On 2007-04-24 09:09:14 -0700, Cary said:

The problem, as I understand it, was the battery was dead. According
to the POH, the battery is used to start the engine and is used as a
backup during flight for all the electronic gear (including the
FADEC). Although the investigation is still ongoing and other answers
may be forthcoming, when they operated the landing gear they exceded
the power available from the alternators and the backup system (the
battery) was not available so the FADEC (engine computers) stopped.
One of the lessons here is that one should not fly an airplane that
relies on electricity if you don't have a battery to run the
electricity!

Actually, if the battery is dead the alternators will never start
working. This is the excitation battery system that failed. Its sole
purpose is to supply enough current to excite the alternator when the
engine is started. After that it is never used again during the flight.

The DA-42 has considerable redundancy. Click on the electrical diagram
in the article and you can see the problem immediately. This particular
airplane has two alternators *and* a generator. If the alternators do
not work (as is probable) then the generator kicks in. The generator,
however, is not big enough to operate both landing gear and engine -- a
possible design flaw. Also, if you are going to have a generator, why
not use it to excite the alternators if the excitation system has
failed? Apparently the designers assumed that if the generator is being
used that the alternators have failed beyond repair, but here it might
have been possible to get the alternators working with a full
complement of power. On the other hand, maybe the original problem was
the alternators were both dead (doesn't seem likely) and that is why
the excitation system was dead. So even current from the generator
would not have excited the alternators.


--
Waddling Eagle
World Famous Flight Instructor