For Want of a Nail (Part Two)

CCXXXVII

Amelia was absurdly fond of the RDF loop and posed for many photos with it prior to installation

Whether Earhart’s failing voice receiver had any direct impact on the operation of the Radio Direction Finder (RDF) unit aboard the Flying Laboratory is unclear. The answer is “probably” since the two systems worked off the same Bendix RA-1 cockpit unit  What is fairly clear is that, just as with the voice receiver, Earhart was essentially unfamiliar with the more arcane functions of the RDF, which had been installed only shortly before leaving Miami, replacing the previous Western Electric system.*

The loop antenna atop the fuselage was operated manually. Amelia would turn the loop by means of a click wheel on her instrument panel. Indicators on the front of the RA-1 unit would then give her readings of the relative strengths of radio signals being received by the unit. After “locking in” on the strongest available (“high gain”) signal, Amelia would then turn the loop either full forward or full backward to set a “null signal”. The difference between the “high gain” and the “null” would provide a bearing. The system worked well, except for one critical shortcoming, that being that the “nulls” at 90 degrees and at 180 degrees tended to cancel each other out.   Thus, it was necessary for a pilot to know whether they were approaching the locked-in signal or moving away from it to confirm the aircraft’s relative position. Usually, this could be determined by the pilot, observing whether the “high gain” was increasing in strength or decreasing over a fixed period of time before choosing a “null” position. But for a pilot who was unsure of their location relative to the signal and running out of fuel (and hence time), or for a pilot whose receiver was malfunctioning, or for a pilot who was not familiar with the system, the RDF of the era was ineffective. This inherent technological shortcoming of mutually-cancelling “nulls” would be erased by wartime advances in RDF design, but they would come too late for Amelia Earhart. 

The problem was compounded in the case of Amelia’s Bendix receiver. At only 50 watts, there was only a moderately measurable difference between the “high gain” and the “null,” and if the receiver / transmitter was not working properly there might be no discernable difference at all.  Then too, Amelia (who had trained a little on the Western Electric gear but not at all on the Bendix) would have been a bit familiar with the RDF frequencies of the former but not at all the latter. And nineteen hours into the flight there was no time to read the Owner’s Manual.**

*Like so much else on the Lae-to-Howland leg, even the identity of the radio equipment is up for debate. No one really knows what frequencies Earhart could use. Most sources say that Bendix completely replaced all the radio / RDF gear on the Electra while it was on the hardstand in Miami. TIGHAR argues that Bendix only switched out the RDF loop. If true, the competing non-standardized technologies of Western Electric and Bendix would have been incompatible and suffered problems --- as the RDF did.   

**Another point made by TIGHAR is that Bendix did not provide an Owner’s Manual for the system, a strange oversight if in fact they replaced the entire communications suite.