As part of the research for the new Electra book, Bob Brandenburg and I have been trying to pin down how the radio systems aboard NR16020 evolved over time and exactly what radios were present for the second world flight attempt. It's an important question because Earhart's failure to find Howland was essentially a failure of radio navigation.
In piecing together what happened we've uncovered another mystery and I'm hoping the Forum can help us. Here's a quick overview. (Sorry if this gets a bit down-in-the-weeds but there's no avoiding the technical aspects of the problem.)
In Burbank, on or about March 7, 1937 as NR16020 was being prepared for a planned March 15 departure from Oakland, the Hooven Radio Compass (installed back in October) was removed and replaced with a Bendix loop, loop coupler, and receiver. The loop and loop coupler appear to have been a Navy RDF-1 system and the receiver seems to have been an early version of the RA-1. This new receiver was installed on top of the fuel tanks behind the cockpit bulkhead behind the copilot - the same location where the Hooven receiver had been situated.
One advantage of the new receiver was that it covered 500 kHz, the marine calling and emergency frequency. The Western Electric 20B receiver under the copilots seat did not include that frequency. In February, W.C. Tinus at Bell Labs had convinced Earhart that it would be good if she could communicate with ships on 500 kHz and he had modified her Western Electric 13C transmitter to include a 500 kHz crystal. 500 kHz requires a long antenna and the airplane was equipped with a trailing wire for that purpose. 500 kHz is also a code-only frequency but Earhart had Harry Manning aboard as navigator and radio operator. Manning was adept at Morse code.
With the new receiver Manning would have two-way communication in code with ships at sea.
Helping Earhart with radio matters at this time was United Airlines technician Joe Gurr. Gurr later said that he helped Manning learn how to use the Bendix DF system but had little luck getting Earhart to pay attention long enough to really become familiar with it.
Manning left the team after the accident at Luke Field. Gurr later said that he reinstalled the radios in early May while the plane was in the Lockheed repair shop. Gurr was under the impression that the plane had the same radios aboard for the second world flight attempt as for the first. Based primarily on Gurr, Elgen Long was quite sure there was a Bendix receiver aboard for the second attempt. That does not seem to be the case. In the time between when Gurr reinstalled the radios in early May and when the plane came out of the shop on May 19 the Bendix receiver was apparently removed and the loop and loop coupler were attached to the Western Electric 20B receiver. There are two sources to support this conclusion:
1. A statement made by Chicago Tribune reporter and Earhart friend Carl B. Allen in a manuscript reportedly now in the Smithsonian National Air & Space Museum archives. The following is from East to the Dawn - the Life of Amelia Earhart by Susan Butler (page 387):
"When Allen first arrived in Miami and caught up with Amelia at the airport, one of the first things he did was to go over the equipment list to see if there had been any changes since Oakland. he noted one change that he was;t sure he approved of - the elimination of the marine frequency radio that operated on the 500-kilocycle bandwidth. “Oh,” she said, ‘that was left off when Manning had to drop out of the flight. Both Fred Noonan and I know Morse code but we’re amateurs and probably would never be able to send and receive more than 10 words a minute …The marine frequency radio would have been just that much more dead weight to carry and we decided to leave it in California.”
This is an extremely important quote and I want to check the source myself. I’ve put in a request to NASM to confirm that they have the manuscript and whether it’s at the Udvar-Hazy archives at Dulles or at the NASM Library downtown.
2. In "Last Flight, chapter titled Karachi, Earhart described the "Bendix direction finder" as an instrument on her "dash." She says the Western Electric receiver is under the copilots seat. No mention of a separate Bendix receiver. In fact, nowhere in the 1937 literature (Earhart, Chater, etc.) does anyone say anything about a Bendix receiver.
The mystery is who made the changes in those last days before the plane left the Lockheed shop?
As an aside, there is a rather strange quote via Fred Goerner:
"Lieutenant Commander William Van Dusen, U.S. Naval Reserve (ret.) (who worked with Pan American Airways for many years) has written, "The simple reason ships and shore stations were unable to communicate with the Earhart plane or to transmit bearings, is that the radio transmitter for these marine frequencies was left sitting in the corner of a hangar in Miami when Earhart and Noonan left the U.S. At the last minute she decided to leave it behind because it weighed so much: it meant more work paying out and reeling in an aerial line."
It's not clear where Goerner got this. The Bendix radio was a receiver not a transmitter. This may have been the start of the myth that the trailing wire was removed in Miami. Photos of the plane in Burbank prior to departing for Miami clearly show that the trailing wire was already gone.