Old Wive's Tale
Read what
Bob Nuckolls has to say Bob wrote the book by which many RVs are wired.
Below is quote from Bob's post:
Actually, the av master was borne on a compelling
but flawed premise . . .
About 1966, when transistors were starting to show
up in aircraft radios (pnp germanium with 30v ratings)
Cessna (and I suspect others . . . don't know for sure
but I was at Cessna) were experiencing a rash of radio
failures in new, ready-to-deliver airplanes . . .
. . .and yes . . . it seems that radios left ON during
engine start were the most affected. It was surmised
that a 'spike' from the starter got 'em. It stood to
good reason. That starter thingy draws hundreds of amps,
was very inductive . . . it HAD to be a potential source
of inductive 'kick back' deleterious to those relatively
fragile.
The Avionics Master was borne . . . and yes . . . the
failures went away. In retrospect, it was not an
inductive 'spike' that got the radios but brownout.
The 'Airplane Patch' east of the plant would park hundreds
of airplanes waiting for delivery . . . none of which had
guaranteed battery integrity . . . a 6v sag to the 14v radio
during a starter 'grunt' was the most likely cause. Further,
while DO-160 (and its predecessors DO-138 and DO-108)
was definitely around, it was not as rigorous with
respect to bus voltage aberrations.
In years since, solid state devices have become very
robust in that they are designed to EXPECT the very
abuse that rolled a NavCom 300 T.U. in 1967.
By the time yours truly was designing transistors into
airplanes in the 1975 time frame, the idea that I would
want to 'take it off the bus' during an engine start
was rather humorous . . . the industry had learned
how to live in the vehicular DC power environment.
It was no big deal . . . yet the seeds of concern for
'spiking' a radio persisted as did the avionics master
switch.
Bob . . .