An Aviation Safety Staff Report
Common wisdom holds that since multi-engine piston airplanes have at least two engines, they must be safer than a single. Unfortunately for the common wisdom, statistics demonstrate thats not true after an engine fails. Then, the bean counters tell us that the people aboard the single are more likely to live through the experience. But operators still buy and fly twins, knowing that engine redundancy is not the only reason to move up from a single. Instead, the extra horsepower available in the average twin often means it can haul around more stuff, including two vacuum and electric sources as well as other equipment designed to minimize the operational impact of a systems failure. While pilots flying single-engine airplanes dont have the same number of powerplants a twin driver enjoys, they can often fly with the same-or better-redundancy in other aircraft systems. And theyll burn a lot less gas doing it.
These days, many new singles leave the factory with much better systems redundancy than was the case even five years ago. And, if you have a single of the same vintage as the average store-bought airplane, any systems redundancy is probably the result of something you or a previous owner added.
Which Systems?
Which systems are we talking about here, and what do we mean by redundancy? The basic systems were concerned with are those responsible for powering the gyro-based flight instruments and radios. Depending on your airplane and its vintage, two of your flight instruments-the artificial horizon, directional gyro/heading indicator-are probably powered by either a vacuum or a pressure system. The third and final gyro instrument is your turn coordinator (TC) or turn and bank indicator (T&B) and is probably electric. This arrangement meets some long-gone FAA staffers idea for redundant flight instruments required under IFR. Even so, the accident record is littered with examples of pilots who tried and failed to keep the shiny side up using only such an electric-powered TC/T&B instrument after a vacuum- or pressure-system failure. The other three basic flight instruments-airspeed indicator, altimeter and vertical speed indicator-are powered by the pitot/static system. But you knew that.
Beyond the flight instruments, your airplanes radios and other systems are powered by good old-fashioned electricity and, unless you have a miniaturized nuclear reactor stashed in the nose, depend on a battery recharged by at least one alternator or generator.
Backup Vs. Redundancy
One thing needs to be stated up front when discussing system redundancy: Unless you have two complete and separate systems, you really only have a backup in case of failure. An example of a separate system would be a set of electrically driven gyros somewhere in the panel, located so as to minimize inducing spatial disorientation when using them. An example of a backup vacuum system would be one designed to use the difference between the engines manifold pressure and ambient air as a vacuum source to drive those instruments. Even then, however, such a system does not provide a backup for all vacuum failure modes.
To achieve true redundancy here, two vacuum/pressure pumps, two filter systems, two regulators and two sets of gyros would be necessary. While components of vacuum/pressure systems other than the engine-driven pumps themselves rarely fail, it has been known to happen, including the gyros themselves. As a result of the components necessary to achieve a truly redundant vacuum or pressure system, most singles flying around with some attention paid to this system failure simply feature an alternate source, like a standby vacuum pump.
Electric systems are usually more complicated and involve many more failure modes. Belts pulling a generator can jump off their pulleys; gear-driven alternators can seize; voltage regulators can quit regulating and circuit breakers break. While backing up an electric system can be as simple and as cost-effective as buying a handheld comm radio, a portable GPS and two fistfuls of AA batteries, a truly redundant electric system will feature a separate power source (alternator or stand-by battery) as well as a second bus to power essential equipment. A schematic diagram of how Cirrus implemented its all-electric SR-22 is reproduced in the sidebar.
Improve Vacuum Reliability
But before getting into ways of achieving greater redundancy, lets discuss the many ways to enhance reliability of your existing vacuum system for little or no money.
Perhaps first and foremost is the simple expedient of proactive maintenance. How often do you replace the vacuum systems filters? Theyre fairly inexpensive, but can be difficult to access. Technically/legally, they should only be replaced by a certificated technician, but it doesnt take long and can be done during an oil change or other inspection.
Correctly installing the vacuum pump can make a world of difference in its reliability. Mounting hardware should be torqued to the correct values, low-loss fittings should be used to minimize restrictions that make the pump work too hard and cooling air should be provided, especially for dry pumps-consider the RAPCO cooling kit.
Finally, consider prophylactically changing the vacuum pump before it fails. Pump manufacturers have established life limits; changing the pump before those limits are reached can be slightly more expensive but helps prevent the most common failure mode.
Backup Vacuum Systems
Dry vacuum pumps fail with some regularity. One manufacturer, Parker-Hannifins Airborne division, has published numerous safety warnings about their dry vacuum pumps. These warnings include mandatory replacement times of a few as 300 hours time in service. The company, which no longer markets pumps for aviation use, published a series of AFM/POH supplements stating, in part, A back-up pneumatic power source for the air driven gyros, or a back-up electric attitude gyro instrument, must be installed in all aircraft which fly IFR. Although that statement doesnt have the force of law, it probably should.
Precise Flight (www.preciseflight.com) markets a Standby Vacuum System which uses the difference between engine manifold pressure and ambient air to supply vacuum to the primary aircraft instruments. However, the system is most effective below 8000 feet AGL and can be problematic with turbocharged engines.
Aero Safe Corporation (www.aerosafe.net) markets an electric motor to power a separate pump. The system can be activated by either an automatic switch or by the pilot. This system has the advantage of not being altitude or power-dependent, but does exact a weight penalty.
Before the dry vacuum pump came on the scene, the so-called wet pump was in widespread use. Wet pumps use engine oil for their internal lubrication (dry pumps use a self-lubricating material, like graphite) and usually last at least as long as the engine before needing overhaul or replacement.
Wet pumps often cannot be used with pressure systems, like those in late-model Bonanzas, and usually require an air-oil separator. But, Airwolf (www.airwolf.com) recently began marketing a new-manufacture wet pump it says can be used with pressure systems and aircraft with pneumatic deicing boots.
Perhaps the best solution to achieving a redundant system of vacuum-powered flight instruments is to remove all the vacuum instruments, the related plumbing and the pump(s), and move to an all-electric airplane. Removing the vacuum system will require installing a redundant electric system, however, and force you into the never-never land of the FAAs field approval system. Until, that is, someone comes up with an FAA-approved way to nuke all vacuum pumps driving primary flight instruments without spending more money than the airframe is worth. In the meantime, an electric artificial horizon paired with a wet vacuum pump is a cost-effective and reliable compromise.
Electric Systems
Even vacuum systems installed in twins can be relatively simple when compared to the ways in which electrons flow through an airplane. In addition to the alternator/generator, there is usually at least one battery, master solenoid, regulator, related wiring, circuit breakers and devices, all of which have different maintenance requirements and failure modes. Additionally, the electric system powers a host of devices, not just a couple of flight instruments. As noted in our July 2004 article, The Other Partial Panel, electric system failures present a completely different challenge to a pilot whose training has focused on vacuum-driven instruments.
Achieving true redundancy in the average airplanes electric system can mean some serious surgery and even more-serious bucks. To get there, youll need a separate generator/alternator as well as a distribution system-the essential bus-isolated from the rest of the airplane. The first place to start is to assess the systems you want to power from the essential bus.
Depending on how complicated your airplane is, how you use it and what devices youre willing to do without in the time it takes to get on the ground, this could include only a single nav/comm and the transponder/encoder. Of course, if you want to remove the vacuum system and go all-electric, thats not nearly enough; youll need to add the gyro instruments. Even if you retain the vacuum system, you may want to swap a simple nav/comm for the GPS and add the autopilot, the engine analyzer, annunciators and the turn coordinator to the essential bus.
Once youve identified the load you want to power from the essential bus, you then need to find an alternator with enough juice to power all this stuff. One source is B&C Specialties Moving back from installing a second electric system-and toward financial reality-means you may not need an essential bus. In the event of a primary system failure, you will, however, need to shed enough load to allow the backup system to do its thing. This means knowing your electric system and which circuit breakers control which equipment. In any event, be prepared to land without electric flaps and to pump or crank down the landing gear. Just as proper maintenance of the vacuum system can help prevent failures, so it is with the electric system. Conventional wisdom holds that an aircraft batterys nominal lifespan is four years; replacing the battery after three years helps prevent its failure. Similarly, if your belt-driven alternator throws its belts from time to time, you probably have an alignment problem. While finding the misalignment can be devilishly difficult, its probably cheaper than some other options. Gear-driven alternators have their own problems, especially in some Continental engines. Proper maintenance here can save your bacon on a dark and stormy night. Especially with older airplanes, flaky wiring, circuit breakers, regulators, switches and solenoids can ruin your day. Any recurring problems in this area is definitely reason to consider biting the bullet and replacing the offending components on a wholesale basis. This is especially true since some-but not all-newer components can be more reliable. Conclusion Also With This Article
Achieving true systems redundancy in a piston single can be both difficult and expensive. Only you, the owner, can decide if the juice is worth the squeeze. At the end of the day, proper maintenance and some good troubleshooting skills-along with some battery-powered portables-may be all you need. Never putting yourself in the position of absolutely, positively needing all non-redundant systems to do their thing works, too.
“How Cirrus Does It”
“Vacuum System Failure Modes”
“Electric System Failures”
