With the increasing use of single-engine aircraft for actual IFR flight, it is important to understand and properly maintain the pneumatic system that operates the gyroscopic instruments.
Reliable aircraft operations require customary service and system replacement at regular intervals according to manufacturer specifications. Ignoring this system until it fails could end in disaster and at the least could become quite inconvenient and costly.
The concept of safe, legal and smart also applies here in that while aircraft operated under FAR 91 legally require no overhaul or replacement (except for AD compliance), it is very smart to adhere to the replacement and maintenance procedures set up by the manufacturer.
In addition to regular maintenance of the pneumatic system, those who fly “hard IFR” in single-engine aircraft should invest in a backup pneumatic system, a low vacuum/pressure warning system or electrically powered gyros. Practice flying with the turn and bank or turn coordinator would also be an excellent plan.
Regular maintenance of the system is necessary, and during an annual inspection this system should be carefully examined for 1) cleanliness around the pump (correct any fluid leaks), 2) gauge readings verified with a properly calibrated instrument and system vacuum/pressure adjusted to the correct value, 3) replacement of central air filters at 500 hours or annually, whichever comes first, 4) inspecting for deteriorated hoses, loose fittings, substandard system components and replacement according to airframe manufacturers’ guidelines.
Additionally, if your aircraft has a backup system installed according to an STC or Field Approval, instructions for continued airworthiness will spell out specific inspection and maintenance procedures that must be followed. Ensure that your mechanic has this information.
Scheduled maintenance should also include replacement of the vacuum or pressure pump according to the manufacturer’s time table (1,100 hours or six years, according to one major manufacturer). Check with your mechanic on this, as the numbers change depending on the manufacturer and aircraft make and model.
Keep the pressure/vacuum adjusted to manufacturer’s specs with a calibrated gauge, not the aircraft gauge. Check for system leaks on an ongoing basis. Record the vacuum/pressure readings on a regular basis and obtain the service of a mechanic for maintenance if any significant changes are observed.
Why Systems Fail
Premature failure of a pump really comes down to one issue—that of poor or no maintenance. Service life of pneumatic pumps can be greatly affected by leaks in the system making the pump work harder to supply the proper vacuum/pressure to the gyroscopic instruments. Dirty or clogged filters (indicated by decreasing vacuum readings) also cause the pump to work harder, decreasing the life of the pump.
Fluid contamination is another cause of premature failure; most vacuum pumps are DRY, and rely on dry conditions for proper operation. Any oil residue, hydraulic fluid, fuel or even soap and water introduced during an engine wash that enters the system will destroy the internal components of a dry pump.
Foreign object damage is another killer of vacuum pumps; small pieces of hose that break off and are drawn into a pump break the carbon vanes and shut the system down in short order. Old, hardened rubber hoses are prone to internal deterioration and should be replaced with the proper aircraft grade hose somewhere between the five- and 10-year marks.
Some pumps are designed to be operated in only one direction and are manufactured to a specification requiring a specific volume. If the incorrect pump is installed and/or is operated in a reverse rotation, you may get shortened pump life and possibly insufficient volume to operate the gyroscopic instruments.
Install only an approved pump intended for use in your aircraft and engine. Normal vacuum pump life, while not an exact science, could be well above 1,000 hours if these recommendations are followed. If you have experienced one or more pump failures in the 300- to 700-hour range or every two to four years, then it is time for some serious troubleshooting before throwing on another pump.
Airborne (tech service phone 800-382-8422) has available for lease, loan or purchase a test kit that is very useful for troubleshooting and normal maintenance as well. Ask your mechanic to use this kit to make sure your system is operating according to the specifications of the airframe manufacturer.
When replacing a vacuum pump for any reason, always inspect the system for possible air leaks and general cleanliness, including deteriorated hoses. Replace all filters, and adjust the regulator with a properly calibrated gauge, and finally make the proper log book entry.
Replacement pump considerations: Several enhancements to the manufacture, materials and safety of the overall vacuum/pressure system have come about in recent years. Improvements in pump design and additional manufacturers have made vacuum pumps more reliable and better able to withstand severe conditions. However, this should not be used as an excuse to defer maintenance. Overhauled or new pumps are now available at competitive prices, so the changing of a pump prior to failure has become more feasible.
While on the subject of overhauled pumps, overhauling a pneumatic pump is not a job for an aircraft owner. Some pumps were never intended to be overhauled; however, some are successfully rebuilt using exacting procedures and specifications and finally testing (off the aircraft) to be sure the pump is providing full volume.
Most A&Ps do not have the equipment or test facility to properly overhaul and test a pump, thereby compromising the reliability of the pump we must depend on. If you are intent on purchasing or exchanging an overhauled pump, buy from a reputable firm with a warranty. As the saying goes—garbage in, garbage out!
Serious IFR flyers should consider choosing from a variety of (generally more reliable) new pumps, some only slightly higher in price than an overhauled exchange unit.
Low Pressure/Vacuum Warning Systems
First and foremost to consider is a timely warning of a vacuum pump failure. 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 (about $400 installed) that will warn instantly of low vacuum or system pressure.
Properly operating gyros with correct system pressure/vacuum prior to failure will only operate two to three minutes before they become unreliable. Unfortunately gyros do not die quickly, but linger on enough for an unsuspecting pilot to follow the incorrect information spiraling into the earth! Knowing instantly when a malfunction occurs and taking immediate action can often make the difference between life and death.
Changing altitudes, finding a clear area between layers, descending or diverting to VFR weather and declaring an emergency are all options to consider that a quickly recognized warning will make easier to deal with.
Backup Systems
Several backup pneumatic systems are available at reasonable costs, with some manifold vacuum systems in the area of $1,200 installed. Check with your mechanic as to the options available and prices for your specific aircraft.
If you have a standby vacuum system, review the operating instructions occasionally and ensure that all scheduled maintenance procedures are followed.
Finally, electrically operated gyros are an option that should be considered, but remember, electrical systems sometimes do fail, and with electrically powered gyros you could end up in a more serious emergency (no radios, lighting or gyro instruments) than just a pneumatic system failure.
Also take into account that some autopilots are dependent on pneumatic power to operate, so this should be taken into consideration when faced with a pneumatic failure or when purchasing or installing a backup vacuum system. System demands vary, and some backup pneumatic systems may only produce enough volume to power gyros, not de-ice boots, pneumatic door seals or autopilot servos.
If you own or install a backup system make certain you know how to operate the system and verify on a regular basis that the backup system does in fact work and you know how to operate it.
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. However, act responsibly, land and have the problem corrected.
Also consider that gyroscopic instruments can be damaged if they are “spun down” or stopped and are subjected to shock damage from rough air, or even rough runway surfaces; restore the pneumatic system before operating the aircraft again to protect your gyros.
Flying single-engine IFR can be done safely if proper procedures are followed, if you are familiar with what to do when a pneumatic systems failure does occur, and finally, apply a little safe, legal and smart. Enjoy your flying, keep it safe!
Michael Berry – ATP multi – 757 / 727 – commercial single land and sea – 15,000 + pilot hours -Turbo Jet Flight Engineer, A/P and IA mech – former aircraft repair shop owner, Piper Apache owner, 121 Air Carrier Captain.
