Backup vacuum and electrical systems
It actually was a dark and stormy night when Al Yecny's alternator-out light came on a few minutes after taking off from San Francisco International Airport in his Cessna Turbo 210. If the loss of the alternator is discovered immediately and the electrical load is reduced, a healthy airplane battery should be able to power enough avionics equipment to safely continue the flight for at least an hour. But the loss of a vacuum pump under the same conditions is a life-threatening event.
Because of the demand created by a change in FAA regulations, reliable backup vacuum and alternator systems are now available for almost any general aviation airplane. So on that dark and stormy night, Yecny flipped a panel-mounted toggle switch to select his standby alternator, cross-checked the system operation with the ammeter, and continued toward home.
Single-engine IMC air-taxi regulations
Yecny had installed standby vacuum and alternator systems on his T210 to take advantage of 1997 amendments to FAR Part 135. They allowed air-taxi businesses to carry passengers during instrument meteorological conditions (IMC) in single-engine airplanes. The rule changes also require an engine monitoring program and improved recordkeeping, but that's another story.
The change prompted Yecny and other air-taxi operators to aggressively seek hardware to comply with the new regulations. Backup vacuum systems had been readily available from many sources. Alternators were a different story. Today there are at least two alternator options, with a third offering on the horizon.
The availability of reasonably priced backup systems has convinced more and more pilots that the latest avionics are nice, but only after these vital backup systems are installed.
Vacuum pump failures
Dry vacuum pumps are not particularly reliable and fail for various reasons. If engine-cleaning solvent is sprayed in the vicinity of the coupling at the base of the pump, the coupling will almost certainly fail within the next five hours of operation. This preventable failure contains a valuable lesson — launching into IMC immediately after under-cowl maintenance is not a particularly safe practice. Of course, a backup vacuum system removes this restriction.
Pumps also fail because of what Darrell Allison, a good old boy from Texas who designed Aero Safe's first electrically driven backup vacuum system in March 1982, calls "shake, rattle, and roll." Allison is describing the environment of an engine-driven vacuum pump.
STCed cooling kits reduce stress on the pumps, which translates to longer life. The load on a pump also affects its life span. One popular airplane manufacturer found this out when it installed deicing boots on its high-performance single-engine airplanes in the early 1980s. The boots were inflated and deflated by a single, large-capacity vacuum pump — the same pump that provided vacuum for the flight instruments. Turning on the deicer boots increased the load on the pump. At this inopportune time, a number of the pumps failed, causing loss of vacuum to both the artificial horizon and the directional gyro. The solution was separate vacuum pumps for each system.
Since vacuum pumps are twitchy, superstitions have arisen concerning system maintenance. Some mechanics operate under the theory that if the system works don't touch it. Don't touch the filters, don't touch the hoses, don't touch anything. Others meticulously change the central filter at its prescribed 500-hour interval, and the garter filter (which protects the vacuum relief valve) every annual or 100 hours. There's merit to both theories. One time when the system definitely needs work is after a pump fails.
The failure caused a failure
Between the filter and the pump — including all the hoses, the gyro instruments, and the vacuum regulator — is a low-pressure area. If the failure of the pump is catastrophic (i.e., the vanes in the pump disintegrate to powder), then this failure opens up the pump end of the system to ambient pressure. The ambient pressure, which is always higher than the lower pressure in the system, drives any fine contaminants back into the system. This contaminates the hoses and can deposit some of this nasty crud in the gyro instruments.
If the system isn't cleaned before the installation of the new pump, the result is an expensive pump failure. This presents another reason why owners should not blast off into IMC immediately after maintenance.
Life-threatening failure
A vacuum pump failure is a life-threatening event during IMC flight. How many instrument pilots are really up to the task of immediately transitioning to needle, ball, and airspeed while maintaining an instrument flight plan? According to Finagle's Law of Dynamic Negatives (often mistaken for Murphy's Law), "Anything that can go wrong, will." The FAA has even suggested that a single-engine airplane with no backup systems should not be flown under IFR.
For Beechcraft airplane owners with pressure gyro systems, many of these backup systems also will work on your airplanes.
Backup vacuum systems
There are five varieties of backup vacuum systems. One of the simplest — and least expensive — is the Precise Flight Inc. Standby Vacuum System SVS III. This system taps into the engine induction system between the carburetor throttle plate and the cylinders. When the engine-driven pump fails, a warning indicator light is illuminated. When the pilot sees this light, pulling a knob opens a simple on/off valve, connecting the engine vacuum to the instrument vacuum system. An automatic shuttle valve isolates the inoperative engine-driven part of the system, and the standby system takes over.
Since a vacuum system requires at least three inches (minimum) of vacuum to drive the instruments, and this vacuum is created by the difference between the atmospheric pressure in the cabin (pressure at the central vacuum filter) and the less-than-atmospheric pressure in the engine intake manifold, at some higher altitudes it is necessary to close the throttle to maintain the necessary differential. For more details on this system, visit the Web site ( www.preciseflight.com). More than 16,000 of these simple, inexpensive backup vacuum systems have been sold.
Rapco Inc., of Hartland, Wisconsin, sells a system that senses low vacuum and automatically engages an electrically controlled clutch that is mounted between an engine accessory drive pad and a standard vacuum pump. One of the advantages of this system is that the pump is not rotating until it's needed, and then it's turned on automatically. A manual override circuit allows for testing, and control of the clutch if the automatic part of the system malfunctions for any reason. More information can be obtained on the Web site ( www.rapco-rfs.com).
AeroSafe, of Granby, Texas, sells its Guardian I backup vacuum system. This popular, STCed system consists of a firewall-mounted vacuum pump/electric motor and an instrument panel-mounted operating switch. Like other systems, this system uses shuttle valves to isolate the standby system from the primary system. Unlike other systems, the manufacturer recommends turning on the standby system at all times when in IMC. More information can be found on the Web site ( www.aerosafe.net).
Airborne, of Elyria, Ohio, a division of Parker Hannifin, sells its own version of an electric motor-driven backup vacuum system. This version is unique in that its panel-mounted warning light doubles as a system on/off switch. When the primary pump fails, a red light comes on — by pushing on the red light, the pump is turned on. When system vacuum is restored, the light turns green. Visit Airborne's Web site ( www.parker.com/airborne/).
Instead of installing a backup vacuum system, some pilots install rate-based autopilots. These systems receive roll information from an electrically driven turn coordinator. S-Tec's System 20 or System 30 autopilots are popular choices. For more information on this option visit the Web site ( www.s-tec.com).
Another option is a standby electric attitude gyro. This is not only a good backup for the vacuum system, but also provides a backup in case the gyros in the vacuum-driven artificial horizon fail. With electric AH prices coming down below $3,000, this idea is gaining acceptance.
Shuttle valves and lost vacuum warnings
Shuttle valves are critical to the safe operation of dual-source vacuum systems. A shuttle valve — sometimes called a check valve — automatically prevents system leakage into the inoperative system when the backup system is operating. Inoperative or leaky check valves can be detected during pretakeoff checks. If there are any discrepancies noted during pretakeoff checks, be advised that taking off into IMC with a faulty or inoperative backup system is not recommended.
For those pilots who have polished their needle, ball, and airspeed flying to the point that the thought of a vacuum pump loss in IMC doesn't start their upper lip twitching slightly, at least invest in a low-vacuum warning light. The small, one-inch vacuum gauges located at the far edge of the copilot's instrument panel are vertigo-inducing if included in an IFR instrument scan. It's not unusual for IFR pilots — especially those with an autopilot that receives its roll information from the vacuum-driven artificial horizon (AH) — to find out that their vacuum pump has failed by being forced to recover from an unusual attitude that the autopilot has put them in as it follows the slowly dying AH.
Electrical–system backups
Not too long ago, an electrical system backup consisted of a flashlight, a fistful of AA batteries to power a handheld communications radio, and a handheld GPS. This system, which can be bought for less than $1,000, still works well.
To improve in-flight performance, a panel-mounted plug is required to connect the handheld com to an airframe-mounted communications antenna. Any avionics shop can install this connection.
Av-Tek, of Kent, Washington, advertises a standby electrical power source. This source is a lead acid battery in a carrying case. According to the owner, there's a circuit in the case that monitors and controls power flow. Plugging into the airplane cigar lighter supplies power for a limited amount of time. The unit does not require approval for installation and does not have to go through an FAA certification process. For more information, visit the Web site ( www.avtek2.com). Those considering this system should get a qualified aircraft electrician to verify that the wiring in the cigar lighter circuit is large enough to handle the proposed electrical load, and that the wiring is protected by a circuit breaker or fuse.
Basic Aircraft Products Inc., of Evans, Georgia, sells an STCed auxiliary power supply. This high-quality unit, termed a turboalternator by owner Ron Cox, is a manually deployed air-driven wind generator. When the primary system fails, the pilot pulls a T-handle release lever, causing the self-regulated turboalternator to deploy into the slipstream. The output of the system varies somewhat with the airplane's speed, but 15 amps for 14-volt systems and 12 amps for 28-volt systems are average output values at cruise airspeeds. With modern avionics, this is more than enough power to communicate and navigate with a little left over. The units come with a two-year parts and labor warranty. For more information visit the Web site ( www.basicaircraft.com).
B&C Specialty Products Inc., of Newton, Kansas, manufactures an STCed engine-mounted standby alternator capable of 20 amps. Part of this system — which is the one installed on Yecny's airplane and selected for the 2001 AOPA Sweepstakes Bonanza — is a load-monitoring sensor that helps the pilot determine when the electrical load is greater than the capacity of the standby system. Reducing the load keeps the airplane battery as a reserve. Visit the B&C Specialty Web site ( www.bandcspecialty.com).
Both the Basic turboalternator and the B&C alternator are self-exciting. This means that even if the airplane battery is totally discharged, both of these backup systems will still be able to power up the airplane's electrical system.
At the present time these are the only FAA-approved backup systems for light single-engine airplanes. However, at General Aviation Modifications Inc., in Ada, Oklahoma, the same group that brought flow-matched fuel-injection nozzles and lean-of-peak engine operation to general aviation, has a standby alternator in the works. The Supplenator, GAMI's 30-amp self-exciting alternator, can be investigated on the Web site ( www.gami.com).
Conclusion
Pogo, the famous philosopher who masqueraded as a comic strip character in the 1950s and 1960s, once said, "We are confronted with insurmountable opportunities." Pogo must have been an instrument pilot without a back-up system. The modern aircraft owner can install separate top-of-the-line vacuum and alternator backup systems for less than $5,000. This is a small price to pay for the peace of mind, added safety, and the added utility that backup systems provide.
E-mail the author at [email protected].
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