Despite their increasing obsolescence, vacuum pumps remain installed aboard the vast majority of piston-powered aircraft, providing the energy to spin the gyroscopes in flight instruments like attitude indicators and directional gyros. Even newer aircraft sporting factory-installed “glass” panels may have a vacuum (or pressure) pump powering one or more backup instruments. Odds are, the airplane you fly has at least one, and the odds that it rarely gets serviced are even better.
A common theme in many of my previous articles for Aviation Safety is that aircraft systems require regular service and system component replacement at regular intervals, according to manufacturer specifications. This also is true for vacuum/pressure pumps, which can be impossible to inspect without disassembly, which typically can’t be done in the field. Perhaps more so than many other aircraft systems—except perhaps the powerplant(s)—ignoring this system until it fails could lead to catastrophe, or at least significant inconvenience and expense.
The vacuum/pressure system of a modern piston-powered aircraft typically consists of an engine-driven, rotary-vane pump, a regulator and filters, the instruments and accessories powered by the system, and hoses or tubing needed to connect everything. Essentially, the pump creates a vacuum downstream of the instruments, sucking air through the system and spinning a rotating wheel with integral cups that turn when there is sufficient vacuum. The rotational speed of the gyroscopic instruments is governed by the amount of vacuum supplied to the instruments.
A typical pump can be installed to provide either pressure or vacuum. When other equipment, like pneumatic deicing boots, autopilot servos and/or inflatable door seals also are aboard the aircraft, the pump typically is installed as a pressure system. Piston twins generally have one pump on each engine, with associated filters, valves and gauging providing redundancy.
During an annual or 100-hour inspection, this system should be carefully examined. Things to look for include security and cleanliness around the pump, including any fluid leaks, which must be corrected. Verify panel-mounted gauge readings with a calibrated instrument and adjust the system to the correct value. Inlet air filters should be replaced at 500 hours or annually, whichever comes first. Inspect also for deteriorated hoses, loose fittings, and installation of system components according to airframe manufacturer guidelines.
Like many components, vacuum/pressure pumps have a defined service life, measured in operating hours and/or years. Even if the pump is working well and shows no signs of failure, it’s not a bad idea to replace it prophylactically on a regular basis to guard against failure on a dark and stormy night. Do your research and due diligence on replacement as manufacturer’s recommendations may vary according to the engine and airframe the pump is installed on as well as how it’s used. Keep the pressure/ vacuum adjusted to the airframe manufacturer’s specifications. Check for system plumbing leaks on an ongoing basis by looking for carbon dust as a telltale sign of a leak. Record the vacuum or pressure readings on a regular basis and consult a mechanic for maintenance if any significant changes are observed.
WHY VACUUM SYSTEMS FAIL
Other than normal wear, premature failure of a vacuum/pressure pump really comes down to not following manufacturer instructions regarding maintenance and replacement. A pneumatic pump’s service life can be greatly affected by air leaks in the system, which make the pump work harder to supply the proper vacuum, which generally is 4.5 to 5.5 inches Hg to the gyroscopic instruments.
Dirty or clogged filters may be indicated by decreasing vacuum readings, which also cause the pump to work harder, decreasing its life. Other factors that may decrease a pump’s longevity is insufficient cooling air, frequent operation at moderate-to-high altitudes and frequent use of deicing boots operated by the pneumatic system.
Fluid contamination is another cause of premature failure. So-called “dry” vacuum pumps—not the “wet” style lubricated by engine oil—incorporate vanes made from a graphite compound and rely on dry internal conditions for proper operation. Any oil residue, hydraulic fluid, fuel or even soap and water entering during an engine wash can destroy the internal components of a dry pump.
Foreign object damage is another killer of vacuum pumps, typically caused by small pieces of hose that break off internally and are drawn into a pump. This can break the carbon vanes and shut down the system in short order. Old, hardened rubber hoses such as those behind the instrument panel are prone to internal deterioration and should be replaced with the proper aircraft-grade material somewhere between the 5-and-10-year mark.
Some pumps are designed to be operated in only one direction and are manufactured to require a specific volume. If the incorrect pump is installed or is operated opposite its specified rotation direction, shortened pump life and possibly insufficient volume to operate the gyroscopic instruments can result. Install only the pump intended for use in your aircraft and engine combination. Normal vacuum pump life, while not an exact science, could be 600 hours or possibly up to 1000 hours, depending on the quality of maintenance received and the demands placed on the system.
If you have experienced premature pump failures, then it is time for some serious troubleshooting before throwing on another pump. When replacing a vacuum pump for any reason, always inspect the system for possible air leaks and general cleanliness, including deteriorated hoses and associated small chunks of rubber from failing hoses.
Always use the proper torque for mounting a pump as well as replacing the gasket and the lock washers with new ones each time. Some installations require a special wrench to properly tighten the mounting nuts as access may be limited. Use the correct tool; do not improvise. Replace all filters and adjust the regulator with a properly calibrated gage, then finally make the proper logbook entry.
Several enhancements to the manufacturing process as well as materials used in the overall vacuum or pressure system have come about in recent years. Improvements in pump design and additional manufacturers have made vacuum pumps more reliable, competitively priced and better able to withstand severe conditions. The addition of a wear indicator port has allowed for inspection of the internal components of the pump, thus making it possible to determine when to replace the pump prior to failure. See the sidebar above for more details.
New ways to peer into a pump’s innards should not be used as an excuse to defer maintenance. Overhauled or new pumps are available at competitive prices, so the changing of a pump prior to failure has now become more feasible. It’s not a difficult or lengthy job, and any A&P should be able to change a pump..
While on the subject of overhauled pumps, some were never intended to be overhauled. Others can be successfully overhauled using exacting procedures and testing to be sure the pump is providing full volume after the work. A local mechanic may not have the equipment, workplace or parts to properly overhaul and test a pump, which kind of defeats the purpose of overhauling the pump so it won’t fail.
If you are intent on purchasing or exchanging an overhauled pump, buy from a reputable firm with a warranty. As the saying goes, you get what you pay for. Serious IFR fliers should consider choosing from a variety of generally more reliable new pumps, which really are only slightly higher in price than an overhaul exchange unit.
Even with proactive maintenance and monitoring, pumps will still fail. That’s when a timely alert or warning can come in very handy.Gauges are not always easy to read, or are not kept in a frequent scan pattern during IFR flight. Low-vacuum warning flags in gyro instruments are now available when purchasing new gyros and should be considered. A low-vacuum indicator switch and light is also available for low cost that will warn instantly of low vacuum or system pressure.
Several backup pneumatic systems are available at reasonable cost, with choices including a second engine-driven or electric pump, or an intake manifold tap. Check with your mechanic as to the options available and prices for your specific aircraft. If you have a standby vacuum system, review its operating instructions occasionally and ensure that all scheduled maintenance procedures are followed.
Electric gyros also are an option, and the FAA allows replacing the turn coordinator with a suitable one (see Advisory Circular AC 91-75). Keep in mind that an electric gyro will have its own maintenance requirements. That said, a new generation of multi-function electronic instruments is available that can be configured with their own backup batteries. If money is no object, all your pneumatic instruments can be replaced with electric ones, allowing you to do away with the vacuum pump entirely. However, some autopilots and deice systems depend on pneumatic power to operate, so it may not be feasible to go all-electric and remove the pump.
Finally, do not continue to operate an aircraft even VFR with a pneumatic system failure. Should a pneumatic system fail when operating day VFR, it may not be an emergency, but you should act responsibly and land to have the problem corrected. Keep in mind that the failure may not be something simple like a broken drive coupling or fractured vanes. Also consider that gyroscopic instruments can be damaged if they are spun down or static and are subjected to shock damage from rough air, or even rough runway surfaces. If it fails, restoring the system to operation before flying the aircraft again will help protect your gyros.