Vacuum suction

NealRomeoGolf

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Airplane: Piper Archer II PA28-181
Issue: suction settings for vacuum instruments
POH recommendation: suction gauge reading of 5.0 +/- 0.1
Instrument overhauler recommendation: reading of 5.5
Actual reading in airplane: 5.0

I recently had my AI overhauled. It is taking a while for it to spool up to normal. By a while I mean it doesn't seem normal until cruise. Overhauler says I need to increase my suction to 5.5 by adjusting the regulator. POH says not to. Anyone have experience running the suction higher? Does this seem right?
 
The instrument should operate correctly at the manufacturers specification. How do you know the aircraft's vac indication is correct? That said, I think you have a bad AI.
.
 
Another vote for checking the accuracy of the gauge as a first step to troubleshooting.
 
Something my A&P can do with a calibration device?

Yes.
Check it at the gauge, and at the connection to the AI. A leak anywhere in the old rubber hoses or connections could be the causing your symptoms.
 
Manometer: you can get handheld gizmos that resemble hand held radios etc. or you can make your own. This one is measuring pressure. Measuring vacuum just lifts the fluid in the other arm of the manometer.

 
Your gauge measure suction. Not flow. And it measures it downstream from your instruments. Presumably the filter on the inlet to the AI got changed. Presumably.

You can have all the suction in the world, but if the intake side is restricted, then you ain't gots no flow and you ain't gonna get no spinning going on in your gyro.

Of course your A&P knows all this and would have made sure that this was not an issue.
 
The avionics shop said they did, yet here we are. After a long flight after landing and I pull to idle I can get a vacuum pump warning light too, but I thought that was normal. Maybe my pump ain't sucking so well.
 
Once the bag gets half full, I find changing it and making sure the hose and beater bar are cleaned out, the vacuum works much better. Of course if you have a Shark or Dyson, empty the container and wash the filter.

( Sorry, just couldn't resist. )
 
Once the bag gets half full, I find changing it and making sure the hose and beater bar are cleaned out, the vacuum works much better. Of course if you have a Shark or Dyson, empty the container and wash the filter.

( Sorry, just couldn't resist. )

Isn't that women's work? o_O

[Pause here to duck, as Cajun, Everskyward and my long suffering wife launch heavy projectiles in this direction. I was joking! Really!! :D ]
 
So I went through my scanned logs and found my vacuum pump was installed in 1990 (Airborne 211CC if you care). I can't find any overhauls in the books from then until now. I know my scanned copies are missing some pages from the early 90's so I need to dig out the physical books to look. But even then, my vacuum pump hasn't been overhauled since at least 1996. That seems like a long time ago. How is my pump even still working? Engine is at 1300 hrs since overhaul, so if they overhauled the vacuum at the engine overhaul (is that normal to do?) I would've thought this thing would've failed by now. More to talk about with my A&P....
 
So I went through my scanned logs and found my vacuum pump was installed in 1990 (Airborne 211CC if you care). I can't find any overhauls in the books from then until now. I know my scanned copies are missing some pages from the early 90's so I need to dig out the physical books to look. But even then, my vacuum pump hasn't been overhauled since at least 1996. That seems like a long time ago. How is my pump even still working? Engine is at 1300 hrs since overhaul, so if they overhauled the vacuum at the engine overhaul (is that normal to do?) I would've thought this thing would've failed by now. More to talk about with my A&P....
Vacuum pump is a factory overhaul thing. It's r&r for you. 1300 hours is a bunch on a dry pump. It's about half the life of a wet pump....and yes I think most dry pumps get replaced at overhaul unless the old one is really new. They are relatively cheap.
 
Unless they missed a log entry, that means my pump hasn't been overhauled in 1300 hours. Holy cow. I know the standby pump has been serviced in the mean time, just not the main one.
 
So I went through my scanned logs and found my vacuum pump was installed in 1990 (Airborne 211CC if you care). I can't find any overhauls in the books from then until now. I know my scanned copies are missing some pages from the early 90's so I need to dig out the physical books to look. But even then, my vacuum pump hasn't been overhauled since at least 1996. That seems like a long time ago. How is my pump even still working? Engine is at 1300 hrs since overhaul, so if they overhauled the vacuum at the engine overhaul (is that normal to do?) I would've thought this thing would've failed by now. More to talk about with my A&P....

Neal,

In my experience the average life of a dry vacuum pump is five hundred hours give or take a few hundred; less on those that power deice boots. Some survive longer, some die a premature death. There are a multitude of factors involved in their longevity including maintenance practices, the overall health of the vacuum system, and the number of instruments they are powering. The longest I've seen one last was 2,000 hours in a Mooney where it was only powering the AI. When changing the engine I noticed the vacuum pump had not been replaced since the engine was new. Owner said "Replace it" without equivocation.

There is also credibility to the thought of turning them against the direction of rotation. I recall once hearing a snap when an A&P checking mag timing. It was one of the carbon vanes taking a dump. Not the fault of the mechanic but simply an indication of the wear the pump had encountered in it's service life. A review of the logs proved that observation correct.
 
Ok, found the log entry where they replaced the pump in year 2000. It's a Stratus AA211CC pump and currently has 950 hours on it. Could not find any time where it was overhauled. From my research is appears Stratus pumps were marketed by Aero Accessories. I found a service bulletin that recommends TBO on these pumps at 800 hours. This model doesn't have an inspection port like some of the newer pumps do. Sounds like it's time for an overhaul or maybe upgrade to a Rapco 215CC.
 
This past weekend on startup the suction was showing 4.75 at 1000 rpm but would then move to 5.0 at 2000 rpm (run up). Is that just the age of the pump showing or is it normal to underperform at lower rpm?
 
Wait a minnit, wait a minnit. IF the gauge is correct and IF the pump is pulling 5" why in the world would you overhaul a pump that is producing the correct vacuum? Sure, if the flow is heavy and the pump can't hold 5", then yes, OH the pump. But not if it is pulling its weight.

As to the water manometer idea, as I vaguely recall 1" Hg = 13" H2O, so that damn manometer is going to be at least 5.5 feet tall. That's a moose of a manometer.

Jim
 
The warranty n a Rapco 216 is 3 years, 1300 hours from date of installation or 4 years from date of manufacture whichever comes first.
 
So you would prefer to have a past TBO pump fail in IMC before I overhaul it? Why doesn't it hold suction at low rpm?
 
This past weekend on startup the suction was showing 4.75 at 1000 rpm but would then move to 5.0 at 2000 rpm (run up). Is that just the age of the pump showing or is it normal to underperform at lower rpm?

Sounds normal to me.
 
Well thanks to a pitot leak (causing us to pull the AI to get to the other instruments), we think we've found the AI problem. When the shop (not my A&P) reinstalled the AI after overhaul, the filter didn't go on right. A&P says that is restricting air flow to spool up the AI.
 
Well thanks to a pitot leak (causing us to pull the AI to get to the other instruments), we think we've found the AI problem. When the shop (not my A&P) reinstalled the AI after overhaul, the filter didn't go on right. A&P says that is restricting air flow to spool up the AI.
That would only make sense if the suction gauge was only plumbed to the suction side. Most of those gauges have two ports and are plumbed across the gyros to get the actual differential pressure between the vacuum and air sides, and a dirty filter will cause a decreasing suction reading as the flow decreases through the instruments and the drag within them therefore falls off, decreasing the differential.

As far as how many instruments are being driven by a pump: that has little to do with pump wear. The regulator is just a pop-off affair in the suction line (it has a small foam garter filter on it and is normally found on the inside of the firewall) and it just lets air into the line when the vacuum setting is reached. The vacuum pump is a positive-displacement pump and pumps a given volume at a given RPM (unless it's totally shot) and it doesn't care if that air came through an instrument or through the regulator.
 
That would only make sense if the suction gauge was only plumbed to the suction side. Most of those gauges have two ports and are plumbed across the gyros to get the actual differential pressure between the vacuum and air sides, and a dirty filter will cause a decreasing suction reading as the flow decreases through the instruments and the drag within them therefore falls off, decreasing the differential.

As far as how many instruments are being driven by a pump: that has little to do with pump wear. The regulator is just a pop-off affair in the suction line (it has a small foam garter filter on it and is normally found on the inside of the firewall) and it just lets air into the line when the vacuum setting is reached. The vacuum pump is a positive-displacement pump and pumps a given volume at a given RPM (unless it's totally shot) and it doesn't care if that air came through an instrument or through the regulator.

Well here's my setup. Are the 2 ports sort of marked by 17 and 20 what you're referring to?
IMG_2533.JPG
 
This past weekend on startup the suction was showing 4.75 at 1000 rpm but would then move to 5.0 at 2000 rpm (run up). Is that just the age of the pump showing or is it normal to underperform at lower rpm?

When is the last time the filter was replaced? The pump could be fine and the filter clogged..... -Skip
 
I think I need to consult with my engineer father more often. He agrees with @Dan Thomas. So here's what I think I'm going to do:

Test suction gauge; recalibrate if necessary
Install new Rapco 215CC
Replace all filters
If none of that fixes the problem, increase suction to 5.5 inches
If none of that fixes the problem, sell plane and buy a Mooney. :cool:
 
Sounds normal to me.

You might be right.

It looks like the PA28 series doesn't specify an RPM at which its supposed to make suction at 5.0, the manual I looked at just states "medium RPM" lol.

The C177 manual states "...a range of 4.6 to 5.4 is acceptable. To adjust remove the control air filter and run the engine to 1900 RPM and adjust the relief valve to 5.3-+.1. Ensure filter is clean before installing. If reading drops noticeably replace the filter."

At full power on the ground mine reads about top of the green band. At 5000 feet or so its near the middle at 2400ish RPM. I think around 10k feet its closer to the bottom of the green. (C177)
 
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I think I need to consult with my engineer father more often. He agrees with @Dan Thomas. So here's what I think I'm going to do:

Test suction gauge; recalibrate if necessary
Install new Rapco 215CC
Replace all filters
If none of that fixes the problem, increase suction to 5.5 inches
If none of that fixes the problem, sell plane and buy a Bo. :cool:

FTFY

No point in exposing yourself to any more PoA abuse than necessary by choosing the WRONG airplane. ;)
 
I hear an Aspen would fix the issue with the vaccum pump. Prolly be cheaper than the Mooney or Bo. ;)
 
I hear an Aspen would fix the issue with the vaccum pump. Prolly be cheaper than the Mooney or Bo. ;)
Aspen has some nifty boxes to drive some autopilots. If your autopilot isn't on their list ya might still need the vacuum...yes your plane might still suck with an Aspen.
 
Well here's my setup. Are the 2 ports sort of marked by 17 and 20 what you're referring to?

Those are the ones. But someone may have left off the air side hose and just vented the gauge to the cabin, which throws everything off. It would show good suction even with the filter totally clogged. A visual check of the plumbing might be in order.
 
Step 1 is done. Suction gauge was reading a bit high so we adjusted the regulator valve to increase the suction. Plane needs to be put back together (was working on a pitot leak too) so we can run it and see if the AI spins up better. Still going to order a new pump and install it with new filters.
 
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