A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2015 Nov 20, 09:59 -0800
Drift Check for Landfall
Chichester may have independently evolved the deliberate offset or "Landfall" method of finding an island. Taking his 6 hour flight from Norfolk to Lord Howe as an example, he deliberately aimed 90 miles right of destination, to be sure to know which way to turn for the Landfall sun PL run in. He deliberately tried to fly a track about 10 degrees right of the direct line to the island. This required some CN check on the outbound track.
Chichester assessed drift on 3 headings, to find wind, and flew a drift corrected heading, and amended ETA. The Dalton computer had not then been invented. His own method required plotting on his roller knee pad map, and was most successful, even though he had not swung his compass, and occasionally inadvertently went into cloud because of the head-down plotting. He did not have a turn needle to permit flight in cloud. Strangely, he does not mention surface sea state, an easy way for yachtsmen to assess the Beaufort scale. I did not try to do this in my Stearman.
After about 2 hours during his Norfolk to Lord Howe crossing, the sun was near the beam, well placed for a track check. Having taken off late due to a leaking float, he had to recalculate the sun shot in the air using a Bygrave as described before.
Although he took a conventional shots using 2 hands, and got into some trouble, it might have been done more easily. If the pre-set sextant is viewed at the right time, as described before, the intercept can be seen immediately, using the sun as a scale, and track correction made immediately ,without plotting. It would be nice if the sextant showed the sun lower limb on the horizon, but that never happens! Pilots are taught or learn the "1 in 60 "rule, and mental DR techniques. Chichester had already flown from London to Sydney.
Suppose the sextant had shown the upper limb just on the horizon, instead of the lower. The aircraft is thus too far from the sun by 32 miles , left of track, the dangerous way! After about 180 miles of flight, the drift angle is 32/3 or about 10 degrees too much left. Immediately turn 20 degrees right from the previous heading, and track should be regained in 2 hours. No plotting.
Purists may find the above somewhat slipshod, but it caters for unknown compass and height errors, and it works. The aim is merely to avoid going more than 90 miles left of track, and not too far right either because of fuel. Somewhere in between. Not too onerous. Continue assessing wind, and do it again after another 2 hours. The viz and cloud cover have to cooperate.
Next, Chichester had bigger fish to fry, namely flying a seaplane solo, and planning the all important sun PL to run in to the island.