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At the risk of being roasted here I will give you my two cents.
First, I am an aircraft accident investigator-- so I get to see the results
of vacuum pump failures. Currently, I have three open cases involving
fatalities as the result of vacuum pump failures. I have no cases involving
total electrical failures, although years ago I nearly ejected out of an Navy
A-6 Intruder after a total electrical failure IMC-- the result of a
electrical supervisory panel fault-- the place where all the electical wiring
came together.
Second, I am also a CFI and Designated Pilot Examiner so I get to see a lot
of pilots out there and can tell you that the average level of skill among
non professional pilots that I routinely observe leads me to conclude that
the average private pilot with an instrument rating that flies 100 hours a
year (most don't fly that much) and gets maybe 10-15 hours a year in the goo
couldn't diagnose a vacuum failure correctly and quickly enough to transition
to the turn coordinator to save themselves. Hey, its reality and reality
bites. Flying is an extreme sport.
A vacuum failure is not going to happen like you were trained in a C-172. An
instructor is not going to announce "vacuum failure" and slap a post it note
over the gyro. Instead, you are going to notice that the aricraft instruments
don't seem to agree, the airspeed is increasing above red line and the
altimeter is unwinding like crazy. You check the AI and it is level but the
HSI is spinning fast to the right. You look at the turn coordinator and it
is pegged to one side. Cold sweat breaks out profusely and ATC asks if you
are having a problem. Now you look at the vacuum pump guage stuffed way over
on the right side of the panel by the cigarette lighter and it is showing
"0". You transition back to the turn coordinator but at 7000 FPM and close to
280 knots the 7500 cruise altitude you started with is gone in a minute.
Vacuum pumps are notoriously the weak link in the vauum/ gyro system. The
bulk of the failures occur with less than 500 total hours on the vacuum pump
and many occur within a few hours of installation of a new pump. (I've done a
study of vacuum pump failures for a notable case) Contamination of the vacuum
system is often the culprit. The carbon vauum blades of the dry vacuum pump
are intolerant of any dirt swilling about the chamber. BTW you won't find
vacuum pumps on many biz jets or military aircraft.
With the above in mind I elected to go with an all electric system. I have
the 100 amp alternator with B&C 25 amp backup. The system is wired that
should the main alt fail the backup will automatically power the essential
bus containing the #1 navcomm, xpdr, backup elec gyro, etc. I am also
planning on a small backup battery that will provide power to essential items
as well -- so that one failure cannot take down the whole system.
There are a number of different ways to wire you panel to accomplish a good
level of redundancy. I have chosen a method that allows an automatic switch
to a backup source WITHOUT any pilot action should the main alt fail-- in
some of the vaccum failure accidents I have investigated the pilot never
selected the backup vacuum source even though it was installed--probably
because it was not a normal frequently used/ practiced operation -- hence my
requirement for an auto backup. Also keep your backup power source and wiring
apart from the main for redunancy sake.
Just my 2 cents.
Jeff Edwards
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