Standby Vacuum?

Scott Moore wrote:
: I agree with most of what you say, but this mystical belief by pilots that because
: VAC is a "different" system than electrical it is more secure to have both.
: Electrical systems can be made fully redundant, with separate generator, battery
: and even distribution system and breakers. The fact that your airplane is not
: comletely redundant may or may not degrade its reliability to LESS than the
: inferior reliability of a vac pump.

Even without full redundancy, single point failures must still be eliminated.

: For my own part, I'll be glad to see the vac components go. I'm tired of worrying
: about the low replacement times on the vac pump, the posibility of contamination,
: the lower general reliability of the components, etc.

I've done a number of systems designs where critical components, failure
modes, and redudancy were of major concern, so I am familiar with what goes into such
a system. I'm not saying that a fully redundant electrical system can be done... it
certainly can. My point is that for your average single-engine spam-can, fully
redundant electrical systems are generally overkill. Both financially and from
additional weight, a single-engine plane is not the right platform for a fully
redundant electrical system.

A vacuum pump, regulator, and a few feet of tubing provide relatively
inexpensive, completely redundant attitude information. Yes it's much less sexy and
less reliable than a fully redundant electrical system, but it's *MUCH* simpler,
cheaper, and lighter.

-Cory

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

I have an electric DG and AI in my Extra 300, which can be installed in a
removable panel for cross country flying. Neither has tumbled with gentle
acro (rolls, loops, spins), although it's probably hard on the bearings.

I suspect it would take more than an unusual attitude to tumble the gyros.

wrote:
A vacuum pump, regulator, and a few feet of tubing provide relatively
inexpensive, completely redundant attitude information. Yes it's much less sexy and
less reliable than a fully redundant electrical system, but it's *MUCH* simpler,
cheaper, and lighter.

Not only that, but it's already installed in most aircraft and doesn't
require a new Act Of Bereaucracy to make it legal.

-Luke

Dave Butler wrote:

Did I miss any categories?

Redundant engine driven vacuum pumps.
Quite common on twins and some of the bigger singles. The rub on
singles is having a place to mount the second one.

Jose wrote:
I think that if you really feel the need for redundancy, the sensible
solution is replacing the T&B with an electric AI.


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.

Jose
If I remove the T&B the autopilot won't work!

Ron Natalie wrote:

Redundant engine driven vacuum pumps.
Quite common on twins and some of the bigger singles. The rub on
singles is having a place to mount the second one.

Newer 172s (172R, 172S) even have dual vacuum pumps. That doesn't help
retrofitting, though.

- Andrew

Ron Natalie wrote:
: Redundant engine driven vacuum pumps.
: Quite common on twins and some of the bigger singles. The rub on
: singles is having a place to mount the second one.

Now, even *I* say that's silly for a single. For a twin, sure... since it's likely already in place.
There's still a single point of failure (instrument itself)... An electric AI is more effective. Too bad they're
extra-ridiculously aviation priced.

-Cory
--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
Scott Moore wrote:
: I agree with most of what you say, but this mystical belief by pilots that because
: VAC is a "different" system than electrical it is more secure to have both.
: Electrical systems can be made fully redundant, with separate generator, battery
: and even distribution system and breakers. The fact that your airplane is not
: comletely redundant may or may not degrade its reliability to LESS than the
: inferior reliability of a vac pump.

Even without full redundancy, single point failures must still be eliminated.

: For my own part, I'll be glad to see the vac components go. I'm tired of worrying
: about the low replacement times on the vac pump, the posibility of contamination,
: the lower general reliability of the components, etc.

I've done a number of systems designs where critical components, failure
modes, and redudancy were of major concern, so I am familiar with what goes into such
a system. I'm not saying that a fully redundant electrical system can be done... it
certainly can. My point is that for your average single-engine spam-can, fully
redundant electrical systems are generally overkill. Both financially and from
additional weight, a single-engine plane is not the right platform for a fully
redundant electrical system.

A vacuum pump, regulator, and a few feet of tubing provide relatively
inexpensive, completely redundant attitude information. Yes it's much less sexy and
less reliable than a fully redundant electrical system, but it's *MUCH* simpler,
cheaper, and lighter.

-Cory

I disagree. A battery backed up electric AI is not cheap, but compared to all of the
components in a full vac system, it is, and its far more reliable. Even HAVING a
current VAC system does not eliminate that cost, since the MTBF for a vac pump is
down below 1000 hours, meaning you are going to have to change it soon in any case.

Jose wrote:

No other instrument provides turn rate
information.

Why is rate of turn important compared to having another AI?

Why is rate of turn important compared to having another AI?

Because then you know how fast you're turning, which is new information.
Another AI only tells you the same thing you already know from the
first AI.

If you are asking which partial panel technique I prefer, I have found
that in the airplanes I fly (which are not very high performance), I am
more precise focusing on the TC than on the AI anyway. I'll defer to
those in high performance aircraft that in that case it's different, but
in an emergency, dropping the flaps and extending the gear should turn a
slippery high performance aircraft into a more stable platform to get
down in.

Jose
--
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