Vacuum Pump Replacement

sky1949

sky1949

Pre-Flight
Joined
Aug 16, 2008
Messages
30
Location
Billings, MT
Display Name
Display name:
skym
My dry vac pump has about 650 hours on it. No signs of failure, but there is some opinion out there that after 500 hours one is living on borrowed time with one of those. I have a backup pump, but I'm wodering what the consensus is on just replacing the main pump before the inevitable failure, even though there is no sign presently of any problem.
 
Since it will likely cost just as much now as later, and you have a backup pump, I'd run to failure.
 
Ron Levy said:
Since it will likely cost just as much now as later, and you have a backup pump, I'd run to failure.
Click to expand...
It would be a good idea to check the operation of the backup and vacuum loss detection occasionally if you're not doing that already. Rapco makes a pump that allows testing the vane length providing a way to predict the remaining time. I'd also recommend purchasing the replacement now and carrying it with you so you won't have to pay list.
 
I got less than 300 hours out of my last vacuum pump. Usely if I find myself thinking or worrying about something, I replace it. It's one less thing to have on your mind while your flying.
 
Ron Levy said:
Since it will likely cost just as much now as later, and you have a backup pump, I'd run to failure.
Click to expand...

A proper functioning pump creates a vacuum in the system lines. When the pump fails internally, the carbon rotor and vanes break into several pieces creating very fine particles of carbon and carbon dust. The vacuum that is present will suck carbon debris back up into the inlet hose possibly contaminating the regulator. In some cases instrument contamination can occur. It is very important to remove the hoses from the aircraft and clean them out thoroughly, making sure to remove all loose debris. It is imperative to clean the entire system after a pump failure. This preventive maintenance procedure will eliminate carbon F.O.D. from entering your new pump.

Run it till failure if you want, just be prepared to pay more in the long run.
 
Rotor&Wing said:
A proper functioning pump creates a vacuum in the system lines. When the pump fails internally, the carbon rotor and vanes break into several pieces creating very fine particles of carbon and carbon dust. The vacuum that is present will suck carbon debris back up into the inlet hose possibly contaminating the regulator. In some cases instrument contamination can occur. It is very important to remove the hoses from the aircraft and clean them out thoroughly, making sure to remove all loose debris. It is imperative to clean the entire system after a pump failure. This preventive maintenance procedure will eliminate carbon F.O.D. from entering your new pump.

Run it till failure if you want, just be prepared to pay more in the long run.
Click to expand...
I've heard people say that, but I haven't seen it happen, including when my vacuum pump went belly-up in flight. It's just not worth spending the money for a new pump every 100/300/500/??? hours when the one I have is working fine and there's no accurately predictable MTBF and it won't do more than inconvenience me if it quits.
 
Ron Levy said:
I've heard people say that, but I haven't seen it happen, including when my vacuum pump went belly-up in flight. It's just not worth spending the money for a new pump every 100/300/500/??? hours when the one I have is working fine and there's no accurately predictable MTBF and it won't do more than inconvenience me if it quits.
Click to expand...

You got lucky, once. Overhauling both the AI and DG, replacing filters, cleaning out the lines and regulator cost money. I've seen this happen several times by owners like yourself trying to save a few nickels only to wind up paying much more. False economics.

But don't take my word for it, just call any reputable instrument shop and ask them their opinion.

Kinda like Dirty Harry asking "Well, do you feel lucky?"
 
Last edited:
sky1949 said:
My dry vac pump has about 650 hours on it. No signs of failure, but there is some opinion out there that after 500 hours one is living on borrowed time with one of those. I have a backup pump, but I'm wodering what the consensus is on just replacing the main pump before the inevitable failure, even though there is no sign presently of any problem.
Click to expand...

http://flash.aopa.org/asf/mechanics/swf/flash.cfm?
 
sky1949,
Here is my experience with vacuum pumps. In 3300 hrs I have had three failures of new pumps. Two of the three had cooling kits. I flew them all till they failed. I have one now that has 900hrs. Two went over 1300hrs and one lasted about 50hrs. I had dual pumps on my T-210 (they lasted over 1300 each). I am thinking about buying a replacement to have around for the 900hr pump but right now I am not getting in a rush. My conclusion, if you buy new pumps I think you can count on 1000hrs. I fly fairly high (turbo aircraft) and still get more than 1000hrs. In the last 20-30hrs of a pumps life I have noticed lower than normal suction when idling the engine. Bend radius of hose also affects pump life as do other variables.
 
Walt said:
sky1949,
Here is my experience with vacuum pumps. In 3300 hrs I have had three failures of new pumps. Two of the three had cooling kits. I flew them all till they failed. I have one now that has 900hrs. Two went over 1300hrs and one lasted about 50hrs. I had dual pumps on my T-210 (they lasted over 1300 each). I am thinking about buying a replacement to have around for the 900hr pump but right now I am not getting in a rush. My conclusion, if you buy new pumps I think you can count on 1000hrs. I fly fairly high (turbo aircraft) and still get more than 1000hrs. In the last 20-30hrs of a pumps life I have noticed lower than normal suction when idling the engine. Bend radius of hose also affects pump life as do other variables.
Click to expand...

We buy pumps with the indicator ports and find that they run until around 1100 hours on Lycomings. We don't have any Continentals but they spin their pumps faster and they'll wear out sooner. Both Tempest and Rapco sell pumps with indicator ports; the Rapco uses a small probe to determine vane length via the side of the pump so you need some room there, and the Tempest has a port in the back for direct visual observation but you'll need a little room behind the pump to get a mirror behind it to see.

We didn't have indicator pumps when we had the O-200s, but I can imagine the Tempest will be harder to check since the O-200's pump is vertically mounted and the vane could be anywhere in the slot when at rest, especially after the shaking at shutdown. You'd need to stick a small wire in there and try to coax the vane up against the wall. On the Rapco the probe will shove them to the bottom of the slot and do its thing OK.

Dan
 
Rotor&Wing said:
You got lucky, once. Overhauling both the AI and DG, replacing filters, cleaning out the lines and regulator cost money. I've seen this happen several times by owners like yourself trying to save a few nickels only to wind up paying much more. False economics.

But don't take my word for it, just call any reputable instrument shop and ask them their opinion.

Kinda like Dirty Harry asking "Well, do you feel lucky?"
Click to expand...

You have a difficult time convincing me the debris that destroyed the istruments came from the pump.

do the math, your vac system operates at 4.8 to 5.2Hg, that isn't much pressure but the flow is about 5-6 cubic feet of air passing the inlet filter into the system.

when you figure the a standard day is 29.92 Hq which equals 14.7 PSI, your Vac system's 4.8-5.2 Hq doesn't equal enough pressure to cause the back flow you think brought the debris to your instruments.

Dry vac pumps fail in two ways, they will break a vane and cause the shear coupling to break and vac stops immediately. or they just wear out which cause the pressure to slowly drop over about 100 hours. You should see this and know it is about to quit.
 
Tom-D said:
You have a difficult time convincing me the debris that destroyed the istruments came from the pump.

do the math, your vac system operates at 4.8 to 5.2Hg, that isn't much pressure but the flow is about 5-6 cubic feet of air passing the inlet filter into the system.

when you figure the a standard day is 29.92 Hq which equals 14.7 PSI, your Vac system's 4.8-5.2 Hq doesn't equal enough pressure to cause the back flow you think brought the debris to your instruments.

Dry vac pumps fail in two ways, they will break a vane and cause the shear coupling to break and vac stops immediately. or they just wear out which cause the pressure to slowly drop over about 100 hours. You should see this and know it is about to quit.
Click to expand...

Just call a reputable instrument shop and ask them. I have a good friend who ran an Instrument repair and avionics shop here in Central Florida for years and he showed me several cases where it happened. I bought an airplane and had to have both the AI and DG overhauled and when he opened them they were full of carbon deposits from a previous failed vacuum pump. He had me remove and clean the hoses as well as the regulator before reinstalling the instruments.

Another little known factor is owners not replacing the hoses on a regular basis. As these rubber hoses age they deteriorate and start contaminating the instruments leading to premature failure. Owners would rather "save the money" by using old instrument hoses.

Hey, if you want to run vacuum pumps to failure, go ahead. The instrument repair shops would prefer you use this method since it guarantees work for them. :D
 
Last edited:
Rotor&Wing said:
Just call a reputable instrument shop and ask them. I have a good friend who ran an Instrument repair and avionics shop here in Central Florida for years and he showed me several cases where it happened. I bought an airplane and had to have both the AI and DG overhauled and when he opened them they were full of carbon deposits from a previous failed vacuum pump. He had me remove and clean the hoses as well as the regulator before reinstalling the instruments.
Click to expand...

How do you know those instruments weren't used in a pressure system (vs suction)? I can see how pump failure debris could end up in the lines and subsequently cause an early pump failure if not removed, but it's hard for me to see how it could make it all the way into an instrument when the pump fails. In any case it seems to me that if this were a real problem, a small filter on the pump input would eliminate it.

In my case (B55 Baron with suction driving one gyro), when one pump fails, the other's suction would prevent any debris from getting past the common manifold and hopefully the check valve on the failed pump side would block any debris from even entering the manifold.
 
gismo said:
In my case (B55 Baron with suction driving one gyro), when one pump fails, the other's suction would prevent any debris from getting past the common manifold and hopefully the check valve on the failed pump side would block any debris from even entering the manifold.
Click to expand...

So whens the last time you checked the check valve? It's fairly common to see those fail on older airplanes and usually the owner never checks them for proper operation.

Cessna had a SB on these a while back.
 
Rotor&Wing said:
.

Another little known factor is owners not replacing the hoses on a regular basis. As these rubber hoses age they deteriorate and start contaminating the instruments leading to premature failure. Owners would rather "save the money" by using old instrument hoses.
Click to expand...

you may have something there, because there are old black rubber hoses between the inlet airfilter and any gyros. and will appear as carbon debris inside a gyro.

but it ain't vane material.
 
Tom-D said:
you may have something there, because there are old black rubber hoses between the inlet airfilter and any gyros. and will appear as carbon debris inside a gyro.

but it ain't vane material.
Click to expand...
That is what I was thinking. There is just an incorrect diagnosis as to what the debris is.
 
If I've got it right...

If you have a failed check valve and you have old rotten hoses and your vacuum pump fails you could have backwash into the gyros.
 
The FBO I worked for bought back the 500 hr vacuum pumps for the VFR trainers for 1/2 of new and everybody made out.
Dave
 
Tom-D said:
you may have something there, because there are old black rubber hoses between the inlet airfilter and any gyros. and will appear as carbon debris inside a gyro.

but it ain't vane material.
Click to expand...

That parts not vane material, but vane material can show up with a failed vacuum pump.

Like I said, don't take my word for it, just call an instrument shop and discuss it with them.
 
Last edited:
I could understand vacuum pump debris getting into an instrument. The air passes through the filter and into the gyro's gimbal and wheelcase to spin the wheel. There's quite a bit of drag there, as you can see if you've ever sucked on one to make it go. Then the air leaves the gyro wheelcase and enters the instrument case, which the pump evacuates to drive the whole thing. This case consists of considerable volume effectively acting as a vacuum reservoir when the pump blows up. This vacuum is not going to instantly bleed off back through the wheelcase and filter due to the drag in that system, and so it could conceivably suck garbage back from the pump, especially if the pump isn't too far away. And two instruments doubles the storage volume, making it even more likely that something will get there that shouldn't. The regulator is a simple bleeder affair and won't stop anything.

Dan
 
Rotor&Wing said:
So whens the last time you checked the check valve? It's fairly common to see those fail on older airplanes and usually the owner never checks them for proper operation.

Cessna had a SB on these a while back.
Click to expand...

I do check it on startup and shutdown fairly regularly. Easiest method is to always start the left first and always shut the left down first.
 
Steve said:
A little light reading on the subject

http://www.aerotechcomponents.com/products.html
Click to expand...


"When pump failure occurs during engine operation the pump coupling separates to prevent damage to the internal parts of your engine. Just prior to the rotor binding and coupling separation however, it is not uncommon for the rotor and vanes to fragment into small pieces creating large amounts of carbon dust. With a vacuum still present in the gyro instruments and connecting lines, along with the positive pressure in the engine compartment, this carbon dust can be moved backward, past the vacuum regulator and into both gyro instruments. (Ref Fig. 2)"
 
With a vacuum still present in the gyro instruments and connecting lines, along with the positive pressure in the engine compartment, this carbon dust can be moved backward, past the vacuum regulator and into both gyro instruments.
Click to expand...

Its a difficult concept for folks to visualize.... and their tiny pic does not help. I have often wanted to draw one out with colors indicating pressure changes as the pump quits and the sequence of events that leads to stuff getting sucked from the pump backwards into the instruments.
It's a split second event; when the pump fails, suddenly the lowest pressure in the system is not at the pump, it is upstream. A very fleeting thing, but long enough to suck things up the tubing.
Maybe a slow mo cam, and clear tubing showing a pump shedding a vane, crap exiting both the inlet and the outlet.
I don't think it happens every time. Lots of failed pumps are replaced without instrument fod.
 
Let'sgoflying! said:
Its a difficult concept for folks to visualize.... and their tiny pic does not help. I have often wanted to draw one out with colors indicating pressure changes as the pump quits and the sequence of events that leads to stuff getting sucked from the pump backwards into the instruments.
It's a split second event; when the pump fails, suddenly the lowest pressure in the system is not at the pump, it is upstream. A very fleeting thing, but long enough to suck things up the tubing.
Maybe a slow mo cam, and clear tubing showing a pump shedding a vane, crap exiting both the inlet and the outlet.
I don't think it happens every time. Lots of failed pumps are replaced without instrument fod.
Click to expand...

I can quite easily visualize this but given the small pressure differential the instrument's empty volume would have to be fairly large relative to the volume in the tubing between the pump and the instrument. The AI's I've seen apart didn't have much unoccupied volume but I suppose that in some installations the tubing volume could be quite low. I see that Mooney began installing an extra filter between the pump and the gyros so their plumbing must have been of that sort, but it seems like this would be the standard setup if pump failures regularly contaminated the gyros. I suppose that having a dirty system filter would make the problem a lot worse since it would significantly delay the equalization inside the suction lines and gyros.
 
Vacuum pumps are cheap and easy to replace. I arbitrarily replace mine every 500 hours.

- Russ
 
Good on you for NOT doing the "fly until failure" GA philosophy.....
 
write-stuff said:
Vacuum pumps are cheap and easy to replace. I arbitrarily replace mine every 500 hours.

- Russ
Click to expand...

Great idea.

But if it's every 500 hours, it hardly sounds, "arbitrary," does it? :D
 
SCCutler said:
Great idea.

But if it's every 500 hours, it hardly sounds, "arbitrary," does it? :D
Click to expand...

If it's not backed up by some hard data, then 500 hours is arbitrary. That is, you could just as "arbitrarily" pick 400 hours, or 600. He's doing it regularly, but arbitrarily.
 
You must log in or register to reply here.
Back
Top