A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2015 Nov 24, 05:29 -0800
4. Landfall Procedure
Deliberately flying his unstable biplane 90 miles right of the direct track to Lord Howe, Chichester refined his ETA at the turn-in point. Having taken off late, because of a leaking float, he recalculated the azimuth and altitude of the sun, as seen at the island at that same ETA time. A Bygrave rule can use real coordinates obviating assumed positions and plotting. His aim was to reach the same sextant altitude in his Moth, at the same time.
Chichester frequently mentioned adjusting sextant readings for time variations, without explaining how. This sounds like using Motion of Body (MOB) tables, or the equivalent. These may have been some of the correction tables he pasted to his cockpit sides. I pasted excerpts of MOB tables to my Bygrave, to replicate this. Aircraft cannot stop, but they can do orbits to lose time. It would be a good idea to be early, or, put another way, calculate a time a few minutes late. Head-down time and plotting must be minimised, especially low over a featureless sea.
Having pre-set the sextant to the angle expected at Lord Howe, and sighting early, he would have seen the sun too high in his sextant. Using the MOB tables and the azimuth given by the Bygrave, he would note that the afternoon sun , almost directly ahead , was descending at about 10" per minute. His ground speed of about 120mph would cause the sun to rise at 2" per minute, a combination of 8" down per minute. Mental DR again. As an example, if the sun was 1 diameter above the horizon in his sextant, but ETA was 5 minutes to go, he was 1 minute early. A gentle 25 degree bank turn would lose 1 minute.
If the sextant angle and time did not come together, he would turn on time, 90 degrees left of the azimuth +/- drift, and note the sextant error which would equal the new cross track error. No plotting is required here.
He did not know his time to destination, which could be any time between 0 and 2 hours, but most likely about 1 hour. If the viz was good, Lord Howe hills should have shown up from 500ft at about 40 miles. He actually had time to complete another Bygrave calculation.
In the event, afternoon orographic cloud formed, and this produced rain or poor viz. This might have been foreseen. Nevertheless, Chichester's CN technique was perfectly good, and he nearly flew straight into the islands on both Landfall legs.
Many of the alarming CN situations he got into in flight might have been reduced had he fitted a turn needle. Notwithstanding the difficulties of operating a solo biplane, clearly his unique performance was audacious and astounding, especially given the lack of preparation.