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Author Topic: Drift in the Dark part 6  (Read 3736 times)

Colin Taylor

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Drift in the Dark part 6
« on: March 21, 2023, 04:46:30 AM »

Drift in the Dark
Part 6 of 8

What were the options for the final approach to Howland?

1.  If they failed to observe the sunrise and calculate a sun-line, the only option would be to continue with the DR heading and timing and then do an expanding square search. There would be no reason to mention the sun-line on the radio. Since Earhart did refer to the sun-line we know they did get a sun-line. With a sun-line there are three options:

2. Offset track North: a 20 degree left offset over a distance of 200 NM will give a 66 NM offset and, on timing, a turn to the right onto a Southerly heading, correcting for drift on the sun-line track. However, I think they were already 170 NM North of Howland and so, if they had committed to this, the offset would have placed them about 236 NM N of Howland with three hours of fuel remaining and an unknown (but we know two hour) slog South.

 The radio calls to be expected will be ‘200 out’, ‘100 North-West’, ‘On line 157/337 going South’, ‘On you’. In that order. That is not what was received by Itasca.

3. Offset track South: If they had got a second fix en-route, confirming they were on track, then they could turn right 20 degrees and fly 200 NM with a turn on timing to the left on to a Northerly heading on the Howland sun-line. But if they were already 170 NM North, this would have put them 104 NM North and a left turn would just take them further away.

 The radio calls should be, ‘200 out’, ‘100 South-West’, ‘On line 157/337 going North’, ‘On you’. In that order. That also was not sent.

4. Direct approach: a direct approach would require searching North and South on the Howland sun-line.

 The radio calls should be, ‘200 out’, ‘100 out’, ‘On you’, ‘On 157/337 North and South’. This is what was heard by Itasca in that order but with a question mark over the ‘100 out’ call. Therefore, I think they did a direct approach, descending to 1000 ft in preparation for landing.
 Once they descended below the clouds, the wind could be measured using the drift sight.

 To measure the wind, take three drift measurements on three headings 45 to 60 degrees apart. On the Triangle of velocities side of the Dalton computer, set the airspeed and on each heading draw the drift line with a pencil. The three lines cross at one point. Move this point onto the centre line and read off the wind direction and speed speed.
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