I’m getting ready to replace my vacuum pump next annual. I fly in IMC and would like to have the best on the market, plus any backup vacuum system I might add to the plane. My plane is a 1967 A23-24 Super III 200 HP. Looking for any suggestions.
Search strings: vacuum pump replacement, replace my pump, failed vacuum pump, new vacuum pump, best pump, vacuum pump failure
Technical Editor:
I can highly recommend the Precise Flight Standby Vacuum System. It requires proper installation, but is not difficult to install or use. My Sierra can maintain 90 KTIAS at 10,000 feet and 2,700 RPM, while maintaining four inches of vacuum for the gyros, when in back-up mode. There was a new dual-chamber pump on the market for a while, but the company halted production and closed their doors due to reported experiences of simultaneous failure of both pumps; something that wasn’t supposed to be able to happen. I’m not sure the pros outweighed the cons anyway, on the Sierra. For near the price difference, you can put on a standard RAPCO and the Precise-Flight back-up system, and you aren’t faced with the much higher ongoing pump replacement costs. The SVS also provides a selling point some day, more so than the type of pump, and it completely separates the primary and back-up vacuum sources (no commonality anywhere except at the shuttle valve that selects the working source). The dual-chamber pump would also have made oil filter changes a real challenge on my Sierra, as the aft chamber would largely block my filter access.
CAUTIONARY NOTE! The SVS works on the “differential pressure” between atmospheric pressure and the manifold pressure in the engine. If you have the engine at full throttle, there is no differential pressure, therefore no vacuum to run the instruments. If you cannot keep at least 3.5 to 4.0 inches of difference between the pressure seen at WOT, and the actual throttle setting, you cannot rely on the vacuum gyro instruments. When might this matter? Perhaps on a full-throttle go-around from a missed approach in hard IMC and a high density altitude. You have to keep this in mind. If you are depending on the SVS to get you down from a vacuum failure, the best option will always be to head toward reported visual conditions. If you are forced to attempt an instrument approach, you must remember to remain at less than full throttle should a go-around be necessary. It carries a risk, but so does an out-of-control roll into terrain due to inadequate vacuum on the gyros. You also have the option of spending a couple of grand or more on an electrically-driven standby vacuum pump. They carry their own set of risks, and cost much more than SVS. While they have the capability of providing full vacuum despite the engine’s power setting, they also have the same limitation as any other dry-vacuum pump; i.e. unpredictable failure at a bad time. In contrast, the SVS is mechanically very simple, and is highly unlikely to fail at a bad time.
As far as choosing a replacement pump, below is an Aviation Consumer article on the subject from last November. I’m not sure whether all of it will display correctly in the List posting. Send me an email if you need it forwarded as email content. I can’t just send the link, as it will prompt for a log-on. If you are VFR-only and are willing to use a rebuilt pump, the overhauled Tempest looks like it may be the best value. If you are IFR and use only new pumps (like me), and change it every 1,000 hours (before it fails), the RAPCO may be the best buy. None of these pumps have the long service history of the Airborne pumps, so it remains to be seen what their true average service life will be. My personal intent is to change the pump before it reaches the median age of failure, less a cushion. You can read this article and decide what makes the most sense for your circumstances and preferences.
Most cases of premature vacuum pump failure are related to maintenance and installation issues (hoses, debris, oil or solvent sprays, etc.). This is especially true if the previous pump failed during operation, and the lines were not thoroughly cleaned and inspected, filters replaced, hoses and instruments inspected, etc. The pump on my Sierra when I bought it lasted 1,400 hours (before I knew better), and failed during ground taxi. I preemptively changed the next one at 900 hours (I was headed on a long and remote trip). I plan to change this one at 1,000 hours.
There is also now a new wet-style vacuum pump on the market:
http://www.m-20turbos.com/pr17.htm
In the past the wet pumps have had life spans that matched engine TBO, and rarely had in-flight failures. Their chief drawbacks were price and oil on the belly of the plane. I’m not sure that the price situation has improved any, but the M20 Oil Separator should certainly solve the oily belly problem. Check the Q&A section for a separate post on that subject.
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Buy New Before It Quits by Coy Jacob
That’s the best way to avoid unexpected vacuum pump failures. Wear inspection ports now make that practical.
Tempest (left) and Rapco pumps (far right) have wear inspection ports. Aero Advantage dual chamber pump, center, is a good, albeit expensive, back-up option.
Is the vacuum pump—one of the more irritating accessories an aircraft owner has to suffer—an endangered species? Cirrus, Lancair and Diamond have converted to all-electric instruments for their new airplanes. Cirrus has even canned vacuum-driven mechanical gyros entirely in favor solid-state primary flight display technology.
These developments, and the fact that owners grow increasingly disgusted with dry vacuum pumps, have touched off significant changes in the vacuum pump industry. Also, one former major player in the field, Parker Hannifin’s Airborne Division, has abandoned the pump business due to liability worries and Champion also bailed out after dabbling in the business.
Against this backdrop, we think we can say vacuum pumps are getting better, although we wonder why it has taken so long. Two companies now offer a means of inspecting an in-service pump for wear while a third is making inroads with a dual-chamber design that combines two pumps on a single shaft. These developments make a case for proactive pump replacement, something not many owners have done in the past. The dual pump offers automatic redundancy, without an additional gyro or back-up system.
Why Pumps Fail
The list of reasons for pump failures is long but probably not definitive. Contamination by oil and grit, collapsed vacuum lines, excessive operating temperatures and long periods of inactivity—especially in an adverse environment—are given as common causes for failures. Improper installation and failure to follow manufacturers’ recommended maintenance guidelines is also high on the list of failure causes. We’ve seen more than one pump done in by collapsed hoses that should have been replaced but weren’t or by clogged filters that starved the pump.
However, Tim Henderson, of Gibsonville, North Carolina-based Aero Accessories, believes the primary cause of pump failures is simple wear and tear. The pumps wear out, jam up and quit. (Aero Accessories sells pumps with a wear port inspection feature. More on that later.) Henderson says when the graphite pump vanes wear past an acceptable limit, they tend to wedge in the slots in the pump rotor, break off and disintegrate. Pump failure follows, often shearing off the disposable drive coupling to the accessory case.
If no other factors are at work, such as a defective oil seal or extreme heat, the natural rate of wear and eventual failure of a pump is somewhat predictable. The length of the vanes relates directly to the amount of life remaining, says Henderson, which is the reason he developed the inspection or “wear port” idea for his Tempest pumps as well as the Velocity line of pumps he briefly produced for Champion before it got out of the pump business.
The Tempest pumps have become the mainstay for OEM airframers, now that Parker Hannifin’s Airborne pumps are out of the picture. When the wear port idea first surfaced a couple of years ago, Rapco, who got into the pump business itself after years of providing overhaul kits, didn’t think much of the idea and even published a sarcastic ad with a mechanic hanging from the ceiling trying to inspect the back of the pump with a mirror. But it has now introduced its own wear measuring device, a dipstick gadget called “Smart Stick.” This is a notched plastic stick inserted into a hole in the side of the pump that allows gauging the amount of vane material remaining via a sliding collar on the stick.
We inspected vanes in both the Rapco and Tempest pumps. Although the Tempest port is harder to get to at the back of the pump, it’s easier to see the vanes and assess wear than it is on the Rapco pump, in our view. The Tempest port is much larger than the Rapco’s. Further, on the Rapco, you have to align the vanes by shining a flashlight into the tiny inspection port, then insert the Smart Stick. You have to know what you’re looking at to get the vanes properly aligned.
Vacuum Pump Life
Various pumps operate at different RPMs primarily due to differing accessory case gear ratios specific to each model engine. This, as much as anything, may determine pump life. Both Aero Accessories’ Henderson and Rapco’s Mike Lotzer believe the pump’s rotational speed impacts the failure/wear rate but there isn’t always a direct correlation that either company can confirm.
Typically, a pump’s vanes wear at a rate of about .025 inches per 100 flight hours. New vane length is about .875 inches
(Aero/Rapco/Airborne) and a vane becomes prone to failure once it wears down to about .650 inches in length. This translates to about 900 hours life expectancy on a typical 200-series pump across a range of engine types. But wear rate is dependant on other variables that can substantially reduce the life of a new pump.
On normally aspirated engines, pumps should last an average of 600 to 900 hours before replacement. Rapco claims theirs may last up to 1,800 hours, especially on Lycoming four-cylinder engines, which turn the pumps more slowly than other engines do.
Admittedly, our research isn’t conclusive, but we doubt this claimed long life is realistic. Typically, most pumps aren’t used frequently enough nor is the vacuum system maintained proactively to allow the pump to operate in an ideal atmosphere.
Turbocharged aircraft flown at higher altitudes tend to be harder on pumps, probably because of heat issues. If you get 500 hours out of pumps in these circumstances, you’re doing well.
Many owners report what they consider to be premature failures, which they attribute to either bad quality control or bad luck. In reality, when a pump fails prematurely, there’s usually a reason. These early failures can often be traced to one of three causes: system contamination, operational overstress or a faulty installation.
Dry pumps are vulnerable to contamination, particularly by liquids. The graphite vanes are designed to operate bone dry and the introduction of any liquid can quickly destroy a pump. So wrap the pump in plastic if you’re spraying water around the engine compartment to degrease it.
Since most pumps—except Sigma Tek’s—are directional, make sure you have the right pump for the application. Installing the wrong model is something you’d think only boneheaded mechanics would do but it happens often enough to be an issue.
Insist that the manufacturers instructions be followed and this generally means installing new filters and blowing out the vacuum lines. When this is overlooked, gyro instruments pay the price because when a pump fails, it belches carbon debris into the hoses and contaminates the system.
Replacement Strategies
Are pumps getting better? In a word, yes. For this reason, owners now have more options in developing a proactive replacement/maintenance program for dry pumps. Furthermore, Rapco markets a relatively inexpensive cooling shroud kit ($79) that we think is a good idea for extending the life of any pump, especially those in turbocharged engines, which tend to run hot. A cooling shroud strikes us as cheap insurance and it’s a permanent, reusable accessory.
Rapco’s Smart Stick measures vane wear.
With proactive replacement in mind, there are three relatively new options: the aforementioned Aero Accessories and Rapco pumps with provisions for measuring wear and the new dual-chamber pump from Aero Advantage, which doesn’t address the proactive replacement idea but gives you redundancy without extra gyros or a spare pump or induction
manifold-type back-up.
Let’s consider the Aero Advantage dual chamber pump first, which we have reported on previously but have not examined in detail. The concept is that a single shaft drives two sets of dry pump vanes, each in its own independent chamber. If one rotor fails, the other, theoretically, keeps on pumping in reserve. While we think this pump represents an advance in technology, owners should understand that the second chamber is meant to provide vacuum only long enough to reach an airport or your destination, not to land and take off again to fly back home. Second, an inherent weakness of
this design is that if one pump chamber has failed due to wear, can the second be far behind?
Aero Advantage told us that testing revealed that when one pump failed, other chamber also failed within 25 to 50 operating hours. Because of that, Aero Advantage’s STC specifically says the second chamber is intended solely as a limp-home back-up; once you land, the pump should be overhauled or replaced. The dual-chamber pump, having nearly twice the component count of a conventional pump, isn’t cheap, at $795, with an overhaul cost of half that. It also won’t fit every aircraft because it’s longer than the pump it replaces. Initial installation of the dual chamber pump system is more than just bolting it on because it also includes panel-mounted warning lights that illuminate when either chamber fails. Projected time to initially install the pump kit is 4 to 6 hours so allowing for everything, count on an invoice of about $1000.
While the dual-chamber pump is obviously an advancement from a safety standpoint, it doesn’t address the inconvenience factor of a Saturday afternoon pump failure that might keep you grounded until Tuesday when FedEx can deliver a replacement. This fact may encourage pilots to fly home on the second chamber but doing so isn’t recommended by Aero Advantage. One other worry: would a failed chamber dump debris into the lines, just as with a single-chamber pump? No, says Aero Advantage’s Dave Boldenow. He told us there’s a check valve which springs closed the instant either chamber fails, keeping debris out of the remaining chamber. He also explained that testing had been done to assure continued operation via independent shear couplings, either of which protect each chamber’s failure from affecting the other.
As for better conventional pumps, we think the Aero Accessories Tempest Tornado and Rapco’s new model are improvements, although we await field experience in estimating wear. The Tempest Tornado series has an extra seal to help eliminate one of the main causes of premature failures: oil contamination.
Another feature of the improved Tornado pump is through bolts which extend from the front to the rear and sandwich the stator, rather than the usual shorter bolts that thread into the stator itself. This provides better alignment throughout the range of operating temperatures, something that should improve longevity. We like the wear port in the Tempest pump. Although its location at the back of the pump may be difficult to get at in some applications, it does provide practical means of checking wear. This product is too new to have developed long-term service history but Aero Accessories’ Tim
Henderson told us something interesting. These days, the trend is that pumps are being returned to his facility for overhaul before they have failed, something that was unusual in years past. Previously, very few pumps were overhauled prior to hard failure.
Rapco’s Smart Stick wear inspection port is another new idea that we think looks good on paper. At $420 new, it strikes us as a good value and once you learn its quirks, the Smart Stick provides a degree of positive measurement of wear.
Recommendations
In the past, our view of proactive replacement of vacuum pumps has been mixed. If it ain’t broke, why fix it? The problem with this is that when a pump does break, it’s almost certain to be at an inconvenient time when ready replacement isn’t easy. One solution is to carry a spare pump in the baggage compartment. For under $500, that’s not a bad idea. You’re still vulnerable to an inflight failure, of course.
Replacing a failed pump with either the Rapco or Tempest makes the most sense, dollar wise.
Of the two, we like the Tempest Tornado better because of its improved front-end oil seals. The fact that it can be inspected for vane wear and replaced before failure is icing on the cake.
Aero Advantage’s dual-chamber pump provides inflight back-up—good—but it doesn’t address proactive replacement, if you think that’s a good idea. You could always care a spare Aero Advantage pump but at $795, we don’t think it’s as good a value as the Tempest. One other option is to install the dual chamber pump with a spare Tempest to get you home. That would allow time to overhaul the dual pump after a failure and you could put the Tempest back to spare duty.
Follow-up comments from Bob Steward:
Precise Flight (the standby vacuum people) sell just the warning light and the pressure switch with the STC for under $100 (last time I bought one).
This is a GREAT tool to make you aware instantly that the pump has failed,
and give you ~2 minutes of time to get your act together on partial panel
before the gyros spin down to where they are inaccurate.
It also makes a nice Master ON warning. The red light is on whenever the
Master is on and the vacuum pump is not working.
Technical Editor Addendum 2009:
My opinion, based on articles in Aviation Consumer, Light Plane Maintenance, and 20 years of personal experience, follows.
Any replacement pump will fail if the installation is not properly performed. This is especially true if the replacement is the result of a failure during operation. The likelihood of contamination in the hoses is very high, if they are not cleaned or replaced. Ditto if the cause of the failure is not the life limit, but is instead due to oil contamination from a leaking Garlock seal in the drive adapter.
The ‘old days’ of Airborne vacuum pump rebuilding caused early pump failures, and gave the rebuilt pumps a bad name. Those rebuilds only replaced the drive coupling, the rotor, and the vanes. Those parts were installed in the original (worn and scored) housings. They naturally did not achieve a fraction of the life of a new pump; failures occurred in as few as 100 hours.
If a modern rebuild from Rapco is used, the only reused parts are the end plates, after refinishing. The housing and all rotating parts are new. Assuming proper installation, they appear to have the same service life as a new 215-series pump (but without the inspection capability). This is what I run in my own Sierra. I know of several that have passed 800 hours TIS. You can buy two of these for the price of any other pump, and put one in your travel kit ‘just in case’. If you make proactive pump changes somewhere between 500-1000 hours (I have historically changed mine at the 1,000 hour point), you can then carry the removed pump as an emergency spare. This is naturally more practical if you do proactive replacement at 500 hours rather than 1,000 hours. Financially, the worst situation is to encounter a pump failure while far from home and a technician. You wind up paying full List price or more for a replacement, and very high service charges if someone has to fly in to change it. You could probably buy five of the Rapco pumps (or more), for what that event would cost.
I have encountered cases of interference between the tach cable nut and the outer cooling ribs, on the 215 series pumps. I have not dealt with enough of the 215s to know whether this is a chronic problem, or is instead the result of non-original tach cable nuts. I have never seen it occur with a 211 pump.
The AEON piston pump from Sigma-Tek is the one I like the best in principle, but it won’t fit on the 19/23/24. There is mounting tube structure interference no matter how you orient the drive, even if you try to use the clocking adapters that are available.
I don’t know how useful the inspection ports are, on the Tempest and Rapco pumps. If someone intends to operate their pump to the brink of failure, the ports probably let you get closer. If someone intends to proactively replace their pump, then it does not matter what the ports tell you. They will only show progressive wear. They won’t protect from a sudden failure that is the result of debris in the hose or a corner chip breaking off. Just for perspective, a $200 Rapco rebuild that gets changed at 500 hours only costs $0.40/hour to operate, with no labor time for port checking. Even a new Rapco 215CC is only $270, or $0.54/hr at 500 hrs and $0.27/hr at 1,000 hrs, not including any paid maintenance time to check any inspection port. Keep in mind that if your pump has the port, a shop may tell you that it is obligated to check the wear. Then they get to make the call on whether you must replace the pump, while you are at their mercy.
The Sigma-Tek equivalent is $484, or $0.97/hr at 500 hrs and $0.45/hr at 1,000 hrs. The Sigma-Tek vane pump has an aluminum rotor and composite vanes. I am unconvinced that they will have the same life expectancy as the traditional carbon parts. They can’t possibly be as heat-tolerant; and the vacuum pump goes well over 200 degrees during cruise operation. I strongly suspect that the reduced heat resistance is the reason why Sigma-Tek excludes their vane pumps from applications that drive deicing boots. The Tempest pumps fall between the Rapco and Sigma-Tek, if I remember right. The wet pumps last forever, but have their own set of drawbacks; not least of which is the price. They also add large plumbing behind the engine, for which there is very little room on the Sierra. In turn, that drives up the costs of time required for maintenance.
Keep in mind that, regardless of the pump used, any IFR airplane really must have a standby gyro capability. The evidence is overwhelming that even very experienced pilots tend to lose control in IMC, following a pump failure. This happens even when they catch it in time, and are talking to ATC (and receiving instructions intended to keep them upright and level). Either a standby vacuum system or an electric AI is needed, and preferably both. The Precise-flight system works fine on our non-turbocharged planes. The PF system includes a lost-vacuum warning light, which I believe you can also buy separately. Every vacuum-dependent plane should have a lost vacuum warning light, as almost none of them have the vacuum gauge high up in the panel where a rapid instrument scan will always see it.