Standby Vacuum?

Mitty wrote:
Airplanes: We have 4 different flavors of PA-28s, six total, plus a
Cherokee Six. The two being upgraded right now are Archers.

Type of Flying: We do not fly a lot of hard IFR, panel space is limited
as is money. Hence, we are not looking at backup horizons, etc. Also,
one of the airplanes will be getting an S-TEC 2/ or 30, which replaces
the TC.

Hence, something like the Precise Flight at $400-500 kind of numbers is
the candidate. Possibly an electric pump, though I don't know anything
about the cost.

I once owned an Archer that was leased to a flying club, and I installed the
Precise Flight system. I wouldn't do it again. I *might* feel differently if I
were the only pilot, but probably not.

Pilots need to understand how it works and be prepared to accommodate its
peculiarities. The peculiarities can be explained in a few paragraphs, but won't
be appreciated by the pilots unless they have actually experienced the way it
works. You can't experience the way it works unless you physically disconnect
the vacuum pump, cap it off, and go flying. You can't do that kind of training
for every pilot in the club.

If you think just pulling out the knob is sufficient to experience how it works,
you are mistaken. The result of pulling out the knob is quite different when the
vacuum pump is operating and when it is not operating.

Unless you preflight the system before takeoff, you can't be sure it will work
when you need it.

If you don't exercise the shuttle valve often it can get stuck, rendering the
system inoperative and requiring extra maintenance. You exercise the shuttle
valve every time you do the preflight checks, *if* you do the preflight checks
correctly. I found most pilots didn't understand how to do the preflight checks
correctly, didn't follow a written checklist, and often failed to preflight the
system at all. If they did preflight it, they didn't understand how to do it
correctly so that the shuttle valve is exercised and you know it's working.

In operation, at best, you must reduce engine power (thereby reducing manifold
pressure, or, put another way, increasing manifold vacuum) in order to make the
system work. So if you need to climb, or at some altitudes, even maintain
altitude, you are going to require more power than the reduced amount that will
keep the gyros spinning. So in that circumstance you are going to be partial
panel anyway.

In the high workload environment of say, a missed approach, as you add power,
the gyros start to spin down. That adds another work item to your already high
workload: recognizing that the backup vacuum has "failed". Operating under the
stress of a vacuum pump failure and a missed approach, you might forget that
will inevitably happen as soon as you add power.

In short, IMO you might as well just learn to fly partial panel.

Dave

I have the precise flight setup. I've never had the failure where I had
to use it but it gives me peace of mind. So does a new pump.

I'm not familiar with typical IFR setups but do most a/c have a vacuum
failure light to tell you when the vac pump fails? I believe that the
vacuum failure light that was installed with the Precise Flight system
is as valuable, or perhaps more valuable than the actual backup function.

That series of tests where the vac was failed while filming pilots
reactions seem to indicate that the main problem was detecting the
failure and slow roll over of the AI in time. A red light beaming at
you from the center of the panel seems to be to be pretty damn valuable.

But like I said, I haven't had the failure so I just don't know.

Mitty wrote:
Our club is looking at upgrading a couple of airplanes to Garmin 430s, etc.

It seems to me that standby vacuum would be a good thing to add, too.

1) Good idea? Do these systems really work?

2) Recommendations on type/brand/model?

TIA

Seems that the best redundancy is a rate based autopilot (read STEC).
Since it is based on, and runs on, a turn indicator the issue of
whether yo have a good one is solved. However I still have an electric
backup vacuum pump in my C172.

wrote:
Seems that the best redundancy is a rate based autopilot (read STEC).
Since it is based on, and runs on, a turn indicator the issue of
whether yo have a good one is solved. However I still have an electric
backup vacuum pump in my C172.


I really believe this. The TC won't tumble. It comes down to two things:

1. If you have the presence of mind to engage the AP if the AI goes south.
Of course, in most IFR situations, the chances are good the AP is already
on.

2. Although the standard "ethic" says that engaging the AP during a loss
of control is a good idea, I tend to doubt it. The STEC has limits on what
control pressures it will tolerate, and the result can be for it to
simply disengage if it cannot overcome them.

Its a good idea if you can't afford a standby electric attitude
indicator. I've had more indicators fail then pumps myself.

-Robert