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The TwinStar
DA-42 is a highly complex, diesel powered aircraft that
relies on the alternators and batteries more than any other
general aviation aircraft produced to date. An
accident in this aircraft occurred on March 4th, 2007 in
Germany that indicates just how important electrical power
is to the DA-42
The article
below is not meant to frighten or make a pilot feel bad
about flying the DA-42, instead it should justify the need
to check all systems on the aircraft before departing on a
flight. The article results in the requirement of the
pilot in command to verify that the Volts and Amps being
produced by the aircraft are well within the green during
preflight.
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In the german magazine ‘Pilot und Flugzeug” ( pilot and
plane ) of April 19th will appear an article by reporter
Jan Brill, which was published on a german forum on
Friday, April 13th. This posting was found by fellow
DAPO member Wolgang Daiser, and while my german is a
little rusty and my technical profiency regarding these
systems virtually non-existent ,I’ve tried nonetheless
to summarize the following salient points :
Airworthiness of the DA42 under scrutiny due to problems
with its electrical system.
Following the accident of a new DA42 (D-GOAL ) on
takeoff in Speyer, Germany on March 4th,2007, all pilots
of DA42s need to be made aware of weaknesses inherent in
the design of the electrical system that can lead to a
catastrophic shutdown of both engines. The redundacy of
systems on twins normally excludes failures from “single
points of failure”, however a defect in the airplanes
main electrical bus can cause a total shutdown of both
powerplants.
The article continues by describing the accident, i.e.
both props feathered on take-off upon switching on the
gear retraction, and how the engine failures were
directly attributable to an electrical failure although
the exact root of the problem hasn’t yet been
determined. It is known however that the load required
for the gear retraction could not be isolated from the
ECUs, and the ECUs shut down. The alternators require
excitation to remain online and to provide a charge.
Should this breakdown, the battery would serve as
backup, however if the battery for any reason becomes
unavailable to serve this purpose, a full shutdown of
both engines will ensue.
Within five days of the accident, Diamond issued
SB42-050, forbidding any day-VFR or IFR flight following
an EPU initiated engine start.
The deeply depleted status of the main battery may be
only one of many possible causes leading to a failure,
such a main relay failure or oxidized contacts. The
article questions the wisdom of this type of electrical
syatem when other all-electric aircraft such as the
Columbia 350/400 have a fully redundant electrical
system and battery.
Diamond has stated to “Pilot und Flugzeug” that the
problem is a powerplant issue and up to Thielert to find
a solution. The Austrian government aviation agency (
Austro Control ) supports this position. Other European
aviation agencies don’t agree.
Diamond is proposing the backing up of the ECUs with
additional dry batteries. Frank Thielert states that
while the engines would continue running, this solution
adresses only the symptoms rather than the root cause.
The article states that a comparable airplane such as
the Piper Seminole, has a much larger battery. ( however
a post by another forum member points out that the
Seminoles’ usage is also much greater and that
proportionately, the DA42 has a larger capacity to usage
margin than the Seminole )
It also makes another point about the configuration of
the electrical system being poorly protected against
overloading, stating that an alternator can overload
through the bypass even when the alternator switch is
off and its circuit breaker pulled. The only way to
isolate this alternator would be to pull the
corresponding ECU breaker, an undesirable dependancy.
A final point is that the battery relay can only draw
from the battery hot bus. This means that the electrical
bus can be put under load by a ground power unit without
the battery being charged. ( I can confirm this as
Diamond people have told me a about a slow depletion of
the battery occuring while connected to an EPU during
demonstrations of the G1000 ). A pilot could
consequently go flying without being aware of an
inherently unstable configuration.
The article goes on to say that the best solution would
be a conversion to a dual-bus configuration. In the
meantime, every pilot should be aware that care should
be taken with the battery and to know that any glitch in
the battery relay could make for a very bad day.
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