DA 42 accident

More to the point, if all the power goes away, what happens to all
the "settings" the FADEC controls?

Do they go to zero, full, stay where they are?

Without electrical power, apart from the fact that the FADEC is a computer,
and no computer will run without power, on a common rail diesel the
injectors cannot open. A purely mechanical "limp home" mode is physically
not possible. Multipoint or direct injected gasoline engines are the same.

It appears that they go to zero, which is a damn unhandy failure
mode.

Yes it is, but that's just the way it is. It's no different from the failure
mode you get when your fuel supply or both magnetos fail or something
similar. That's why the power supply of a FADEC should be held to similar
stringent redundancy requirements. Actually, the FADEC itself IS double
redundant but fed from just one battery. Go figure.

regards,
Friedrich

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

On Mon, 23 Apr 2007 21:35:03 GMT, wrote:

It appears that they go to zero, which is a damn unhandy failure mode.

It's the new, revolutionary fail unsafe mode.

--
Dallas

On 2007-04-23 14:35:03 -0700, said:

Gig 601XL Builder wrDOTgiaconaATsuddenlink.net wrote:
Neil Gould wrote:
A simple voltmeter with a "red line" should suffice, along with a
caution; "Don't take off with the needle outside the green arc". Of
course, that won't prevent someone from insisting on making a bad
decision.

I again agree but if you are going to have an sytem with FADEC it ought to
have the authority to to clearly tell you that it is about to use its'
authority to shut the engine off.

From the description it sounds more like the FADEC didn't have the
authority (or power) to do anything.

More to the point, if all the power goes away, what happens to all
the "settings" the FADEC controls?

Do they go to zero, full, stay where they are?

It appears that they go to zero, which is a damn unhandy failure mode.

If you are talking about the props feathering when the engines quit, it
is absolutely essential in a twin.
--
Waddling Eagle
World Famous Flight Instructor

On 2007-04-23 13:44:11 -0700, "Gig 601XL Builder"
wrDOTgiaconaATsuddenlink.net said:

Neil Gould wrote:
A simple voltmeter with a "red line" should suffice, along with a
caution; "Don't take off with the needle outside the green arc". Of
course, that won't prevent someone from insisting on making a bad
decision.

I again agree but if you are going to have an sytem with FADEC it ought to
have the authority to to clearly tell you that it is about to use its'
authority to shut the engine off.

The FADEC cannot tell you anything or control anything if it doesn't
have power. There would be warning systems, but all they would tell you
is that your engines have quit. :-)

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.
--
Waddling Eagle
World Famous Flight Instructor

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.

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

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