DA 42 accident

"Neil Gould" wrote ...

If the alternators weren't excited, wouldn't the pilot be looking at two
alternator warning lights prior to take off?


According to the AFM there are amber caution lights for alternator failure,
so yes.


Does the aircraft have an
ampmeter?



Again according to the AFM, yes, but notably, it is not on the MFD's default
engine display page but on the "System" page together with the voltmeters.
You need to push a button to see it. However, the checklist requires you to
have a look at the "System" page after engine start as well as before
take-off, so if the checklist is followed a fault would not go unnoticed.

In the other scenario posited by the article, i.e. an unconnected main
battery, things become interesting. In this scenario the ammeters would
presumably show "normal" values, i.e the instantaneous consumption of the
electrical devices. In this case the voltmeter would really be essential .
The AFM says about the voltmeters: "Under normal operating conditions the
alternator voltage is shown, otherwise it displays the 'main'-battery
voltage." So the voltmeters presumably measure the bus voltage, and in this
scenario (battery disconnected) they would probably show an abnormal voltage
which could alert the pilot. I'm no expert here, but I seem to recall
stories of batteries failing in-flight and how that can be seen from the
voltmeter.

Also, in this case, there seems to be no way of positively checking the
actual, pure main battery voltage, because according to the schematic the
main battery relay needs power from the battery itself to operate and
connect to the battery bus. Or then I'm missing something......

Recently, Snowbird posted:

"Neil Gould" wrote ...

Does the aircraft have an
ampmeter?



Again according to the AFM, yes, but notably, it is not on the MFD's
default engine display page but on the "System" page together with
the voltmeters. You need to push a button to see it. However, the
checklist requires you to have a look at the "System" page after
engine start as well as before take-off, so if the checklist is
followed a fault would not go unnoticed.

I would think that this preflight requirement has implications for this
accident.

In the other scenario posited by the article, i.e. an unconnected
main battery, things become interesting. In this scenario the
ammeters would presumably show "normal" values, i.e the instantaneous
consumption of the electrical devices.

If the alternators are working, the ammeters should show a higher than
normal postive value, as the battery is not putting a normal load on the
charging system. That, too, would stop me from taking off.

In this case the voltmeter
would really be essential . The AFM says about the voltmeters: "Under
normal operating conditions the alternator voltage is shown,
otherwise it displays the 'main'-battery voltage." So the voltmeters
presumably measure the bus voltage, and in this scenario (battery
disconnected) they would probably show an abnormal voltage which
could alert the pilot. I'm no expert here, but I seem to recall
stories of batteries failing in-flight and how that can be seen from
the voltmeter.

Hmm. I'd think the alternator voltage would always be shown unless the
engines aren't running, and then the bus voltage is shown. A look at the
schematic could reveal which case is correct.

Also, in this case, there seems to be no way of positively checking
the actual, pure main battery voltage, because according to the
schematic the main battery relay needs power from the battery itself
to operate and connect to the battery bus. Or then I'm missing
something......

I'd think that checking when the engines are not running would show the
main battery voltage level. If the battery is dead, of course, the relay
wouldn't have power to connect to the bus.

Neil