Troubleshooting, "lost vacuum"

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Dave Palacios

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I wanted to post something but only had this; wondering if others know this: When the vac gage and inop dg. ai. indicate "vacuum lost". I know it may be obvious to most, probably, to replace the usual suspect "the vac pump" and this method is the "shotgun approach" if you are the kind that prefers to use the investigative approach before shelling 300 to 400 clams, because there can be other parts of the system that can be at fault (especially after under the panel work). With the engine top cowl open and the pump still installed, look at the base of the vac pump at the narrow part that has openings. The internal part that you see thru the openings is the pump shaft, watch that while turning the prop (by hand, be careful! of course) if it doesn't move while turning the prop, the pump shaft is sheared. this only works with a dry air pumps and i think very recently some new pumps may not have these openings.
While talking about these dry air pumps, watchout when doing an engine wash, guard against even water getting into those openings. Another problem would be oil leaking thru the garlock seal in the pump drive pad if the pump gets contaminated internally the carbon dust will turn to paste and jam the pump, in turn the shaft shears. It won't happen immediately could take a few months but will be suden. wet vac pumps are not susceptible to this and I think Sigma Tek pumps use aluminum vanes.
 
If you are properly maintaining the vac system, what exactly do you believe could have failed other than the pump? The flexible hoses need replaced every 10 years and it is a good time to replace the regulator. Replace the pump every 500 or 7 years. Do this and vac systems are dependable.

Most vac system failures are from poor maintenance or when running stuff like speed brakes off them.
 
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From my experience it seems the plastic drive is what fails many times from age, not the pump many times.
But if the drive failed then you are best to replace the pump also.
Also like you mentioned the oil seal should be replaced at the same time a new pump is installed.

There are inspection plugs in many of today's pumps to check vane life even 10 years ago. Nothing new.
The rest of the system short of the filters can last the life of the plane even the hoses in some cases.
 
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Every new vacuum pump comes with a handy installation guide some mechanics don't read, and almost never give to the aircraft owner. On the Tempest document, we see stuff like this:

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#3 there: Pumps are often replaced after the old pump tore itself apart internally due to vanes worn way past limits. The vacuum in the whole system (not just the hose), including the instrument cases and hoses behind the panel, can suck carbon debris out of that busted pump. In extreme cases it might even get into the instruments. If the system is not thoroughly cleaned out, meaning taking the hoses out and cleaning them (and checking them for age and internal crumbling, etc.) and making sure there isn't anything that will get sucked into the nice new pump and chomp it up before the owner even picks the airplane up from the shop. Pump life expectancy gets shortened that way.

So there's this:

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Both Tempest and Rapco have vane wear checking ports. Why do people still buy Airborne or any other non-inspectable pumps? You either end up running the pump to failure (dumb) or replacing it at the recommended (and very early) 600 hours or whatever (expensive). If the vane wear is within the limits shown (Rapco, and some Tempest pumps, uses a little plastic dipstick measuring tool) the vanes won't jam in the rotor and shatter the rotor, killing the pump. This inspection feature gets the most hours out of the pump without undue inflight-failure risk.

More good stuff:

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And this:

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Regulators last a very long time. I only ever replaced one, and it was because the valve seat was corroded and leaking, dropping the vacuum way down at idle/taxi RPMs. The airplane was 50 years old.


Lots more on this Tempest sheet: https://aeroaccessories.com/wp-content/uploads/2019/01/AIA114-100110.pdf

One more thing, from https://aeroaccessories.com/wp-cont...-Acc-Serv-Letter-SL-001-Revision-F-120122.pdf

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A lot of airplanes sit most of their lives, and that plastic coupling ages. Heat from the engine ages it much faster. It can fail simply because it's tired, not because the pump seized up, and replacing a low-time pump is an awfully expensive way to get a new coupling.

New pumps, like the Tempests, have drives that are mostly closed off so that oil and solvents don't get at the coupling or into the pump itself. More on that in that first document. It allows any oil leaking from the engine's pump drive to drain out, but it also doesn't let any cooling air at the coupling, so I would expect the coupling to suffer sooner. Compromises.

Your pump will last longer if you keep it cool with a pump shroud.

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The plastic shroud snaps onto the pump. The hose flange attaches over a hole cut in the engine rear cooling baffle.
 
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