Vacuum and TC failure in IMC

DavidWhite

DavidWhite

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This was posted by Matthew Hammer, a CFI and board member and I'm reposting it here with his permission. Please note the suction gauge and red flag on the TC - as he said "Not what you want to see in actual IMC"

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There was a warning sticker on the last vacuum pump I installed that warns "...may cause death". Comforting hu?
 
"L VAC," not "L VAC R," and the vacuum gauge is still showing well above zero with the AI/HI apparently still working properly. Also, the airplane appears to be climbing through 3000 feet, which makes me wonder why the are still in the air and climbing, rather than on the ground (or at least headed that way), which is where they should have been going when the first thing (either left vacuum or the TC) failed.

Or was the party involved in this one of those misguided souls who thinks a vacuum failure in IMC in a vacuum-AI/HI airplane is not a reason to declare an emergency?

And while it's theoretically possible, I'm just not buying simultaneous failure.
 
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As my CFII is fond of saying, when you have two independent failures at the same time, it means that today just isn't your day (or is your day, depending on how you look at it).
 
Ron Levy said:
"L VAC," not "L VAC R," and the vacuum gauge is still showing well above zero with the AI/HI apparently still working properly. Also, the airplane appears to be climbing through 3000 feet, which makes me wonder why the are still in the air and climbing, rather than on the ground (or at least headed that way), which is where they should have been going when the first thing (either left vacuum or the TC) failed.

Or was the party involved in this one of those misguided souls who thinks a vacuum failure in IMC in a vacuum-AI/HI airplane is not a reason to declare an emergency?

And while it's theoretically possible, I'm just not buying simultaneous failure.
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L = Low? IE no Right...
 
Ron Levy said:
Also, the airplane appears to be climbing through 3000 feet, which makes me wonder why the are still in the air and climbing, rather than on the ground (or at least headed that way), which is where they should have been going when the first thing (either left vacuum or the TC) failed.
Click to expand...

I can think of at least one valid reason. Could be over an area below minimums, know that the tops are just a couple hundred feet away, climbing for VMC, then head to where it's VFR, because he did his homework and knows where VFR conditions can be found.
 
If the gauge isn't suck there as in INOP, then I would say there must be a large leak or the regulator crapped out.
 
Ron Levy said:
Also, the airplane appears to be climbing through 3000 feet, which makes me wonder why the are still in the air and climbing, rather than on the ground (or at least headed that way), which is where they should have been going when the first thing (either left vacuum or the TC) failed.

Or was the party involved in this one of those misguided souls who thinks a vacuum failure in IMC in a vacuum-AI/HI airplane is not a reason to declare an emergency?
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If I had a vacuum failure I would want to come down out of the clouds on my own terms. As mentioned before there are a hundred reasons for him to be climbing. A climb on top is a good one. Its much easier to do a wings level climb partial panel than it is to navigate around and shoot an approach. Perhaps ATC is vectoring him to a larger airport that can give him a no-gyro approach, and he needed to go to 4000 to clear terrain....
 
bnt83 said:
L = Low? IE no Right...
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Those models have dual vacuum pumps. If the left fails, you see "L VAC." If the right fails, you see "VAC R." If both fail, you see "L VAC R". The picture shows "L VAC" with about 4.0 inches on the vacuum gauge, and that says the right pump is still working and the AI/HI are still reliable. But I wouldn't continue IFR flight after one of the pumps fails even if the other is still working fine -- put it on the ground and sort it out there. And without hearing from the pilot of that flight, I'm just not believing they lost both the left pump and the TC in quick succession over an area where they could not land and had to climb -- odds are too much against it.
 
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Ron Levy said:
"L VAC," not "L VAC R," and the vacuum gauge is still showing well above zero with the AI/HI apparently still working properly. Also, the airplane appears to be climbing through 3000 feet, which makes me wonder why the are still in the air and climbing, rather than on the ground (or at least headed that way), which is where they should have been going when the first thing (either left vacuum or the TC) failed.

Or was the party involved in this one of those misguided souls who thinks a vacuum failure in IMC in a vacuum-AI/HI airplane is not a reason to declare an emergency?

And while it's theoretically possible, I'm just not buying simultaneous failure.
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Ron,

To answer your questions: yes, the failures were simultaneous in that they took place within 10 minutes of each other. No, I am not one of those "poor misguided souls" who thinks a vacuum failure is not grounds to declare. ATC was notified of the situation. I knew where the tops were based on my previous flight and a careful evaluation of the weather. The reason I'm climbing in the picture is because I knew it was clear up top, and I felt it would be safer to continue to a VFR airport than attempt a landing at the closest airport, which happened to be IFR. It removed us from an overcast layer and allowed us to continue the remaining 20 minutes of the trip in VMC to our destination, which was also known to be VMC.
 
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It does seem like a remarkable (and unfortunate) coincidence; I will, however, wait to learn more before rendering judgment.

I certainly do like my complete redundancy -- electric HS I and AI, an air powered DG and AI. Plus, the S-Tec 30 autopilot, which cares not one whit about what the attitude indicator has to say.
 
Ron Levy said:
Or was the party involved in this one of those misguided souls who thinks a vacuum failure in IMC in a vacuum-AI/HI airplane is not a reason to declare an emergency?
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Which misguided souls would those be? NO ONE in that other thread to which you seem to be referring said that a vacuum failure in IMC in a vacuum AI/HI airplane is not an emergency declaration-worthy event.
 
matthammer said:
I knew where the tops were based on my previous flight and a careful evaluation of the weather. The reason I'm climbing in the picture is because I knew it was clear up top, and I felt it would be safer to continue to a VFR airport than attempt a landing at the closest airport, which happened to be IFR. It removed us from an overcast layer and allowed us to continue the remaining 20 minutes of the trip in VMC to our destination, which was also known to be VMC.
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Ding ding ding! I called that right... Anybody want tonight's lottery numbers? :yikes:
 
matthammer said:
Ron,

To answer your questions: yes, the failures were simultaneous in that they took place within 10 minutes of each other. No, I am not one of those "poor misguided souls" who thinks a vacuum failure is not grounds to declare. ATC was notified of the situation. I knew where the tops were based on my previous flight and a careful evaluation of the weather. The reason I'm climbing in the picture is because I knew it was clear up top, and I felt it would be safer to continue to a VFR airport than attempt a landing at the closest airport, which happened to be IFR. It removed us from an overcast layer and allowed us to continue the remaining 20 minutes of the trip in VMC to our destination, which was also known to be VMC.
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Good thinking.
 
If the vacuum gauge was not in the green on one pump then the other is likely weak as well.

Or it could just be parallax
 
Skylane81E said:
If the vacuum gauge was not in the green on one pump then the other is likely weak as well.

Or it could just be parallax
Click to expand...



It's not parallax. This picture was taken at full power in a climb. It was even worse with power reduced in cruise, and the gyros started getting all messed up during descents.
 
Skylane81E said:
Something is wrong with the system, one pump should easily keep up
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That's what I was thinking back in post #8. dry pumps FAIL and don't make suction after they jam up breaking the platic drive shaft. If that happened to #1 then its dead. #2 will keep up, but the gauge shows a leak in the #2 system.
 
Maybe a dirty shuttle/check valve or something that connects the to systems? I'm not sure how they are plumbed together...
 
bnt83 said:
Maybe a dirty shuttle/check valve or something that connects the to systems? I'm not sure how they are plumbed together...
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Shuttle valve, just think of it like it is a twin with two pumps. However I have seen worn dry pumps get weak shortly before failure, but usually only at low RPM
 
Skylane81E said:
Shuttle valve, just think of it like it is a twin with two pumps. However I have seen worn dry pumps get weak shortly before failure, but usually only at low RPM
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The difference in this case is that unlike a twin, where you should be checking shuttle valve operation every time you shut down/start up, you have no way to secure and then start the pumps in sequence one at a time.

For those unfamiliar, there are both an FAA recommendation and an SB from one of the pump manufacturers (Parker-Hannefin, IIRC) recommending that pilots of twins start and shut down their engines in the same order every time. That shifts the shuttle one way on start and the other on shutdown, giving you a test of its operation in both directions every flight. This is a point of which my observations suggest many twin pilots are unaware.
 
Ron Levy said:
The difference in this case is that unlike a twin, where you should be checking shuttle valve operation every time you shut down/start up, you have no way to secure and then start the pumps in sequence one at a time.

For those unfamiliar, there are both an FAA recommendation and an SB from one of the pump manufacturers (Parker-Hannefin, IIRC) recommending that pilots of twins start and shut down their engines in the same order every time. That shifts the shuttle one way on start and the other on shutdown, giving you a test of its operation in both directions every flight. This is a point of which my observations suggest many twin pilots are unaware.
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That sure is sad design, "safety" of dual vacuum pumps that fails to work as advertised... :mad2:
 
bnt83 said:
That sure is sad design, "safety" of dual vacuum pumps that fails to work as advertised... :mad2:
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I didn't say that. In Matt's case, it appears to have worked more or less as advertised, since he still had his vacuum gyro instruments working well enough to find VMC. The problem is the lack of a means to exercise and check the proper operation of the shuttle valve. What I don't know is just how the second pump on the Cessnas is powered, i.e., if it's electric there is probably a way to shuttle the valve during start so it can be checked and exercised, and if so, that ought to be part of The checklist for anyone flying with that configuration. I know you can do that on Bonanzas with the mechanical primary and electric backup vacuum pumps. I'll have to research that on the Cessnas.
 
Ron Levy said:
I didn't say that. In Matt's case, it appears to have worked more or less as advertised, since he still had his vacuum gyro instruments working well enough to find VMC. The problem is the lack of a means to exercise and check the proper operation of the shuttle valve. What I don't know is just how the second pump on the Cessnas is powered, i.e., if it's electric there is probably a way to shuttle the valve during start so it can be checked and exercised, and if so, that ought to be part of The checklist for anyone flying with that configuration. I know you can do that on Bonanzas with the mechanical primary and electric backup vacuum pumps. I'll have to research that on the Cessnas.
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It's obvious. With duel engine driven pumps on a common manifold and check valve, there should not be a change in gyro performance as stated in post #22. The system barely worked, certainly not as advertized. Is 3 inches of vacuum enough to run the gyros when they were new? How about old gyros? Anything over 3" will prevent the "Low Vac" annuciator from coming on assuming the switch isn't stuck...


matthammer said:
It's not parallax. This picture was taken at full power in a climb. It was even worse with power reduced in cruise, and the gyros started getting all messed up during descents.
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On airplanes without Garmin G1000, the source of vacuum air is in the cabin and is pulled through
the system by the engine-driven vacuum pumps. This air goes through the gyro filter at the cabin
inlet source before it goes through the vacuum gage and gyro instruments. The vacuum is controlled
by the regulator valve. The regulator valve is on the aft side of the firewall. The vacuum air is then
pulled through the vacuum manifold and past the low vacuum annunciator switches and then into the
vacuum pumps"
(1) The vacuum gage gives a direct indication of the system vacuum in inches of mercury (in.hg.).
(2) The low vacuum annunciator switches are part of the panel annunciator warning system.
(a) If the left vacuum switch (SN012) senses a vacuum below 3.0 in.hg., the VAC annunciator
will show L VAC.
(b) If the right vacuum switch (SN011) senses a vacuum below 3.0 in.hg., the VAC annunciator
will show VAC R.
(c) If both switches sense a vacuum below 3.0 in.hg., the VAC annunciators will show L VAC
R.
(3) For more information on the maintenance practices for the panel-mounted annunciator (UI005),
refer to Chapter 31, Annunciator Panel - Maintenance Practice
 
Ron Levy said:
I didn't say that. In Matt's case, it appears to have worked more or less as advertised, since he still had his vacuum gyro instruments working well enough to find VMC. The problem is the lack of a means to exercise and check the proper operation of the shuttle valve. What I don't know is just how the second pump on the Cessnas is powered, i.e., if it's electric there is probably a way to shuttle the valve during start so it can be checked and exercised, and if so, that ought to be part of The checklist for anyone flying with that configuration. I know you can do that on Bonanzas with the mechanical primary and electric backup vacuum pumps. I'll have to research that on the Cessnas.
Click to expand...

Both are engine driven, the left right is a bit of a mis nomer. Top and bottom would be better descriptors. Stand by for pics
 
Skylane81E said:
Both are engine driven
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Guess that's better than just one mechanical pump, but certainly not optimal, and the lack of a means to exercise/test the shuttle valve is just asking for trouble in event of single pump failure. I've been in twins where the owner/pilot had never exercised the valve, and when we tested it, the valve failed to move. That'll make a believer out of someone.
 
FWIW, the MM just calls is a "check valve"... and the system is designed in such a way the pilot will only know if one system or the other is malfunctioning when suction falls below 3 inches on one side or the other, which appears to be well below the green arch on the vacuum gauge.



So it appears to be that you could dispatch with a weak pump and not get an annuciator and when the other pump fails, have inadequate suction to drive gyros but no annunciator on the "good' side
 
bnt83 said:
That sure is sad design, "safety" of dual vacuum pumps that fails to work as advertised... :mad2:
Click to expand...

I think more likely is two pumps masked issues that would have been known on a single pump system.

It has it's limitations but still beats the hell out of all or nothing.
 
bnt83 said:
FWIW, the MM just calls is a "check valve"... and the system is designed in such a way the pilot will only know if one system or the other is malfunctioning when suction falls below 3 inches on one side or the other, which appears to be well below the green arch on the vacuum gauge.



So it appears to be that you could dispatch with a weak pump and not get an annuciator and when the other pump fails, have inadequate suction to drive gyros but no annunciator on the "good' side
Click to expand...

I have never take it apart to find out but it could be just a double check valve. That would probably be less failure prone.
 
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