Of all the things pilots worry about-and thats plenty, if you ask us-they seem to worry most about gyro instruments or, specifically, the vacuum systems which power those gyros. We get more letters and e-mails on this subject than on any other, suggesting that owners sweat gyro failures but don’t exactly know what to do to forestall them or what kind of back-up to rely on when the worst happens.
There’s no easy, all-purpose answer to this dilemma. Owners want cost-effective solutions to the instrument back-up challenge but cost effective is another word for compromised. You can pay a lot and get a lot or you can pay nearly as much and get a lot less. Or something in between.
The choices available for gyro instrument back-up have never been greater and some of the companies plying this field are…umm…passionate about their products. For example, our article on vacuum pump options in the November, 2000 issue of Aviation Consumer drew sharp reactions from Rapco and Aero Advantage, both of whom say we havent been paying attention in class. (See this months letters section.) Fair enough. Were re-examining the issue here in the context of all the gyro back-up options available to the typical owner. Our purpose here is to opine on the best bang for the buck in back-up strategies within a specified framework based on our demons. (You have your own demons, no doubt, and you’ll have to adjust accordingly.)
Some Assumptions
Assumption one: dry vacuum pumps are ancient, dead-end technology that should have been banished years ago. Having said that, thanks to the certification straight jacket, were stuck with these things and were happy to see that Rapco, Aero Advantage and Aero Accessories are at least producing a better class of pump. We think they deserve kudos for making the best of an unfortunate situation.
Assumption two: dual electric systems are the best basic system option. Cirrus, Lancair and Diamond have done away with dry vacuum pumps entirely, installing either dual electric busses with electric back-up gyros or battery back-ups of some kind. Dual batteries and dual alternators are becoming increasingly common in new aircraft.
Assumption three: in a single-engine airplane, its neither practical nor necessary to plan for a simultaneous vacuum and electrical failure, thus if you have back-up vacuum or a back-up electric gyro, that ought to be enough. (If you have both, you probably also have every merit badge the Boy Scouts offered.)
Assumption four: we no longer care if the FAA considers any kind gyro back-up strategy to be legal. Under FAR Part 91, no gyro redundancy is specified for IFR flight and the agency has steadfastly dragged its regulatory feet on safety initiatives from the field, including attempts to replace turn coordinators with attitude gyros and the use of portable electronic solid state systems which have worked quite we’ll in our tests. Were not arguing for bootleg installations of unapproved parts or instruments or stringing a panel together with safety wire, just that a workable portable solution doesnt need the FAAs blessing to be the system of choice, if it meets your needs.
Assumption five: backing-up means two things. The system should pull your fat from the fire after an inflight vacuum failure and address the aircraft-on-ground (AOG) problem of a weekend failure stranding you at some God forsaken pea patch with the closest replacement pump three days away.
Assumption six: youre not made of money. You have a budget.
Vacuum Solutions
The typical GA panel has two vacuum instruments, the attitude indicator and the directional gyro. Some high-performance aircraft have vacuum-or, in the case of Beechcraft, pressure-AIs and electric HSIs. Lose the pump and you lose your critical gyros. Worth mentioning here is both the HSI and AI constitute single-point failure potential of their own. You can have a perfectly functioning vacuum system and still find yourself in extremis due to a failed attitude gyro. The only solution to that is a back-up vacuum or electric gyro, in addition to the back-up power source to run it. (No one said this was simple.)
For the moment, lets ignore the single-point gyro failure worry and discuss vacuum back-up options. Precise Flight makes the cheapest and most cost effective standby vacuum that we know of. Like windshield wipers of circa 1950s cars, the Precise Flight SVS system taps the pressure differential in the engines intake manifold and makes a vacuum source out of it. And like the fast-slow-fast wipers of the 1950s-if you don’t remember, ask your grand dad-these system work we’ll at low power settings but hardly at all under full power, where the vacuum differential in the manifold isn’t great enough.
What this means is that if you have a pump failure at some mid-altitude, say 8000 feet, you’ll need to reduce power to generate enough vacuum to run the gyros. And that might mean descending so the available power is sufficient to maintain altitude, but at a slow cruise speed. you’ll get still more vacuum when you begin the descent for an approach, with reduced power. The downside of this system is that you might not have the altitude to give back due to weather or terrain. Further, this system isn’t recommended for turbocharged engines, although owners install it nonetheless. From a mid-teens cruise altitude, you’ll need to descend quite some distance to keep the gyros happy. If youre crossing the Rockies, you have a problem.
Weve experienced two pump failures with the Precise Flight SVS available as back-up, one in a Mooney and one in a Cessna 172. It performed as advertised, illuminating a little red pump failure light which was the signal to turn on the system by pulling out its choke-like knob. (don’t know what a choke is? Ask Gramps about that, too.) The gyros became a little sluggish in cruise flight but the AI remained erect and its response improved on approach, when the power was reduced and the vacuum pressure peaked. For $395 or about $500 installed, this system is minimal, but its reliable, requires little maintenance and is, we believe, cost effective. To solve the AOG hassle, carrying a spare pump is the best option.
Pros: Cheap, easy to install, reliable.
Cons: Minimal vacuum; requires slow cruise and/or descent; not recommended for turbocharged aircraft.
Aero Advantage
Advancing up the vacuum-only food chain, the choices get expensive by a factor of two. The aforementioned Aero Advantage dual-chamber pump is the new kid on the block and looks quite promising. As explained in our November article, the Aero Advantage drives two pump chambers through a single shaft; a special coupling keeps a failure in one chamber from taking out the shaft, disabling both pumps. If one chamber fails, Aero Advantages testing shows that the other will deliver full vacuum for at least 20 to 50 hours, making this pump a true back-up system requiring no additional plumbing. However, installation is not a simple bolt-up. You have to run wiring into the cockpit for an indicator light warning that one of the chambers has failed but reversion to the second chamber is automatic. The pump retails for about $750 and we think you should plan on about $1000 total to install it.
With what amounts to a full-up pump as back-up, the Aero Advantage produces plenty of vacuum and provides a relatively cheap option to address the single-point gyro failure worry: simply install a second, relatively inexpensive vacuum AI for $600 to $700. (We like the Sigma-Tek models.) While you may lose your autopilot if the primary AI goes, you should be able to fly comfortably on the standby gyro.
Speaking of which, what about the AOG problem? Aero Advantages Dave Boldenow tells us he is revising the STC to allow up to 25 hours of VFR-only flight on the second chamber if the first has failed, meaning you could fly home on the back-up system. Of course, if you think the second chamber is sound enough to fly home IFR, no one is going to dissuade you; there’s no need for a spare in the baggage compartment and AOG isn’t an issue. Personal choice, here.
Pros: Automatic, self-contained back-up; cost effective price; will run a second vacuum-powered AI; reasonable rebuild costs.
Cons: Wont fit every airplane; if youre a stickler for legal detail, you’ll need a spare pump if the Aero Advantage fails on a trip and you want to fly home IFR.
Aero-Safe Guardian
The Guardian system has been around for years and represents a simple solution to a simple problem: its nothing but a conventional dry pump married to an electrical motor. (The Guardian used to have an Airborne pump, it now uses the Tempest.) When the engine-driven pump fails, you simply switch on the Guardian system and youre back in business. However, Aero-Safe recommends that the Guardian be operated continuously as a hot standby while the aircraft is in instrument conditions. Its equipped with check valves that automatically isolate the engine-driven pump from the system, if that should fail inflight. You can also pre-flight the Guardians function by operating it with the engine shutdown. As back-ups go, the Guardian isn’t cheap, at $1995 for a vacuum version and $2095 for a pressure version, plus $200 to $300 in installation costs. Many Guardian pumps are firewall mounted but some are also installed in the tail, with the plumbing passed forward through the cabin. As for the AOG issue, its largely solved; you can fly the airplane home on the back-up system and enjoy normal function of instruments, including an attitude-based autopilot.
Pros: Full vacuum at the flip of a switch; STCs apply to most aircraft; plenty of vacuum to run a back-up AI; you can fly home IFR with a failed engine-driven pump.
Cons: Relatively expensive to buy and somewhat complex to install. Weight is 8.5 pounds, making this the heaviest of the back-up options
Clutched Pump
If youre a Bonanza owner, there’s another option, at least on some models. For engines equipped with a spare accessory pad, a secondary vacuum pump can be installed. It remains in standby mode until manually engaged by a clutch controlled by an electric switch on the panel. Many Bos already have this system and for those that don’t, Rapco sells the kit for $1550 without the pump, $1850 to include the pump in 12- or 24-volts for the control clutch. The kit includes the clutch, wiring and panel switches.
Pros: Provides full vacuum or pressure for all instruments; remains in standby mode until needed; can be used in manual or automatic engagement mode.
Cons: Approved only for the Bonanza; relatively expensive and somewhat complex to install compared to other systems; requires spare accessory pad.
Electric: R.C. Allen
While manufacturers of newer aircraft are trending toward full-electric airplanes with no vacuum systems at all, this isn’t a practical option for aircraft not certified that way. Nonetheless, there are three electric gyro options, none of which are particularly cheap. There are actually more choices but we cant imagine a Cherokee owner would be seriously interested in a $10,000 electric gyro.
On the low end of the price scale is the R.C. Allen electric gyro, a design that has been around for several decades. R.C. Allens experience in gyros extends to World War II, when it was a major instrument manufacturer. Kelly Manufacturing Company-which has been busy buying up aviation niche manufacturers-now owns and makes the R.C. Allen line, which includes vacuum and electric DGs and AI.
The electric AI is the model RCA26 and is available in four variants, one of which has a slip-skid indicator. All of the R.C. Allen gyros have a caging knob and, depending on model, have a blue-over-brown or a blue-over-black display with pitch bars in the blue fields. These gyros are available in standard 3-inch diameter and either 12 or 24-volt. Prices range from $2500 to $3500, which makes them a bargain in the world of electric gyros.
Performance wise, R.C. Allen gyros are adequate, in our view, but not exceptional. Weve seen them slow to erect and somewhat lazy in response. (The slow erection is due, in part, to the nature of electric gyros, which still rely on air-driven jets to self-erect.) We would call the service history of these gyros somewhat mixed. Weve heard from owners complaining of lower long-term durability than they expected.
Pros: Affordable electric gyro that solves single-point failure issue and vacuum pump failure with a single purchase.
Cons: Full-size instrument requires full-size hole; if your DG or HSI is vacuum-powered, youre still stuck without heading reference, other than compass; reliability isn’t the best, in our view; wont run an attitude-based autopilot.
Mid-Continent
Across town from Kelly in Wichita, Kansas, Mid-Continent Instrument makes a competing line of instruments, including a newly introduced electric gyro called the 4300. This product represents some new thinking in gyros because it operates off both ships power (10- to 32-volt) and it also contains a standby battery pack good for an hour of operation.
Like the peanut gyros found in jets, it gives you protection against a complete power or bus failure. The battery pack is a replaceable lead-acid array with a three-year life cycle. Replacement cost is $375. The basic gyro itself costs $2775 without the battery (4300-300) and $3330 with the pack.
We havent examined this gyro and its too new for any widely based field reports, but Mid-Continent predicts the mean time between failures will be 7500 hours which, if accurate, practically makes it a lifetime gyro. Mid-Continent says the gyro has a large mass rotor which spins at a lower RPM than do other electric or vacuum gyros and Mid-Continent predicts this will extend bearing life. As is visible in the photo on page 18, the Mid-Continent has an activation button which the pilot must push if ships power fails. It has a blinking annunciator to indicate power loss.
Pros: Appears to be a high-quality gyro; can serve as primary or secondary attitude reference; standby battery pack is a plus; can be customized to suit customer requirements; can replace turn coordinator.
Cons: On the pricey side; no autopilot pickoffs, if thats important.
AIM Electric Gyros
For years, BFGoodrich made the AIM 1100 line of electric gyros which, although considered low cost in the world of big iron, are hardly cheap. The AIM series, now under the L3 flag, since that company bought out Goodrich last year, consists of 2, 3 and 4-inch models. Only the 3-inch is cost effective for light aircraft, however, and these start at $4712 and go up from there, depending on style, options and lighting. Just for reference, the 2-inch AIM costs $7661. Nice little unit but not competitive for our purposes here.