nuckolls.bob(at)cox.net
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 Posted: Thu Aug 07, 2008 5:40 am Post subject: Serious Incident to Piper PA-34-200T |
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At 10:33 AM 8/7/2008 +0000, you wrote:
| Quote: | Published yesterday by the Air Accident Investigation Unit in Ireland.
http://www.aaiu.ie/upload/general/10887-0.pdf
Concerns a complete electrical failure aboard a twin Piper on an IFR
flight during winter. Safe landing in the end.
Thoughts on lessons learned?
Andrew.
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This is an interesting case. While the symptom was not repeated
by experiment (difficult and hazardous) the deduction drawn by
the investigators is quite plausible. There is only one error
in the data which is the 0.3 ohm contact resistance cited for
the contactor specification . . . this is probably 0.3 MILLIOHMS.
Obviously, a contactor with 300 milliohms resistance would be
useless.
If one tracks the history of system evolution for aircraft,
the first time this airplane (or it's ancestors) flew, it
was fitted with generators. Generators are noteworthy for
their ability to function without benefit of external
power for excitation support. The history of the 6041
series contactors in aircraft and throughout industry
has been exemplary. However, they DO have a service life
and they do WEAR OUT at some point in time.
http://aeroelectric.com/Pictures/Failures/6041_Contactor_Failure.jpg
The MTBF study offered up in the report spoke to "three
failures" . . . of course, these were failures that generated
noteworthy incidents such as the case under study. There
were thousands of failures for both battery and starter
contactors where the difficulty was discovered on the
ground or the contactor was teamed with generators
and the in-flight failure of a battery contactor did not
generate a reportable incident.
This is a GREAT example of how architecture, selection
of components, and plan-a/plan-b operating procedures
can utilize seemingly pedestrian components in a way
that failures are simply maintenance events as opposed
to reportable events (where there was risk of bending
an airplane or occupants).
It was consideration of vulnerabilities demonstrated
by various components that evolved the Z-figures. First
with addition of the e-bus in Z-11. Had the case study
airplane been fitted with an e-bus, they would not have
needed to resort to cell phone communications or augering
around in clouds sans electronic nav aids. The next
step was independent, cross-feed electrical systems,
Z-14. Obviously a giant step above any of the intertwined
architectures that fly in the majority of GA aircraft.
I'm aware of NO production aircraft that features such
an architecture.
The shift to all electric panels vacated the vacuum
pump pad. A second engine driven power source could
utilize the vacated pad and offer up a three layer system
illustrated in Z-11. (1) Battery only + e-bus (2)
battery + e-bus supported by 8A engine driven power
source holding the battery in reserve for approach to
landing and (3) full up battery + e-bus + main bus
supported by ships normal alternator.
With the advent of the SD-20 offered into GA aircraft
and Z-12 architecture, they can come close to a
Z-11 configuration . . . but there is still the specter
of battery contactor failure that could generate a
total loss of system under certain conditions.
We were able to address that issue on the Cessna
337 when for a short time there was a small battery
pack of flashlight cells and a push button offered up
to kick-start a stalled alternator. Later, we took
a 3-phase rectifier off the tach generator to provide
a source of voltage for the same purpose.
Decades later, we're finding alternators that either
by accident or on-purpose design have enough residual
rotor magnetism to come on line self-excited. Given
the speed at which we rotate little automotive
alternators in belt-driven applications, it may
be that most of those installations will come up
without a battery . . . if lightly loaded.
Certainly, there are little features we could explore
but without a broad spectrum of purposeful design
amongst the field of suitable alternators, it's
not likely that we'll be able to recommend exploitation
of this quality . . . hence the configuration
the z-figures as presently published.
Is there value in combing through these reports?
Certainly. There is (1) the potential for making us aware
of previously undiscovered/unconsidered failure
modes and (2) re-enforcement of our confidence in
the state of our art in crafting failure tolerant
systems.
The sad part is the regulatory hurdles that prevent
our brothers on the TC side of the house from exploiting
the simple pleasures of electrical system confidence
we enjoy on OBAM aircraft. Present trends plotted out
into the future do not suggest they will ever be
beneficiaries of such freedoms.
Bob . . .
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