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Date: Sun, 27 Sep 1998 11:53:20 -0500
From: Snowbird <snbird@ibm.net>
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Subject: gyro sources for IFR (was: Student using Katana
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TMetzinger wrote:

> In article <360CEBC9.C10EB916@pinetree.org>, Gordon Dewis <gordon@pinetree.org> writes:
> >Doesn't an IFR-certified a/c require a redundant vacuum system?
 
> No.  I think it does require two seperate sources of gyro info.  
> For most trainers this is the TC (electric) and the AI (vacuum).

The FARs do NOT require IFR aircraft to have gyroscopic instruments
powered by two separate sources.  FAR 91.205(d) requires three
gyroscopic instruments: rate-of-turn, AI, and DG "or equivalent".
Nitpickers will note there are certain exceptions to the above.

The FARs say nothing at all about how these three instruments are to 
be powered.  They can all three be powered by fuselage-mounted venturi
vacuum systems and the plane perfectly legal to fly IFR according
to the regs.  They can all three be electric, or all three powered
by an engine-driven vacuum pump.  Anyone who disagrees, cite regulation.

Now, it is true that most recent GA aircraft have electric 
rate-of-turn indicators, and AIs/DGs powered by an engine-driven
vacuum pump.  But this isn't required by the regs.  It's more or
less a pragmatic choice, since the cost of a vacuum pump and vacuum
driven AI and DG is less than the cost of an electric AI.

I bring this up because it behooves the pilot (all pilots) to 
know for sure how the different instruments in the plane they are 
flying are powered, in order to understand what to expect when a
system fails.  Some planes (early Mooneys) had vacuum-powered
rate-of-turn instruments, for example.  There are a couple of
accidents apparently caused when the vacuum pump in these planes
failed.  There was a post in this newsgroup a few months back
by a new PPL who didn't understand why the DG-equivalent in the
plane he was flying acted weird when he experienced electrical
problems.  The plane was equipped with an HSI.  Many HSIs are
electric.

> Not having the regs in front of me I think in essence they say you 
> have to have enough redundance to fly partial panel in IFR.

That would make sense, but in fact it is NOT what the regs say.

I consider this a prime example of the disparity which can occur
between what is legal, and what is safe.  A plane can be IFR-certified
and legal to fly IFR with all gyroscopic instruments powered by the
same vacuum source, which can even be a notoriously icing-prone
fuselage-mounted venturi.

Snowbird

