Noonan (as recorded by P.Mantz , see Hollywood Pilot by Dwiggins) let the offset track fly by time-distance DR on the ETA via direct course to destination...
I have no idea what that sentence means. On page 100 of Hollywood Pilot, Dwiggins relates Mantz's of the flight from Oakland to Honolulu. With guidance from Manning, Noonan and Mantz, Earhart used the Bendix loop to fly a straight-in course. "For the first time, AE had a chance to try out the Bendix loop antenna direction finder. Noonan asked her to hold Makapu beacon 'ten degrees on the starboard bow' - a heading of 252 degrees. She went about it calmly, professionally, rotating the antenna loop on top of the cockpit until the compass needle swung over to 10 degrees - ten degrees to the right of where the electra was pointed. This, she knew, was a wind drift correction Noonan had figured out."
Dwiggins had that last bit screwed up. The procedure he describes would be correct if Earhart had an Automatic Direction Finder (ADF) but the read-out for the Bendix RDF was a simple left-right needle. Dwiggins had a lot of things screwed up. He has Manning annoying Earhart by coming forward to take star sightings through the hatch above her head. There is no way the cockpit hatch could be opened in flight without it tearing off the airplane.
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Are you sure about that Ric, I thought she had to listen for the null?
I have attached a copy of the 1941 Bendix catalog page that shows pictures of the components of the MN-13 Bendix RDF system, I don't see any left-right needle. It appears that this would have been the system installed in Earhart's plane since it could be used with the already installed radio receiver. There are other RDFs shown in this catalog that do have left- right indicators but those also have dedicated RDF radio receivers with outputs to drive the left-right indicator.
From the picture of Earhart mugging with the RDF loop, is is clear that she had the lower sensitivity nine inch loop, a poor choice for this flight.
I also notice that this RDF had provision for an input from a sense antenna to eliminate the 180 degree ambiguity. It is possible that the belly antenna was this sense antenna.
Hmmm, as I am typing this it just occurred to me that perhaps the loss of the belly sense antenna could have caused AE's RDF problem. The MN-13 has a frequency range up to 1500 kcs but that is probably a limit on the MN-13 loop amplifier but it apparently did work with the 7500 kcs signal but was only able to send a weaker signal to the radio receiver due to that signal being above the designed frequency range of the MN-13 loop amplifier. Earhart's radio apparently was able to receive the 7500 kcs signal but she couldn't get a null. It is just possible, that although not optimized for 7500 kcs, that this loop might have had enough directivity to have provided a bearing to Itasca. The MN-13 has a switch which selects the sense antenna or leaves it out of the process. With the sense antenna connected and the selector switch set in the uni-directional position, this equipment eliminates the 180 degree ambiguity and produces only one null instead of the usual two nulls, spaced 180 degrees apart.
So what happens if Earhart sets the switch to the uni-directional position but there is no signal coming in on the sense antenna terminal because the antenna was lost? What would Earhart hear in her headphones? No null? Two nulls? I don't know the answer to this question. I do know, that with an ADF, that without the sense antenna the needle just spins around and around, never settling on a bearing. I wonder now if Earhart could have been saved if only she had placed that switch back into the bi-directional position and used the normal procedure to manually eliminate the 180 degree ambiguity.
Ric, has Brandenburg looked into this?
gl