Standby Vacuum?

Mitty wrote:

WeÂ*doÂ*notÂ*flyÂ*aÂ*lotÂ*ofÂ*hardÂ*IFR

That's good. My "scary thought" about the precise-flight is losing vacuum
pressure on the missed approach.

- Andrew

Jose wrote
I would not remove the T&B. If you add another AI, put it in a nearby
hole, but not the T&B. No other instrument provides turn rate
information.

After 47 years of civilian, military, and airline flying, I have
yet to see a reason for "turn rate" as long as I have a functioning
attitude indicator.

Bob Moore

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

Interesting that you pointed that out. I was thinking the same thing when
I posed the question but thought that two tumbling AIs would pretty much
guarantee that the pilot would not recover.

I agree with you - with two tumbled AI's and no TC, recovery is
impossible. With a working TC, it is merely highly unlikely.
Therefore, if the panel space is there, it might make sense to keep the
TC. Not much sense, mind you, because if you manage to screw up badly
enough to put the plane into an attitude that would cause both AI's to
tumble, well, I'm willing to give very good odds that you're not going
to recover on the TC.

However, let's say having both the TC and the second (electric) AI is
not practical (probably due to space considerations). Would I rather
have an electric AI, or the TC? I would still prefer the electric AI.
First off, with dual AI's next to each other, I believe that following
a dying AI into an unusual attitude becomes far less likely, and thus
while the chances of recovery from the unusual attitude are reduced
slightly, the chances of encountering it in the first place are reduced
dramatically. Not so with a backup vacuum - you have to engage it.
And even if you do, half the time (in my experience more) the problem
is the AI, not the power source, so backup power for the AI does you no
good.

The problem with this analysis is the reliability (or lack of same) for
electric AI's. I've heard the affordable ones are not good, and the
good ones are not affordable.

Finally, there is the issue of training. If you have dual AI's with
independent power sources, it makes sense to skip partial panel
training. If you have only a single AI, even with redundant power
sources, that's not the case. In that case, a standby vacuum system
seems to be an unjustified expense - the money spent on it is probably
better spent on recurrent training.

Michael

Bob Moore wrote:
After 47 years of civilian, military, and airline flying, I have
yet to see a reason for "turn rate" as long as I have a functioning
attitude indicator.

As a recreational pilot of a simple IFR aircraft, I've recently
discovered a great role for the T&B; staying on heading. After a few
years of struggling to keep my non-AP equipped flivver on a MH, I've
re-discovered what many must know. The T&B is the best instrument to go
straight with. The AI gives you good info, and the heading gyro does
too, but the best way to keep it all centered in my Maule is definitely
the T&B. Especially in calm conditions.

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

Scott Moore wrote:
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


Electric attitude. Vacuum pumps are going away.

But, can't an electric attitude indicator fail? Bearings wear out.
I've suffered that on a hard disk. Are there any MTBF numbers for the
two systems?

Therefore, if the panel space is there, it might make sense to
keep the TC.

I know that FAR 91.205 requires a rate of turn indicator for IFR (except for
airliners with three attitude indicators). Is it common for this requirement
to be waived to allow substituting a second AI for the TC?

I know that FAR 91.205 requires a rate of turn indicator for IFR (except for
airliners with three attitude indicators). Is it common for this requirement
to be waived to allow substituting a second AI for the TC?

In a manner of speaking. No waiver is actually necessary. Advisory
Circular 91-75 details the conditions under which this substitution is
acceptable. Note that 91.205 states:

no person may operate a powered civil aircraft with a standard category
U.S.
airworthiness certificate in any operation described in paragraphs (b)
through
(f) of this section unless that aircraft contains the instruments and
equipment
specified in those paragraphs (or FAA-approved equivalents)

91.75 Details what constitutes an FAA-approved equivalent for a
rate-of-turn indicator.

Michael

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.