I've owned and flown with various kinds of backup systems, so here's my 2 m$.
I've had 2 vacuum pump failures, one in VMC and one in IMC. I had no backup
equipment installed when I had the IMC failure. I was able to descend a little
to get below the bases into visual conditions. I wouldn't like having to to a
full approach with needle-ball-airspeed.

Precise Flight standby vacuum. Least expensive, least effective. Has a shuttle
valve that selects vacuum from either the primary vacuum pump or the intake
manifold. The shuttle valve tends to stick sometimes. There's a recurring AD to
inspect it. Check it regularly during preflight. Provides vacuum only at low
power settings (when manifold pressure is lowest). As another poster noted, that
is actually exactly what you have during most of an approach. If you have to do
a missed-approach or go-around, be prepared to lose your vacuum instruments as
you add power. Have your mechanic temporarily disable the primary vacuum pump so
you can go out and experiment with it under VMC.

Electric standby vacuum pump. These seem to be a regular vacuum pump with an
electric motor attached. The one I've seen (on a club plane I fly occasionally)
takes up a lot of space under the cowling and looks heavy, but I don't know what
the actual weight is. There is a panel-mounted switch to turn it on. Seems like
it would do the job OK, but at the expense of space, weight, and complexity.

Dual-rotor vacuum pump. I have one of these on my Mooney. Two carbon-vane dry
vacuum pumps on the same shaft, with a clever arrangement that avoids breaking
the shaft for the second pump when the first one fails. The company that owned
the STC has gone out of business, but another company (Phoenix-something?) has
bought the STC but hasn't marketed it yet. They are trying to work out some
failures that occurred on certain engines, suspected to be related to torsional
vibration, I think. Last I knew, they were actively pursuing getting it back on
the market. When it was available, I thought it was good solution, and voted
with my wallet. Google groups will show you some earlier postings from me about
this system.

Electric AI. I have one of these on my Mooney, too. It's unfortunately installed
on the right side of the panel and is too far out of my scan to be useful. It
also seems to have a lot of lag in it, so it's not as nice as the primary AI.
It's required repair/overhaul/replacement a couple of times in the 10 years or
so it's been in the plane.

There's a dry vacuum pump on the market with an inspection window so you can
tell how worn the vanes are and hope to replace the pump before it fails.

I'm sure someone will chime in with "just get a wet pump". I don't have any
experience with those, so I'll just wait for that posting.

DGB

Chuck wrote:
Hi group,

Seems to be a lot of experience out on this list, so I thought I might
ask a question.

I have a Cherokee 180 that is IFR cert, but both A.I. and Turn
Coordinator are vacuum driven. With both driven by vacuum, a pump or
engine failure in IMC will leave you guessing if you're right side up,
turning, or pitching.

I've considered replacing the A.I. or Turn Coordinator with an
electric driven instrument. But either would be expensive and
replacing the Turn Coordinator would break my autopilot. So, I've
decided the cheapest way to provide backup insurance is to put in a
back up vacuum pump.

Does anyone out there have any experience with the STC'ed backup
vacuum pump kits? Is one maker's kit better/easier to install that
others? And is there any information available on reliability? I'd
hate to put in a "backup" system only to have it fail to work!

Thanks guys.



Chuck
N7398W
San Antonio, Tx


--
Dave Butler, software engineer 919-392-4367