since 1940

Aircraft Vacuum Pump Frequently Asked Questions

Does a new pump include the coupling

Yes, sometimes when you remove the pump the coupling pulls out of the pump and remains
in the adapter drive on the engine. Reach in and pull the coupling out.

this adapter is not broken - it's brand new. I just pulled it out of pump to illustrate how it can stick in the engine when you remove the pump.

How long should a vacuum pump last

The following letter in Microsoft Word Format is from Rapco and outlines average vacuum pump life for particular engines:

Average Vacuum Pump Life (Microsoft Word file)

Airborne has published Mandatory Replacement Times for their pumps in their letter of May 2002 and published a toll free 800-382-8422 Technical Service Hotline.

Why do vacuum pumps fail?

Inside of vacuum pump showing carbon rotor and vanes

All maintenance personnel when inspecting the aircraft vacuum system should ensure they follow the guidance provided in Parker Hannifin Dry Air Pump and Pneumatic System Maintenance Instruction Manual. Correcting the cause of failure of a vacuum pump is important if a subsequent failure of the replacement pump is to be avoided. Use of the Airborne 343 test kit will help determine the reason for the pump failure.

Since dry vacuum pumps were introduced into service, over 30 years ago, they have been a source of continual premature failure. They fail catastrophically with no degradation of performance to warn the pilot of imminent failure. Loss of vacuum will result in a gradual run down of the gyro and presentation of misleading information and if this occurs in IFR flight conditions could result in an aircraft accident.

A lot of work has been done to try to understand the reasons for failures but, as there are many factors involved, the answers are not obvious. The September 1984 issue of Light Plane Maintenance magazine published an article which summarizes the situation in a comprehensive way and was thought worthy of reproduction.

"WHY DO DRY VACUUM PUMPS FAIL?"
To understand why pumps fail, it is helpful to know something about pump anatomy. Both Airborne and Edo (now Sigma-Tec) pumps utilize carbon-graphite rotor construction, with carbon vanes riding loose in the rotor slots. In normal operation, the vanes are thrown against the pump housing by centrifugal force, rising and falling on the elliptical walls and thus compressing the air trapped in the vane compartments. The Edo rotor has eight slots and eight vanes, it can be turned in either direction. The Airborne design, by contrast, uses six slots and vanes each at a slight angle, giving the pump a preferred direction of rotation. Airborne units thus come in 'CW' (clockwise) and 'CC' (counterclockwise) models, and for long life, the proper sense must be observed on installation.

In either case - Edo or Airborne - the vanes run dry on the aluminum housing walls, the constant gradual wearing away of the graphite is the only lubrication the pump gets hence the term "dry pump".

Note also, that Dry pumps can be used to suck air (provide vacuum) or blow air (provide positive pressure), depending on which side you hook the plumbing. When dry pumps are used to provide pressure (as in deice boot systems), inline filters must be employed to remove carbon dust from the system.

Both Edo and Airborne pumps have a standard AND20000 splined drive mounting for use on Lycoming or Continental accessory cases. Also, both Edo and Airborne incorporate frangible drive couplings which are designed to shear in the event of rotor lockup, thus sparing the engine accessory gears of possible damage. You'll notice, however, that the pump makers differ in their approach to drive coupling design:

Edo's coupling transmits torque straight to the rotor along a thin quill-shaft (which has since been changed to a speedometer-type cable in the so-called "dash three" pump models). Airborne, on the other hand, transmits drive torque to the rotor via a somewhat complicated coupling sandwiching eight sheer pins between a nylon torque plate and an upper torque plate, with the rotor spinning on three finger spools which "grab into" the rotor at about the half-radius point.

If it looks like the Edo (or Sigma-Tec) pump drive is "more frangible" than the stouter (if more complex) Airborne nylon-torque-drive, you're right. The Edo quill-shaft is designed to fail at 100 inch-pounds of torque, whereas it takes more than twice as much torque (250 inch-pounds) to snap an Airborne drive. Accordingly, one often finds Edo pumps failing due to drive-coupling breakage not associated with rotor lockup or vane distress.

The Airborne pump suffers real internal damage before its nylon drive shears. When that happens, odd-shaped bits of carbon can wedge between the rotor and housing wall, jamming the rotor, which probably disintegrates due to the sudden shock of stoppage. As you can imagine, even experts can have a tough time pinpointing the primary cause of a given pump failure.

What causes pumps to fail? Suffice it to say, no one thing. (Even the Federal Aviation Administration (FAA) has failed to come up with an easy answer to this question.) The use of carbon graphite as a structural - as well as a lubricating - material in these pumps certainly seems intrinsic to the problem. And yet, ironically, it's carbon graphite's unique qualities that make current "self lubed" pump designs possible in the first place.

In our research for this article (which included talking to the two major pump manufactures as well as mechanics, owners, and overhauler's), we identified no fewer than ten things that could cause a dry pump to self-destruct.

1. Solvent Contamination
Oil or oil vapor rapidly contaminates carbon graphite, turning the lubricating powder at the rubbing surface into a hostile sludge. Oil from the aircraft engine can enter the pump via several routes: a bad pump mounting gasket, oil blown rearward from untended leaks on the engine, and vapor from the crankcase breather drawn into the pump by its own suction. Another cause of contamination-failure is entry into the pump of degreasing solvent (the type sprayed on the engine for routine inspections). Unless exceptional care is taken, solvent can enter a pump through its exhaust tube or drive seals. Obviously, this is something to watch for during any engine spray-down.

2. Foreign Object Ingestion
Carbon-graphite is brittle and quite fragile. A small sliver of rubber hose (liberated by wiggling the plumbing during pump installation), or even the carbon bits left in the lines by a previous pump, can cause immediate failure. Generally this type of failure occurs shortly after installation of a new pump. Pump manufacturers lay the blame for a large percentage of warranty claims to this cause.

It should be noted that even airborne dust is sufficient to give most dry pumps fits. (A filter change is usually required for warranty coverage to be in effect after installing a new pump.) There is also some concern that particles small enough to pass through filters can mix with the lubricating powder at the rubbing surfaces of the vane and rotor, increasing the wear rate and leading to early pump failure. Cigarette smoke contains particles small enough to pass through filters, and of course in planes with suction-operated gyros, cabin air is the starting point for pump pick-up airflow.

3. Drive Misalignment
One of the more controversial (with manufacturers) aspects of the pump-life problem is parallel misalignment of splined-drive gears caused by "engine drive gears not being where Continental says they are" (in the words of one pump engineer). Poor pump lineup was reportedly one of the reasons Edo/Sigma-Tec went to Speedo-cable-type drive "shaft" on its 1 U128A-03 pump. Substantial efforts are continuing at the manufacturer level in designing more compliant drive mechanisms which (manufacturers say privately) should help pumps cope better with misalignments that can be as much as ten times worse than expected from published engine specifications.

4. Heat and Altitude Stress
The heat of compression developed in a dry pump operating at or near full output is of the same order of magnitude as that produced by a turbocharger. And the higher you fly, the harder the pump works. At 22,000 feet, a 211/212 Airborne pump develops a "delta-P" of eleven inches of mercury, maximum (up to 22 inches for a 441-series pump), which means internal temperatures can easily exceed 200 degrees Fahrenheit. Cooling is usually very poor, however - in part because of low humidity at high altitudes, and in part because aircraft designers often neglect to expose the vacuum pump to ram air, instead sequestering it in a 'dead spot' behind the engine baffling (where temperatures are already high).

Concerned about the increase in turbo traffic at the middle flight levels, the National Transportation Safety Board (NTSB) in 1982 specifically asked FAA to evaluate the reliability of small dry pumps at high cruise altitudes, but FAA efforts since then have been limited to "monitoring manufacturer testing.... (which has so far) proved inconclusive". Pamco Industries, the Milwaukee-based pump overhauler, has tested the 211 CC Airborne pump in backup mode (i.e.. powering gyros only) at altitudes to 30,000 feet, where it performed satisfactorily. But nobody really knows what the reliability of dry pumps is at high altitudes. for most installations, the testing simply hasn't been done.

5. Overspeed
Exceeding engine redline is another proven method for trashing your vacuum pump. Most pumps begin to provide useable suction (or pressure) around 1,500 rpm and provide optimal life at engine RPMs below 2,000. (Not surprisingly, this is the speed chosen by designers of electrically powered backup pump systems.) The maximum continuous operating speed of Airborne pumps is 4,000 rpm (rotor shaft); for Edo pumps, 4,200 rpm. Lycoming pump pads generally turn 1.3 times crankshaft speed. Continental pump pads, however, turn 1.5 to 1.545 times crank speed, which means that any time a Continental operator's engine rpm exceeds 2,588, the Airborne pump limits are being busted; and any time a Continental owner turns up more than 2,700 rpm, Edo's rpm limits are violated. combine high rpm with high demand (as in Continental-powered Cessna P210 with deice boots flying at 20,000 feet), and you can begin to see why some operators experience so many problems with pumps. Add a bonafide prop overspeed incident to the scenario (whether intentional or unintentional), and you've got real trouble.

6. Rapid Acceleration
Rapid engine acceleration (on go-around, for example) can apparently put unusual loads on rotor and vanes, which may be why test-stand pumps often run trouble free for many hundreds of hours, while operators in the field continue to rack up unexplainable premature failures.

Whether the acceleration problem is strictly one of mechanical stress, or also involves thermal shock, is anyone's guess at the moment.

7. Reverse Rotation
As mentioned earlier, Airborne pumps come in clockwise (CW) and counterclockwise (CC) flavors, designed to rotate in one and only one direction. The profile of the elliptical rotor bore is "note" symmetrical in the Airborne pump; also, the rotor slots are cut at an angle. (These design features may improve performance, but some experts feel they make the carbon rotor more susceptible to damage.) Attention to label instructions can eliminate incorrect rotation of pumps in normal use, but avoiding occasional engine "kickback" on startup (or shutdown) is not such an easy matter - yet if vane/slot clearances have opened up, one kickback may be all that's needed to jam a rotor and trash a pump. Pamco Industries' Thomas Zompolas (designer of a new standby vacuum system for Mooney's) feels this is a major reason why Airborne pumps used as standbys tent to last longer than the same pumps installed on engines. "That's why you often hear guys say 'but it was working fine when I shut down'" Zompolas maintains.

8. Rough Handling
The FAA's Service Difficulty Report file abounds with examples of "fresh out of the box" pump failures, where just spinning the drive shaft by hand is found to be enough to lock up the rotor. Assuming (and we do) that the manufacturers exercise a modicum of quality control , what could be the explanation? One likely cause: In transit "ship-shock", which can jar vanes enough to chip a corner (or cause other mischief). Of course, pumps also respond poorly to having their housings squeezed in a vice (which is something many A&Ps do while installing fittings in the inlet/outlet holes of a new pump, against the express earnings of the manufacturers), being dropped on the floor, etc. - but your mechanic never does such things .... right?

It's interesting to note that floatplane operators (who suffer a relatively high incidence of shock-related avionics and panel problems) have reported replacing vacuum pumps every 50 to 200 hours, on average - further evidence that mere shock and vibration can have a profound destructive effect on pumps.

9. Pump Lugging
The recent focus on pump failures in Cessna 210s with wing boots has tended to underscore the fact that deice boots place a heavy burden on vacuum pumps - ultimately subtracting from reliability. Whether the erosion of reliability is simply due to the higher average pump loads, or other factors, is not clear. FAA has received Service Difficulty Reports describing sticking deice boot valves in some aircraft.

Ordinarily, pneumatic deice boots cycle on and off alternately inflating and deflating, at the behest of a small timer and solenoid-actuated deice boot flow valve. If either the timer or the valve hangs up in the "inflate": position, however, the vacuum pump can quickly lug and overheat. Until recently, the loss of a vacuum pump in this manner meant not only the loss of boot action, but gyro instruments as well.

But the NTSB has explicitly called for independent instrument power sources as a requirement for deciding certification. Meanwhile, owners of boot-equipped aircraft should beware of the role of timers and flow valves in possible vacuum-pump problems.

10. Normal Wear
Dry pumps inevitably wear out (nothing made of graphite lasts forever), and - quite naturally - if left in service long enough, any dry pump is eventually going to stop working just from its vanes rubbing down to nothingness.

The question is, how long should a pump last? According to overhauler's' figures, under the best of circumstances, smaller (211-type) dry pumps are unlikely to operate reliably over 600 hours as the vanes will have worn to the point where they are likely to cook and jam. (By contrast, makers of electrically driven standby pump systems are confident that the same pumps - protected from heat, vibration, overspeed, contamination, etc - can be counted on to run 1,000 hours or more.) By all accounts, the so-called "boot pumps" (high capacity Airborne) are unlikely, in most applications, to last more than 300 to 400 hours. Of course, there are always exceptions. Study done by NTSB shows a meantime to failure 475 hours (491 failed pumps were analyzed covering failures from 2 1972 hours).

In short, then, the modern dry pump, by virtue of its design and construction, is acutely sensitive to almost everything in its normal environment: heat, oil, solvents, dirt, water, vibration, mechanical stress, and (some would say) the moon and tides. Even under the best of circumstances - with a new pump installed by experts, with cleaned lines and filters, with adequate protection from solvents and oil, and with pilot effort to keep throttle applications smooth and landings soft - you still cannot expect much more than three years of normal flying before your pump (whether Brand A or Brand E) fails. The only thing certain is that it will fail. You just can't say when.

That, as Ripley would say, is general aviation. Believe it or else."

"PUMP REPLACEMENT CHECKLIST"
The following pump replacement checklist is also worthy of note:

1. Troubleshoot cause(s) of last pump failure.
Booted aircraft: Check for normal deice timer operation. Timer should inflate boots for approximately six (6) seconds (somewhat variable in pressure-dependent deice systems, but should not exceed 15 seconds for any one cycle). Boots should be pressure bleed-down checked for leaks using Airborne 343 Test Kit.
Inflatable door seals: Check that the system inflates and holds pressure without recycling (no leaks).
Pneumatic autopilots: Check autopilot regulators, servos, and filters per manufacturer's specifications.
Other systems: Check pneumatic camera doors, avionics cooling, etc per aircraft service manual.
2. Replace all system filters.
Failure to change filters may void new pump warranties. Pump inlet filters (pressure systems) and garters (suction) should be replaced once a year or every 100 hours. Central gyro filters should be replaced once a year or every 500 hours (ditto for auxiliary inline filters).
3. Verify correct replacement pump P/N.
Do not merely replace existing pump with identical-part-number item. Consult aircraft Parts Catalogue or pump maker's Application List. Also (if using an Airborne pump), conduct a rotation check. Remove old pump, then manually rotate propeller in normal direction while drive pad gear is observed. (Observe proper safety precautions when turning propeller.) If drive gear rotates clockwise, a 'CW' pump should be specified. If gear turns counterclockwise, order a 'CC' pump.
4. Remove old pump and gasket.
Your old pump (even if damaged) has salvage value; many pump overhauler's will pay $20 or more to have it. Do not reuse old gaskets. New pumps should be mounted on the new gaskets that accompany them.
5. Remove fittings from old pump.
Discard stripped or damaged fittings, fittings with rounded wrench flats, etc. Thoroughly clean and dry serviceable new fittings before using them.
6. Install fitting in new pump.
Here, it is permissible to clamp the new pump in a vice at the base flange only (never at the center housing). Spray clean fittings with silicone lube and allow to dry before screwing them in by hand. "Do not use Teflon tape, pipe dope, or unapproved thread lubes". Tighten fittings down one and a half turns maximum, using a box wrench. Align fittings as appropriate for plumbing connections in aircraft.
7. Check the AND20000 drive pad for oil.
The pad should be dry, with no oil leaking out. Replace pad seal if necessary.
8. Install new pump.
Be sure to lay the gasket in place first, then install the pump. If you drop the pump, discard it. (Likewise, if the pump shows obvious signs of damage, exchange it for another one.) Replace all locking devices. Cinch all four mounting nuts alternately to 50 inch-pounds minimum, 70 inch-pounds maximum.
9. Install hoses.
Inspect hoses inside and out for contamination, condition, etc, and replace questionable hoses. (Replaced brittle or aged hoses to avoid separation of inner layers of hose, which can break loose during handling and be ingested by the pump, leading to premature failure.) In a pressure system that has experienced pump failure, be sure to blow out all lines with compressed air from the panel side, to remove any remaining bits of carbon. Make sure hoses are connected to the proper fittings (do not swap inlet/outlet hoses by mistake).
10. Check pressure/suction regulation.
Run engine to 1,500 rpm and check that suction gauge is reading in the green (or per manual specs).

NOTE TO AIRBORNE CUSTOMERS:
Airborne specifies a life limit of six years for nylon drive couplings. Airborne SL 17B dated 16 August 1991 refers. Factory kits are available for replacing couplings. Kit No 350 for 211/212 series pumps. No 352 for 440 series pumps. See your dealer for details or write to Airborne Division, Parker Hannifin, 711 Taylor Street, Elyria OH 41035.

Permission from Light Plane Maintenance to reprint the above article is gratefully acknowledged. It is abundantly clear that the American experience is very similar to that in Australia, but on a greater scale. one further point thought worthy of note is an amplification of the reverse rotation effects. There are some tendencies among Australian pilots/organizations, when carrying out daily inspections, to rotate propellers in the reverse direction. This is inviting problems, particularly with aircraft using dry vacuum pumps. There is no need to carry out this practice and it can cause a number of problems including sucking foreign material into the cylinders as well as damaging pumps. This procedure was common practice with aircraft such as the Tiger Moth and was used to blow-out fuel from over primed cylinders prior to re-starting. This procedure is not appropriate or necessary for modern day engines.

Due to increased usage of aircraft equipped with dry vacuum pumps in the IFR, all weather environment, failure rate trends increase with the higher exposure. The situation is worst for single engine, single vacuum pump applications, particularly those driving deice equipment.

to 1,500 hours on a Continental 6 cylinder engine.

What is the difference between the older style 211 and the newer 216 series pumps

Rapco, Inc. has improved the performance of these pumps by incorporating a new internal cavity design that has precision laser cut air ports improving the overall air flow. In addition, we have equipped each pump with an advanced internal rotor that has a bevel on the inlet side of each rotor vane. This allows some small particles to pass through the pump without failure. The new pump also has an exclusive internal Teflon coating to improve carbon vane wear. To prevent premature pump damage, a unique inlet screen is provided, free of charge with each pump, that fits into the inlet pump fitting preventing most foreign particles from entering the pump.

Please note that there is no core charge on this new pump. In each pump box is a rebate coupon that allows the installer to return the old core for a rebate check. This provides for easier handling of cores and still insures that we have an adequate number of cores left over for the overhaul program. We will continue to overhaul the 211CC, 212CW, 215CC, 216CW, 441CC & 442CW series pumps and now we will add the new pumps to our overhaul process!

How often should I replace the vacuum filters

Vacuum Instrument Central Gyro Filters:
Replace at least every 500 flight hours or annually, whichever comes first, to protect gyros.
Vacuum Relief and Pump Inlet Foam Garter Filters:
Replace at least every 100 flight hours to protect the relief valve seat and vacuum pump.
Pressure / Vacuum System Inlet Filters:
Replace at least every 500 flight hours or annually, whichever comes first.
Inline Filters for Pressure Systems:
Replace at least every 500 flight hours or at pump change to provide a supply of filtered air to the system components


There are at least two filters in the aircraft vacuum pump system: the main filter pictured below (style may vary)
 

What vacuum pump goes on my engine?

The link below is an application guide for the Rapco dry vacuum pumps

Press on link for application guide.

Vacuum pump application guide

I'm frequently replacing pumps - what could be causing this

Inside of dry vacuum pump showing carbon vane. Housing has worn "chatter" marks

1. Make sure that you have the correct pump installed on your aircraft.

2. Check your hoses to make certain that they are not collapsed or kinked.

3. Faulty or improperly set regulator.

4. Replace the system filter(s). A dirty or clogged filter will reduce vacuum at gyro gage and increase load factor on pump causing premature failure.

5. Make sure that no oil contamination is entering the pump
 

When replacing a failed pump what should I inspect for

1) 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.

(2) After you have installed the new replacement pump, We recommend using a commercially available test kit to make sure the aircraft vacuum system is working properly. A faulty regulator , dirty filter(s), or a crimped or partially collapsed hose can cause excessive system vacuum. This increases the load factor on pump and shortens pumps life.

(3) If you have a mid time engine, replace the oil seal in the engine AND 20000 pump mounting pad. This area could be dry now, but, due to age the seal could start leaking in just a few hours allowing oil contamination in your vacuum pump, causing premature failure. Less than $5.00 dollars spent here could save you hundreds of dollars later

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.