We sometimes chuckle at the amount and kind of redundant systems many owners put into their single-engine aircraft; dual everything but only one engine. Were often surprised at the logic of the average IFR operator, too.
Worst case scenario: A single-engine airplane equipped with the latest autopilot, multiple GPS map system-plus a portable back-up-launches into the lowest of the low weather and the vacuum pump fails, as does the back-up pump. It happens more than we like to admit.
As one well-equipped Bonanza owner puts it, If I had $10,000 in my pocket during an in-flight equipment emergency, Id pay it in a second to bail me out.
A back-up electric attitude indicator wont cost quite that much but even the cheapest cant be bought for a song. And, frankly, the pickings are slim with regard to choices: there are only two.
Take Your Pick
The two major players in the standby electric attitude gyro are Goodrich Avionics and Kelly Manufacturing, which offers the old R.C. Allen line of vacuum and electric instruments.
Comparing these products isnt easy or straightforward. There arent any bells and whistles and user friendliness doesnt matter. Either the gyro works as advertised when you need it or it doesnt. What makes the comparison more difficult is that the price difference between the R.C. Allen and Goodrich AIM series is substantial, literally twice. The R.C. Allens sell for around $1800, the AIM for $4000.The units we mention here are full 3-inch size instruments so panel space to accommodate either is about the same. Goodrich also makes a 2-inch electric model to fit into a clock-size cutout but unless you fly a King Air or something richer, its cost isnt practical for private GA buyers.
There are a few off brands that cater to the homebuilt market, but these arent TSOd, and will be problematic from a certification standpoint with most FSDOs when installed in a certificated aircraft. Our shop avoids this issue entirely and only installs TSOd instruments. If your shop wants to try field approval under a Form 337, thats their business.
Goodrich AIM 1100 series
The Goodrich AIM 1100 electric AI is a high-quality instrument with a look and feel to match, not to mention a breathtaking price. To appreciate the AIM 1100 series, you have to understand where it came from. Ever look at the flight deck of a modern airliner or large biz jet? The only mechanical instrument on the panel is the small attitude gyro in the center of the panel. Goodrich makes the JET series of electric gyro back-ups for these applications, as well as for the military.
This product is designed to withstand extremely high vibration and many cycles, most of which youd never encounter in the average Bonanza or Centurion.
For the little-guy application, they make the AIM line. At less than half the cost of the JET gyros, the AIM is trickle-down technology from the heavy-metal side.
The 1100 series utilizes standard blue/brown markings, has a trimmable airplane and is available with optional slip/skid indicator, further replicating its heavy-iron brother from the JET line.
Available in 14-and 28-volt operating models as well as 14- 28- and 5-volt lighting versions-or non-lighted for a several hundred dollar savings-the unit fits most general aviation applications with panel tilts from 0 to 20 degrees.
It can be mounted from the rear of the instrument panel for a cleaner look, especially for applications that still have plastic panel overlays. It weighs just less than 3 pounds and measures about 6 inches from front to rear of the case, making it easy to install in most tight panels. The pull-to-cage knob is on the lower right corner of the case so modification of some plastic overlays will be necessary.
One piece of tell-tale nomenclature is the RTCA DO-160C, Section 8.0 Vibration Curve S and P specification. This helps ensure that the unit will perform well in high vibration applications, the factory tells us, and is one reason for the AIMs high price tag.
Relatively easy to install, the AIM unit utilizes a four-pin rear connector thats of high quality. Power failures are flagged with a red bar on the upper right of the display. On initial power-up, the 1100 series is smooth, with very little vibration, tumble or nutation. Its also quiet without grinding or fluctuation in RPM.
In the air, the units that we have flown didnt miss a beat. Steep turns, rapid descents and rapid changes in aircraft pitch seem to be a non-issue for the Goodrich unit. We feel confident in using the AIM 1100 as a primary reference in cloud.
R.C. Allen RCA26 Series
Kelly Manufacturing has been in the aircraft instrument business since World War II and before that, they built business machines. Today, they offer a full line of flight instruments and also provide instruments for military tanks.
Weve noticed that they supply vacuum instruments in some of the New Piper aircraft, an area previously dominated by Sigma Tek. Their electric back-up horizons have proven controversial compared to the Goodrich product, mainly because of the price difference between the two brands.
The RCA26 series look quite similar to the Goodrich product with blue/brown presentation, trimmable airplane and pull-to-cage knob on the lower right corner. The power-loss flag is in the upper right corner.
This gyro is available in 14-and 28-volt models and is suitable for all panel tilt specifications. It weighs approximately 4 pounds. R.C. Allen claims that the gyro mounted three-phase motor/inverter system operates at lower temperatures, thus extending service life. At power up, the R.C. Allen is relatively smooth, although not as tight as the Goodrich and its noticeably noisier. Although noise level obviously isnt a concern, we think having less noise suggests better design.
In the air, the R.C. Allen performs relatively well and we found no indication of laziness, although weve heard a few complaints that the gyro are somewhat slow to erect. One thing we did see was a tumble at shutdown. When we killed the master switch, the unit slowly spun down and then started tumbling rapidly. We wonder how this effects long-term operation of the gyro.
Long-term operation is perhaps the only real test of these gyros and of course if either of these units saves your butt during a high-pucker-factor vacuum failure, either will become your ultimate hero.
The Goodrich 1100 and earlier model 500 series have performed well in our experience, although for their price Delta over the R.C. Allen unit, we would expect them to.
Out of approximately 15 installed over two years, weve seen one catastrophic failure after only 200 hours, a unit installed in a late-model Cessna 182. It became slow to erect on initial power up and was ultimately overhauled by the factory with no questions asked.
The trends among R.C. Allen electric applications seem to be similar. Owners report that they work, but they note tumble on shutdown and theyre noisy.
What do the overhaul experts say? We quizzed Mid-Continent Instruments, a premier overhaul facility and manufacturer of high-quality gyro products. They say hands down that the Goodrich product is built to higher tolerances and is simply higher quality than the R.C. Allen, as the price tag indicates; pay more, get more.
Our conclusion: If you have an application that routinely sees high vibration-a helicopter, warbird or even some twins-youre probably better off going with the Goodrich. If you want an electric horizon as primary alternative to vacuum, then definitely buy the Goodrich.
If you occasionally fly IMC and are increasingly paranoid about vacuum pump failures, the R.C. Allen will do adequate back-up at a fraction of the cost. We advise against buying a used or early model AIM or other off-brand gyro. Some of the early units are not well supported but are sold at cheap prices. As one of our Beech customers has learned, the cost and time associated with repeatedly removing a gyro for service is far from a good deal.
-by Larry Anglisano
Larry Anglisano is an Aviation Consumer contributing editor. He works at Exxel Avionics, in Hartford, Connecticut.