DA 42 accident

On 2007-04-24 18:45:43 -0700, Jose said:

The problem is not FADEC. It is pilot error -- taking off with a known
electrical problem in an airplane dependent on electricity to fly,
coupled with a poor understanding of how an alternator works.

Maybe the problem in =this= flight was pilot error, inasmuch as the
takeoff would be ill-advised under the circumstances. However, the
accident does illustrate a weak point of the system. There are other
ways to trigger that weak point.

Really? Name one.


I don't know the system, so I can't second guess the engineers
intellegently about it. However, it does seem to be a serious
oversight that the engines themselves can't supply their own juice.

Jose

They do -- with an alternator on each engine. There is also a
generator. How predictable is the failure of two alternators, the
batteries, etc?
--
Waddling Eagle
World Famous Flight Instructor

There are other ways to trigger that weak point.
Really? Name one.

I'm guessing here (as I don't know the system), but it seems like a
short circuit in the landing gear could fail the engine's alternator, if
they are interconnected the way it seems from the postings.

it does seem to be a serious oversight that the engines themselves can't supply their own juice.
They do -- with an alternator on each engine.

Well, those alternators seem to be supplying juice to everything, making
them more vulnerable. No?

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"C J Campbell" wrote ...
Jose said:
I don't know the system, so I can't second guess the engineers
intellegently about it. However, it does seem to be a serious oversight
that the engines themselves can't supply their own juice.

They do -- with an alternator on each engine. There is also a generator.
How predictable is the failure of two alternators, the batteries, etc?

Chris,
I do not see the Generator to which you keep refering. Each engine has a
Starter Motor and an Alternator. The Airframe has a single Main Battery (10
amphour rated) and a series up Alternator Excitation Battery (1.3 amphour)
used "in the event of a main bat failure" (Diamond quote in POH). From the
article's diagram the magazine editor marked the excitation battery in RED.
I am not good enough with technical German to read the article, maybe
another reader can summerize the reason for the red highlight.

http://img.edsb.airworkpress.com/red/da42/esys_big.gif

http://www.pilotundflugzeug.de/artik...12/DA42_Unfall

I am a bit confused how that excitation battery is normally charged and how
the battery is monitored.

I also note that the Ground Power system is pretty standard looking in the
schematic, ie keep the Main Electric Master off and turn one Engine Master
on for starting so as not to have the bad battery connected and draw down
the Cart while cranking. But then what? You got your backup Battery excited
Alternator running your ECU on that engine, but it looks like you need to
keep the APU Cart connected because the power to activate the Alt relay can
only come from the main bus side. Is this normal for other twins?

Recently, Mike Isaksen posted:

"C J Campbell" wrote ...
Jose said:
I don't know the system, so I can't second guess the engineers
intellegently about it. However, it does seem to be a serious
oversight that the engines themselves can't supply their own juice.

They do -- with an alternator on each engine. There is also a
generator. How predictable is the failure of two alternators, the
batteries, etc?

Chris,
I do not see the Generator to which you keep refering.

On the schematic you're referencing, the Generators are the circular
symbols with the 'G' and labeled as such. However, as the relay in the
Generator circuit is labled "Alternator Relay", and there is an excitation
circuit I suspect that the terms are being used interchangeably. Not
technically correct, but...

Each engine
has a Starter Motor and an Alternator. The Airframe has a single Main
Battery (10 amphour rated) and a series up Alternator Excitation
Battery (1.3 amphour) used "in the event of a main bat failure"
(Diamond quote in POH). From the article's diagram the magazine
editor marked the excitation battery in RED. I am not good enough
with technical German to read the article, maybe another reader can
summerize the reason for the red highlight.

http://img.edsb.airworkpress.com/red/da42/esys_big.gif

http://www.pilotundflugzeug.de/artik...12/DA42_Unfall

I am a bit confused how that excitation battery is normally charged
and how the battery is monitored.

The excitation battery has a direct feed from both generators (really
alternators). If the fuse isn't blown, either alternator may be able to
charge this battery from excitation feedback. There is no indication of
how the battery condition would be monitored, but if neither alternator
operates, the excitation battery would be highly suspect.

Neil