive features of the earliest production Spitfires Mk. I: wooden two-blade propeller, unarmoured windscreen, straight cockpit canopy, thin and tall aerial mast and (barely visible) rudder horn balance guard.
Compass swinging was a rather time-consuming task which could be simplified considerably by placing an aircraft on a rotating platform such as this. With a fitter sitting in the cockpit and the aircraft in flight-ready configuration, the engine was started and then the platform aligned so that the aircraft faced the 0 degree (north) heading. Then the fitter would check if the aircraft magnetic compass was in alignment with the magnetic north. If not, he would adjust the compensator screws with a non-magnetic screwdriver until the compass read 0 degrees. Then the procedure would be repeated for the 90-degree (east), 180 (south) and 270-degree (west) headings.
After these adjustments the compass was checked once again by turning it around stopping at each 30-degree heading and recording the compass readings, fine-tuning the compensator screws to ensure that there was no more than a few degrees difference between any of the indicated headings on the compass and the actual heading.
[Crown Copyright, via Jenny Scott]