Peter wrote:
I've had two of the above fail in under 3 years.

I would guess that the failures are related to adverse voltages
coming in or an adverse load on the output. The electrical
environment of a small plane can actually be much more abusive
than a modern car, because of the wide varieties of load dumps.

Both units developed an intermittent, vibration-related, fault (no
400Hz output) which is curious since they are potted and not a lot
can
move about.

How is it "vibration-related"? Stops working in some positions or
in vibrating environments? Power semiconductor packaging can fail
intermittently due to things you cannot see, it's (as I understand it)
mostly related to repetitive thermal expansion/contraction.

Tim.