Instrument pneumatic systems — how they work and why they sometimes don't
Instrument pilots are trained to watch the vacuum gauge like a hawk because, as the sweaty-palmed CFII in the right seat is wont to say, you could lose it at any time. Aircraft owners, also leery of such a suctional letdown, are prone to spend money on backup systems designed to keep the gyros spinning in case the primary vacuum source draws its last breath. Not bad tactics.
As a rule, aircraft pneumatic systems are straightforward and for the most part reliable. But every chain has a weak link, and in the pneumatic system, it's the pump. The vast majority of light aircraft use some form of low-pressure pneumatics to drive the primary gyroscopic instruments; that much is obvious. This source of suction or pressure typically comes from an engine-driven pump. And while some aircraft use this pump to pressurize the pneumatic system, by far the most common method is to have this pump develop a vacuum.
In this way, air-driven gyros actually receive their incoming air from inside the cabin, through a pleated paper filter. The air is then drawn through the gyros — with a smaller line tied to the vacuum gauge on the panel — and then to an inline regulator. This regulator is adjusted by trial and error to achieve between 4.5 and 5.5 inches of vacuum at the gyros. Because the vacuum pump is running at a fixed speed in cruise — normally 1.6 to 1.8 times the engine speed — and because it's a fixed-volume pump, the regulator is necessary. Normally, the regulator's pop-off valve vents ambient pressure into the vacuum-supply line just before the pump itself; a small "garter" filter helps keep contaminants out of the pump.
As for the pump itself, it's pretty basic. In most so-called dry designs, a set of carbon-graphite vanes a little larger than a matchbook cover are fitted into a slotted carbon rotor that is in turn attached via a frangible coupling to the engine. This coupling is designed to help reduce vibration transmitted to the rotor and prevent engine damage should the pump seize. Carbon vanes are used because they are self-lubricating inside the eccentric aluminum pump housing. When a pump fails catastrophically, it's usually because some contaminant has entered the chamber and found its way between the edge of the vane and the pump housing, or when the vane itself breaks or shatters, jamming between the rotor and the housing.
Before dry pumps became common, pneumatic air was provided by, not surprisingly, wet pumps. The critical difference is that the wet pump is fed through an orifice in the pad mount a small amount of engine oil that provides the pump's internal lubrication. As a result, the steel gears are always "wet" and tend to last a long, long time. For this very reason, there is a whole generation of pilots who refuse to exchange older wet pumps for the newer dry designs. The major downsides of the wet system include greater overall weight than a dry system and the need to capture and recirculate the oily mist spent by the pump.
Dry pump technology has evolved since the device first became popular. Airborne recently introduced a replacement for the common 211CC and 212CW pumps called the 215CC and 216CW. (The suffix denotes the rotation of the pump; because Airborne uses canted vanes, the pump is designed to turn only one way.) Airborne's new models include a finned housing in place of the old smooth exterior reportedly for better cooling. They also have beveled rotors that are said to be more resistant to contamination. Meanwhile, Sigma Tek several years ago brought out its Gold Label pump that features an aluminum rotor with straight-slotted vanes so that it can run in either direction. Currently, Sigma Tek offers a two-year, 1,000-hour warranty on the pump.
Still, the pump is the weak member of the pneumatic-system family. We perused a listing of service difficulty reports (SDRs) dating back to 1974 that was compiled by Air Data Research (210/695-2204; www.airsafety.com). In it, there were 1,921 entries referring to vacuum or pressure pneumatic system failures. Many of these describe plain, simple failures of the pump or pumps, and the vast majority of these failed pumps were of the dry variety.
We took a random sample of nearly 400 reported pump failures and ranked them by the devices' time in service. The numbers are surprising. A quarter of the reports listed pumps that had fewer than 100 hours, while some 22 percent failed between 101 and 200 hours. Another 15 percent died before reaching 300 hours. In all, some 47 percent of the pumps failed before reaching 200 hours, and fully 62 percent never made it past 300 hours.
There are a few indicators here. Generally, pumps working in the harshest environments quit earlier. It appears that high-demand systems — such as those on pressurized, turbocharged, and deiced airplanes — operate at a disadvantage. (Aircraft with deice boots use the output of a large vacuum pump to inflate the boots.) Typically, the engine compartment of a turbocharged airplane is quite hot, and the demands placed on the pump by a pneumatic deice system as well as having to spin gyros at altitude are great. And yet, there was a report of a pump on a Cessna 421 failing at a comparatively elderly 1,300 hours. For some reason, Cessna T210s were over-represented in the listing.
The pumps in trainers appear to last the longest; no surprise here. These tend to be modestly powered airplanes running moderate engine-compartment temperatures; they're also flown much more frequently. There were numerous Cessna 150s and 152s whose dry pumps lived well past 1,500 hours.
Dry air pumps either fail catastrophically or die a little at a time. Usually, the pilot's first indication is lack of instrument vacuum upon startup or, perhaps, at some time during the flight. This is the light switch effect — one minute everything's fine, the next it's partial-panel time. No warning at all.
If you discover that the pump has died while on the ground, fix it immediately. Flying with a broken pump can lead to two mechanical problems. The first is that fragments of carbon can be pulled back into the gyro system through differential pressure — it'll be higher in the cowling than in the cabin. This can lead to contamination of the gyros and subsequent failures. (For pressure pneumatic systems, it's a virtual certainty that some of the pump effluvia will get past the inline filter.) The second is that it's possible the pump has pulled free of the mounting pad; there are several reports in the SDR listing of airplanes whose engines were deprived of oil because it was pumped out through the pump-pad orifice.
Of course, if the pump doesn't die quickly, it's sure to do so slowly. Normally, the first indication is failure to make green-arc vacuum at low engine speeds. It's appropriate for the pump to provide sufficient vacuum at 1,000 rpm. If you have to crank up the revs to get good vacuum, you've got either a terminally ill pump or a restriction somewhere in the system. Similarly, if you notice that the pump is failing to provide in-range vacuum at high altitudes, it's also likely to be on the way to the big pump pile at the back of the shop. (That pile is getting bigger now that many of the pump suppliers no longer require the return of cores.) Remember, the pump is trying to deliver around five inches of differential pressure; it's working harder at altitude to maintain that differential. It's also possible, but much less likely, that the regulator is failing.
Many pilots consider pump replacement on a calendar or hour limit rather than on condition — a euphemism for "when it breaks." This is generally a good idea. You can use your own logs as a reference point. If you've been seeing pump failures about every 500 hours, then it makes sense to swap them prophylactically every 400 hours. Then you can take your time to replace or rebuild the old pump and have it ready for the next round. Don't go tweaking up the regulator to compensate for a weak pump. If at normal cruise speeds the system is indicating proper vacuum, the problem isn't in the regulation.
Extending pump life may be accomplished by keeping the pump cooler. Airborne suggests that the temperature of the pump has a profound impact on its longevity. Keep your engine's baffling in top shape and consider fitting a cooling-shroud kit to the pump; Rapco makes a shroud kit that sells for about $50. Also, keep on top of the filter replacements; the small garter filter is supposed to be changed every 100 hours or annually, whichever comes first. The larger main system filter is supposed to be changed every 500 hours or annually. And if you do your own engine washing, be sure to cover the pump's inlet tube to prevent it from ingesting solvents, cleaners, or water.
Most experienced airplane owners take proactive measures to help the pump survive, but also to get it out of there before it expires. Nonetheless, those pilots are also the ones who know that even with the best of maintenance practices and intentions, you'll still need to keep an eagle eye on that vacuum gauge.
Links to additional information about vacuum systems may be found on AOPA Online ( www.aopa.org/pilot/links/links9911.shtml).
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