Back-up vacuum system

Back-up vacuum system

<HTML>I'm an instrument pilot and do a lot of weekly commuting.  I am considering installing a back-up vaccum system in my 1973 C-172. 

The back-up vacuum is supplied by the engine.  A shuttle valve arrangement is used to manually switch from the engine driven vacuum pump supply to the manifold upon loss of suction pressure i.e. vacuum pump failure. A minimum of 3.6 inches vacuum on the vacuum gage is required for reliable instrument operation.  For a 4-cyclinder Lycoming like mine 150 hp, at high altitude say 10,000 ft, it may require a decent to some lower altitude to achieve this minimum.

The aviation shop at the field suggested this system. The aviation shop is used extensively by the FBO and come highly recommended. They did most of the maintenance and annuals on my plane (by the prior owner).  They seem very competent and did not try to sell me a much higher priced electric or second engine driven pump.

I like its simplicity and cost - less than $700 installed.  Comes with the required placards and owners manual updates. However, I could not find an STC on the following website based on a search of the "description" field:
http://av-info.faa.gov/stc/

Has anyone heard about this type of system and any recommendations for or against?</HTML>

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Re: Back-up vacuum system

<HTML>There are several other alternatives you might want to consider.

If you don't have a problem with the aesthetics and slight drag imposed by a venturi, you might consider one of those with a shuttle valve to switch between the vacuum pump and the venturi.  Venturis are stone simple and, depending on the size, can easily power both DG and AI.

An electrically powered attitude indicator is another option (albeit expensive), and you'd have redundancy not only in the source of power (vacuum and electrical system) but in the instrument itself (one vacuum powered AI, one electrically powered AI).  You could position the electric AI on the co-pilot's side of the panel.

Finally, Aero Advantage showed a dual rotor/dual chamber vacuum pump at Sun N Fun this year that's supposed to be STC'd for use in most certificated airplanes later this year.  You can look at it on their website at http://www.aeroadvantage.com/ .

I don't think I'd consider an electric motor powered vacuum pump as they are very expensive and typically draw several amps continuosly.</HTML>

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Re: Back-up vacuum system

<HTML>I don't have a high opinion of the SVS by Precise.  It has some pretty serious operating limitations and the physical installation isn't... shall we say...pretty.

A good heated moose whistle will run both your DG and AI in IFR conditions without adding another component to think about in an already crowded enviroment.

To my knowledge there isn't an STC for venturi driven vacuum systems.
Venturi's require preflight inspection and don't work well at low airspeeds.

Cheers,
RH</HTML>

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Re: Back-up vacuum system

<HTML>Precise Flight, Inc.
63120 Powell Butte Rd.
Bend OR 97701
Has the STC you are looking for: It is SE1779NM and the LYC 0-320 is included.
Spruce and Chief sell it. I have no 172 experience but several 180's and 182's I have knowledge of  have it installed and it works fine. It will get you down safely asap to repair the pump .

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Re: Back-up vacuum system

<HTML>Thanks for all the good info.

Fred,
That's the exact system that was suggested, which Roger doesn't care for.  Thanks for the address and STC.

Roger,
I have no idea what a "heated moose whistle",,, The image it conjures up isn't pretty.
I'm checking into the "Precise" performance limitations and maybe get some digital pictures from them.
I do expect to have a vacuum pump failure at some point.  The idea is to obtain a backup that will work in a pinch. I will accept some performance limitations for this infrequent operation.  If I was flying in the Rockies or Sierra's I would be more concerned.  A C172 in high mountain flying isn't the preferred aircraft, especially IFR.  On the east coast it should be fine most of the time.  When I get the restrictions I'll post them here.

The venturi doesn't appear to be an option if it is not STC'd.  Also why penalize myself all of the time for a very infrequent occurence. 


Stan,
The dual rotor design sounds a little dodgy. I assume that both rotors are operating all of the time and the status of each is known. So the second standby rotor may be in poor condition when it is needed and comes under load.   Otherwise if it doesn't operate, then standby might sieze when you need it.  That has occured with electrical stanby pumps if they are not operated frequently.

Having said all that I'll check into it and its cost.</HTML>

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Re: Back-up vacuum system

<HTML>Got the POP (Poop on Performance) of the SVS system,

Website: http://www.preciseflight.com/standby.html

I spoke with Chuck at precise flight.

In the C172, he said not to expect being able to maintain altitude above 8000' (density altitude).  You need to maintain a 4 inch of mercury difference at the shuttle valve to obtain good reliable instrument operation.
The limitation comes because you have to throttle back to achieve a this difference between manifold pressure and ambient conditions.
I do not have charts that show manifold pressure difference versus versus brake hourse power.   So at standard atmospheric, ambient pressure is 22.22 inches and you need 18.22 manifold pressure. 
He expected that to be about 55% power.

This limitation is not unmanageable.  Additionally, part of the whistle-and-bells that come with the SVS is a panel mounted light indicating vaccum failure.

The old systems SVS III have a costly AD associated with it.  Check out AOPA
http://www.aopa.org/whatsnew/regulatory … cise.html.

The continued Airworthiness Requirements for new system is done during the annuals and as part of your preflight. The annual check involves such items as control cable inspection, installation integrity, and check of the vacuum connections. The intial installation includes a flight test. And the prelight is an operational check at idle when differential pressure is maximized and engine driven vacuum is minimized.

They claim with installation the total cost is under $1,000. 
My shop estimate about 6 hours labor and expect the total cost to be about $700.  I expect a price change as the cost at the site is $459 for the components.</HTML>

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Re: Back-up vacuum system

<HTML>Don't fly in high-elevation areas if you need the manifold pressure.  I disagree with the statement that venturis are unreliable or incapable at low airspeeds.  My 170 has dual venturis as original equipment and they give 4" Hg at 75 mph indicated.  Venturis are better than any other vacuum system in my opinion, when it comes to reliability and endurance.  You just don't have gyros all spun up and working before lift-off, but they work fine after only 30 secs of flight.  (You might spend that $700 for that system to spin 'em up before takeoff if it was real important to you, but I'd question using a single engined airplane for IFR conditions so low you couldn't take off without gyros.)  (Roger---Heated Moose-whistle.....that's hilarious.  I love it!!)</HTML>

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Re: Back-up vacuum system

<HTML>PS.  In fact, venturis are my only source of vacuum.  They don't fail, so I don't need a backup vacuum system.  (Lots of folks like to start imagining problems in icing conditions, but if you're in icing conditions in a 172-type airplane you've got lots more to worry about than un-heated venturis.  Don't fly in ice.)</HTML>

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Re: Back-up vacuum system

<HTML>George,  I incorrectly said low airspeed instead of low ground speed <grin>.

Barry, the SVS by Precise is one of those designs which, like T-tails and ballistic recovery systems, is "good in thoery but bad in practice". 
The installation involves ruining a prefectly good intake manifold and then hose clamping the stem of a bicycle innertube to it.  The installation is not a "fix it and forget about it" affair.  It must be inspected and has life limited parts.

Another avenue you may try is the Champion vacuum pump which allows for inspection of the vanes.  This will let you know when the pump is about to fail as opposed to looking at a slowy rolling AI after the failure.

Also, while I'm thinking of it, purchase the cooling shroud for your vacuum pump.  I've had excellent results with these on the trainers- extended the life a good 200hrs.

An inline filter between the pump and the gyros is another good idea.  This keeps the chunks of graphite from being sucked into your gyros after the pump fails.

I know of at least one 172 with a venturi as a backup vacuum system.  The field approval should be a no brainer especially if you use a FAA-PMA'd parts.


Cheers,
RH</HTML>

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Re: Back-up vacuum system

<HTML>Thanks once again for all the input and wisdom.

I'll look further into the inline filter for instrument protection regardless of what backup I choose.  Additionally, like the idea of a cooling shroud - didn't known they made one for it.

The venturi is starting to sound better - thanks for the unabashed input.  I guess I'm still too much a novice.  I would like to know the gyros are working before I take off in IMC.  I guess I could go around, as my minimums are high enough to land without entering the clouds.

When gyros fail (for other reason than no air flow), does it tend to be a slow process where you get plenty of warning signs?</HTML>

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Re: Back-up vacuum system

<HTML>The inline filter idea.  I'm not sure it's necessary. Roger, have you witnessed  this problem first hand? 

The instruments are on the upstream side of the vacuum systems.  Air is sucked in through a filter then through a restriction upstream of the instruments. The suction gage reads pressure at that calibrated restriction.  Then air goes through the instruments and a Y connection.   The other branch sucks air through an adjustable vacuum relief, which is used to set your vacuum pressure. The main branch collects the air from both branches and directs it to the inlet of the Vacuum pump.

The Vacuum pump discharges the air in the engine compartment about 6 inches of tubing from the pump casing to exit of the discharge.  The pump inlet side is at least 4 feet from the instruments (in my C172).  Since the system is open on both ends with no restrictions on the disharge side except for the failed pump shaft and impeller. On the other end, the path to the instruments is a filter with the one calibrated restriction at the inlet. The reverse flow through both sides has a race to fill the vacuum in the instruments.  I'm sought of surprised you would get enough reverse flow to get any of the particles all the way back to the instruments.</HTML>

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Re: Back-up vacuum system

<HTML>Barry, you might wish to check your installation with the illustrated parts manual for your aircraft.  There should be no "restriction" betwixt the central airfilter and the gyros.  The only "restrictor" ever installed in a vacuum sytem (to my knowledge) is within a dedicated Turn/Bank sytem in order to reduce vacuum to a T&B instrument.  This would never be installed in an attitude or directional gyro system.  In such systems there should only be a vacuum regulator which is situated between the gyros and the vacuum source, which acts as a vacuum  "relief" valve to reduce the vacuum applied to the system.</HTML>

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Re: Back-up vacuum system

<HTML>I can't say there is a direct correlation between vacuum pump failure and subsequent gyro failure.

I can say that I've seen these filters with 500hrs on them and they're full of crud.
I can also say that on the air inlet side of most gyros there is a 0.3 micron filter which is not on the vacuum outlet side.

2-1/2psi (5"Hg) isn't a lot of pressure and the likelyhood of carbon contamination reaching the instruments is slight at best.  An inline filter IMHO is just good preventative maintenance as the filter is around $50 compared to (2) $500 gyros.

Another often forgotten option you have is the wet vacuum pump. 
Most people shy away from these as they're heavier and, even with an air/oil seperator, tend to make the belly a little dirty.

The plus side- they outlast most engines.  They're tough lil suckers (pun intended) that will bolt on to the engine with minimal paperwork headaches.

Good luck Barry</HTML>

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Re: Back-up vacuum system

<HTML>I would take a wet pump any day over a dry air pump.  I have yet to see one  totally fail before engine TBO.  I have seen a couple that slowly decreased suction output and required change, but they were still producing suction and did not fail unpredictably.  The slow degradation was noted and the pump changed.  What more could you ask for?  A heated Moose whistle perhaps?</HTML>

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Re: Back-up vacuum system

<HTML>George,
I stand corrected.  Bad choice of words.  What I  was refering to the fitting to which the vacuum gage hose is connected. The characteristics of the fitting is known where the relation between indicated vaccum pressure and system performance is known.

Roger,
I looked at an SVS installation in a C182. Turns out they do not make a hole in the metal pipe to the manifold.  They connect at a convenient synthetic/flexible hose connection. This would be easily reversible if later I didn't like it or a new owner didn't want it.

I asked about the graphite or any other secondary consequences of vacuum pump failure.  None were identified.  The mechanic thought maybe you are referring to is failure in a Bonaza.  He said that's the only plane that uses pumped air in stead of a vacuum system. 

John,
The previous owner replaced the failed wet vacuum pump with the current dry one.  I don't know why.  Unfortunately he died just as I was submitting an offer on the airplane so I never got to talk with him.  There's that heated moose whistle again!  Do you think there's a market for it?</HTML>

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Re: Back-up vacuum system

<HTML>  Take the $700, buy a $5 set of two suction cup instrument
coverups, and spend the other $695 for 10 hours of no-gyro
instruction (some in a good sim).
  I had a vac failure night IMC after 11 hours of flying solo.
I got the first two steps done:

1.  Because of a good scan, I noticed AI showed 5 deg up
climb but everything else set for cruise.  Checked for ice
(none), and noticed vac gauge was ZERO.  You do have the
gauge between the two instruments, right?  Also a vac off
flag for the AI?

2.  I then covered the AI and DG with covers I ALWAYS keep
suctioned on the left window.

  That was it.  Once the problem was discovered and the
distracting instruments covered, the rest was really easy.
I did timed turns to the turn-and-bank, and my vertical
card compass was good during straight and level.  I finished
the LOC approach and landed.

  OK, if you do daily ILS work, maybe a backup is a good idea,
but for non-precision altitudes (500 AGL) and 10-20% of
your flying in IMC, extra training and coverups are a better
deal because you can use them in any plane.

  Oh, I also put a new 1000 hour warranty vac pump on.

  The biggest problems NASA found were because:

1.  The pilots didn't recognise the problem for a full
six minutes, on average.

2.  Some pilots didnt cover the distracting/false instruments.

  Backup systems don't solve this problem at all.</HTML>

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Re: Back-up vacuum system

<HTML>I do have training in partial panel work. In fact an instructor just pull that trick on me recently.  We train for a lot of situations, but if a can avoid the actual problem and extra work load, why not.  Life is interesting enough!

I also have a backup radio and navigation capability.  I could use old fashioned pilotage (and sometimes do) and land at an uncontrolled field, but I see no problem in having two radio's and making sure I can communicate with those who leave thier's on.  It just makes flying safer.

If I flew every day, I'd probably feel the way you do.  But for now, only flying 8 to 10 hours a week and about a third of the time in IMC I would say I was capable but why press my luck.  Add a couple of other issues, like bad weather and you may end up having a really exciting flight!  And what about the passengers who are oblivious to what's going on and trusts you to get them down safely.  Put all this together and I vote for the extra vacuum system. (It's only money!)</HTML>

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Re: Back-up vacuum system

<HTML>Barry,

  Then go electric.  Get a 3"+ for $1200 or a little
clock size one for $2000, but make sure they're electric.

  Some important info:

  A turn coordinator sucks in turbulence.  Always get a turn and
bank instead.

  If your engine seizes, your AI and DG are gonna fail in 5
minutes too (vacc pump is on the engine).

  You lose the AI before the DG starts spinning.

  Training in a real airplane is somewhat worthless.  Get
a sim that gives you an invisible failure (slow, realistic
failure).

  Even with two AI's, what failed?  Was it the ASI?  The
first AI or the second AI?  I've lost an ASI while IFR also,
that was hard to diagnose.

  So here's the bad news...


1.  During my vacc failure IFR night my AI off flag did not work
and it still doesn't.  When the pump failed the contamination
killed the flag.  I'm gonna need a new AI before any IFR.

2.  If you have a backup with the same power source (one pump
or one engine), they both fail if that source goes.

3.  If you have two AI's, you now need extra scan and you
gotta figure out which one is failing.

4.  Just look at the flag, right?  Wrong.  I recently
did some extra training with an A36 driver who had a
subtle HSI failure and reverted to his portable GPS
in IFR.  Worked fine.  The elect HSI started a very slow
spin.  It seems the flag doesn't activate for gyro failure,
just electric failures or signal failures to the navaid
parts.  Power was still going to the gyro, (so no flag) but
it wasn't spinning.  Bad deal, huh?

  So your best bet is a battery operated GPS on a heading mode
with other fields showing ground speed and altitude.  If
you put this in your scan and split 50/50, you will live.
I've trained a half dozen IFR students this way and it works
better than anything else if the GPS is mounted somewhere
good and the student DOESN'T @!#$ WITH IT!  Otherwise it's
a distraction.  Don't try to spell out MUNSO ifr.

  O, and you can use the handheld for "situational awareness"
on an IFR flight test.  Cool, eh?

  The best thing about GPS is it's independent.  If it stops
tracking you, everything goes at once (knots, track, HDG, etc).
If you lose electric at night, the GPS is already with a backlit
independent screen.

  OK, so there's also a PDA attitude indicator/GPS setup for
$2500 that is totally seperate from the aircraft that I've heard
of, but I don't know any more about it.

  So two engines, an electric backup AI (I like the 2 1/2" idea),
and a backlit handheld GPS.  By the way, you CAN use the VFR
GPS to go IFR direct http://asrs.arc.nasa.gov/callback_issues/cb_259.htm

  So I think that is safest.  Of course, a second pilot is even better, but then you'd be an airline, right?

  One of my airline buddies (ATP with 50K hours and a V35B)
says if you fly IFR in a single engine airplane
you should just squawk 7700 for takeoff...

  After two engine failures day VFR above 5000, I'm lucky
but I know what he means...

Didn't mean to scare you...

Mark Boyd</HTML>

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Re: Back-up vacuum system

<HTML>Barry,

  Then go electric.  Get a 3"+ for $1200 or a little
clock size one for $2000, but make sure they're electric.

  Some important info:

  A turn coordinator sucks in turbulence.  Always get a turn and
bank instead.

  If your engine seizes, your AI and DG are gonna fail in 5
minutes too (vacc pump is on the engine).

  You lose the AI before the DG starts spinning.

  Training in a real airplane is somewhat worthless.  Get
a sim that gives you an invisible failure (slow, realistic
failure).

  Even with two AI's, what failed?  Was it the ASI?  The
first AI or the second AI?  I've lost an ASI while IFR also,
that was hard to diagnose.

  So here's the bad news...


1.  During my vacc failure IFR night my AI off flag did not work
and it still doesn't.  When the pump failed the contamination
killed the flag.  I'm gonna need a new AI before any IFR.

2.  If you have a backup with the same power source (one pump
or one engine), they both fail if that source goes.

3.  If you have two AI's, you now need extra scan and you
gotta figure out which one is failing.

4.  Just look at the flag, right?  Wrong.  I recently
did some extra training with an A36 driver who had a
subtle HSI failure and reverted to his portable GPS
in IFR.  Worked fine.  The elect HSI started a very slow
spin.  It seems the flag doesn't activate for gyro failure,
just electric failures or signal failures to the navaid
parts.  Power was still going to the gyro, (so no flag) but
it wasn't spinning.  Bad deal, huh?

  So your best bet is a battery operated GPS on a heading mode
with other fields showing ground speed and altitude.  If
you put this in your scan and split 50/50, you will live.
I've trained a half dozen IFR students this way and it works
better than anything else if the GPS is mounted somewhere
good and the student DOESN'T @!#$ WITH IT!  Otherwise it's
a distraction.  Don't try to spell out MUNSO ifr.

  O, and you can use the handheld for "situational awareness"
on an IFR flight test.  Cool, eh?

  The best thing about GPS is it's independent.  If it stops
tracking you, everything goes at once (knots, track, HDG, etc).
If you lose electric at night, the GPS is already with a backlit
independent screen.

  OK, so there's also a PDA attitude indicator/GPS setup for
$2500 that is totally seperate from the aircraft that I've heard
of, but I don't know any more about it.

  So two engines, an electric backup AI (I like the 2 1/2" idea),
and a backlit handheld GPS.  By the way, you CAN use the VFR
GPS to go IFR direct http://asrs.arc.nasa.gov/callback_issues/cb_259.htm

  So I think that is safest.  Of course, a second pilot is even better, but then you'd be an airline, right?

  One of my airline buddies (ATP with 50K hours and a V35B)
says if you fly IFR in a single engine airplane
you should just squawk 7700 for takeoff...

  After two engine failures day VFR above 5000, I'm lucky
but I know what he means...

Didn't mean to scare you...

Mark Boyd</HTML>

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Re: Back-up vacuum system

<HTML>Mark,

Thanks for all the good feedback.  You've given me some good ideas and supported other's I was wondering about.

You also got me thinking about the engine failure scenario.  As long as the prop is windmilling and the engine somewhat intact, I can adjust the throttle to take advantage of the any suction being produced to keep the DG and AI spinning as long as possible.  The disadvantage is that the spinning prop works against me due to drag, so I might as well get something out of it since I can't stop it. (Never would have thought about it before you mentioned it.)  That's why I really enjoy this forum - a lot of good ideas are shared and inspired.

I did think of the HSI so thanks for validating that.  I could definitely use the GPS as part of the scan in this type of situation.  It beats using the bouncing compass.</HTML>

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Re: Back-up vacuum system

<HTML>One of the PDA system your referring to is http://www.pcflightsystems.com/

It works pretty well.

Cheers,
RH</HTML>

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