by Coy Jacob
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 Hannifins 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. Weve seen more than one pump done in by collapsed hoses that should have been replaced but werent 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 Hannifins 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, didnt 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, its 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 Rapcos. 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 youre 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 Rapcos Mike Lotzer believe the pumps rotational speed impacts the failure/wear rate but there isnt always a direct correlation that either company can confirm.
Typically, a pumps 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 1800 hours, especially on Lycoming four-cylinder engines, which turn the pumps more slowly than other engines do.
Admittedly, our research isnt conclusive, but we doubt this claimed long life is realistic. Typically, most pumps arent 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, youre 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, theres 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 youre spraying water around the engine compartment to degrease it.
Since most pumps-except Sigma Teks-are directional, make sure you have the right pump for the application. Installing the wrong model is something youd 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.
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 its a permanent, reusable accessory.
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 doesnt address the proactive replacement idea but gives you redundancy without extra gyros or a spare pump or induction manifold-type back-up.
Lets 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 Advantages 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, isnt cheap, at $795, with an overhaul cost of half that. It also wont fit every aircraft because its 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 doesnt 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 isnt 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 Advantages Dave Boldenow. He told us theres 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 chambers failure from affecting the other.
As for better conventional pumps, we think the Aero Accessories Tempest Tornado and Rapcos 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.
Rapcos 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.
In the past, our view of proactive replacement of vacuum pumps has been mixed. If it aint broke, why fix it? The problem with this is that when a pump does break, its almost certain to be at an inconvenient time when ready replacement isnt easy.
One solution is to carry a spare pump in the baggage compartment. For under $500, thats not a bad idea. Youre 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 Advantages dual-chamber pump provides inflight back-up-good-but it doesnt address proactive replacement, if you think thats a good idea. You could always care a spare Aero Advantage pump but at $795, we dont think its 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.
• Aero Accessories Inc., 800-822-3200, www.aeroaccessories.com
• Rapco Flight Support, 800-527-2726, www.rapco-rfs.com
• Aero Advantage Inc., 817-326-6147, www.aeroadvantage.com
• Sigma Tek, 316-775-6373, www.sigmatek.com
-Coy Jacob is an Aviation Consumer contributing editor.