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Author Topic: Drift in the Dark part 3  (Read 3202 times)

Colin Taylor

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Drift in the Dark part 3
« on: March 20, 2023, 03:55:49 AM »

Drift in the Dark
Part 3 of 8

The confusing position reports received by radio at Lae (Figure 3)

In fig 3 the direct (078oT) track and the South off-set track to avoid weather are shown. The dashed line is the track where I think the Electra went. It is calculated using the headings from the flight plan (using the forecast wind) and my calculated actual wind.
What is the significance of Position 150.7oE 7.3oS (POS150)?
 This position is 20 degrees Right (South) of the direct track and South abeam WP1 but was reported by Earhart at 3:19 pm L Lae (05:19 GMT), which gives a groundspeed of 39 kts, which makes no sense.

 To explain this, I have inserted an additional waypoint at 156.9oE 6.1oS (POS156) South abeam WP3. This creates a track parallel to the direct course and about 80 NM South, to avoid the weather East of Lae. The offset track converges at 20 degrees to the direct track and puts them back on track at WP4. The ETA for POS 150 is 01:52 GMT. The ETA for POS156 is 05:09 GMT. At 05:19 GMT it appears that Earhart radioed the wrong coordinates; that is, she gave the coordinates for POS150 instead of POS156.
What is the significance of Position 159.7oE 4.33oS (POS159)?

 At 05:18pm L Lae (07:18 GMT) Earhart reported at 159.7oE 4.33oS (Pos 159) with a wind of 23 kts, compared to the forecast of 15 kts. How did they work this out? I can invent some numbers to show how this worked.

 The Dead Reckoning (DR) estimate for WP4 was 06:53 GMT. At that time, they set course for WP6. After 6 minutes, at 06:59 GMT they passed the Numanu Islands 30 nm short of WP4. The new ETA for WP4, based on current groundspeed 121 kts was calculated to be 07:14. The time difference between the two estimates for WP4 over the direct distance from Lae to WP4 gives an additional headwind of about 8 kts which is added to the forecast of 15 kts for a total of 23 kts headwind. All subsequent ETAs are increased by 21mins. (07:14 minus 06:53).
 Knowing that the wind must be stronger than forecast, what wind should Noonan have used for subsequent sectors? The forecast was for the wind to increase and then decrease, with the direction backing from ESE to ENE giving left drift then right drift. The left and right drift effects should cancel out and the effect of the increased headwind will be easily accommodated by the reserve fuel (3hrs+), corrected by radio or astro fixes, therefore the best course of action was to continue with the flight plan.


 The latest fuel plan, revised by telegrams from Kelly Johnson at Lockheed, required 3 hours at 4000 ft, 3hrs at 6000 ft, 3 hours at 8000 ft and then 10000 ft. According to the radio reports, they climbed to 4000 ft and then to 7000 ft at 04:18 GMT and reported at 10000 ft at 05:19 GMT. They then descended to 8000ft at 07:18 GMT. The most likely reason for this was that they were in cloud at 4000ft and 7000 ft and in icing at 10000 ft and so descended to 8000 ft. (Been there, done that!).
 In the International Standard Atmosphere, the temperature at 10000 ft is 15-(10 x 1.98)= -5 degrees centigrade which will pretty-much guarantee airframe icing when in cloud. They may have tried 10000ft again later, although Nauru reported cloudy and Itasca at Howland had reported alto cirrus. This is not a standard cloud type but does suggest high cloud. The PBY flying boat that was sent to assist with the search later, reported cloud and icing between 2000 ft and 12000 ft.

 Any flight in icing conditions or any deviation from the optimum height profile would have compromised their fuel reserves.

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