September 6, 1998 ELT Basics

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by	Tom Rogers, Ph.D.

About the Author ... Tom Rogers is Avionics Editor for AVweb. Tom operates Avionics West, Inc. at Santa Maria, California, one of the finest radio shops on the West Coast. Tom is an instrument pilot, an FAA Designated Engineering Representative (DER) for avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the doctorate, but we call him "Dr. Tom," and he seems to like that.) You can send Tom your avionics questions at avionics@avweb.com. Tom's company, Avionics West, is one of the worlds largest discounters of handheld and panel-mount avionics for general aviation. They offer all leading brands of aviation electronics at deep-discount prices. Every item sold by AWI is covered by a 30-day no-questions-asked return privilege. Avionics West features handheld GPS receivers from Garmin and Lowrance; headsets from David Clark, LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI stocks these units and generally sells them at prices substantially below the manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not permitted to advertise these prices on its web site, but you can obtain them by sending an email to the AWI auto-responder at avionics-specials@avweb.com, or by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time). Ask Tom an Avionics Question

"ELT" stands for Emergency Locator Transmitter. It's the little "black box" (actually, most are orange) mounted in the tailcone and connected to a flexible whip antenna. The purpose of the ELT is to transmit a distress signal if the aircraft impacts something with high "G" force. This little jewel is probably the least appreciated and the most priceless piece of equipment in the aircraft should you need it.

When triggered, an ELT puts out a distinctive "whoop-whoop" signal on both 121.5 MHz and 243.0 MHz, the standard VHF and UHF emergency (or "guard") frequencies. Their power output is at least 50 milliwatts, which may not sound like much but is usually plenty to be picked up by search and rescue satellites which use triangulation to pinpoint the location of the transmitter, and to be homed onto by search aircraft.

Until recently, the only ELT maintenance required by the FAA was to replace the battery every two or three years. No functional checks were required, so you never really knew if the ELT worked or not until you really needed it. But that's changed. Now FAR 91.217 requires a test of the G-switch and the mounting and antenna of your ELT once a year. This is a really good thing, but it has uncovered some really bad news. At our shop, we've found that nearly two-thirds of the older ELTs we check fail! They simply will not alarm when they should. You carry this unit around for years, and the one time you really need it, nobody's home!

Older ELTs

These older ELTs were built to comply with TSO C-91. This TSO was mandated around 1971, but since that time, many problems have shown up. There have been numerous instances of false alarms, causing many wasted hours for the Civil Air Patrol and others searching hangers and tiedowns for aircraft that might have just touched down a trifle hard. And there have been plenty of cases in which the aircraft did hit a mountain or a tree but the ELT did not go off.

Another problem with the old C-91 ELTs is mounting. Often during a collision with a solid object, the ELT will break loose from the airframe and never activate. Yet another problem is that the older ELTs didn't have to meet a tight frequency standard, and some of them drifted off frequency enough that the satellites can't pick them up.

Enter TSO C-91a

For all of these reasons, the FAA finally came up with a new and more rigorous spec for ELTs: TSO C-91a. The new C-91a are required to have a remote panel-mounted switch (which allows the pilot to manually activate the ELT) and a panel-mounted light or horn to alert the pilot when the ELT is actually transmitting. The new ELTs also have a "G" switch that will activate with an acceleration of 3.5 feet-per-second, a heavy-duty airframe mount, and a frequency tolerance of .005% (among other things).

Now what does the really mean to you? Let's say you presently have a Narco ELT-10 which is covered under the old C-91 TSO in your aircraft and for whatever reason, it flunks its annual FAR 91.217 check-up and cannot be fixed. Unless you can find another (used) Narco ELT-10, you must install a new-style ELT that complies with TSO C-91a. You can not install another C-91 ELT. Keep in mind that when you install a new C-91a ELT, you'll have to add wiring for the new panel-mounted swtich and annunciator required by the new TSO. Often you are looking at eight hours of labor or more for the installation, plus giving up some precious real estate on your panel.

Upgrading your ELT

What should you expect to pay for a new C-91a ELT? Artex Inc. has a fine product, the ELT110-4, which lists for $500.00. For a little extra, they even have a system that will attach to your GPS/Loran and will broadcast your tail number and your fix via synthesized voice. Imagine, you run into a solid object, your ELT now transmits your tail number and location. This could change hours of searching to just minutes. This would really be nice if you were around hungry bears or pesky sharks!

But don't forget that if you're upgrading from an older TSO C-91 ELT to a newer TSO C-91a unit, you'll face installation labor that could easily cost as much or more than the ELT itself.

Should you upgrade to the newest unit? I'd rather wrestle an alligator than try to sell and ELT. Pilots love buying GPSs and moving maps and fuel totalizers, but most would rather not think about ELTs. The next time you load that airplane up for a ski outing or trip to grandmas, take a look at those who are riding with you. Bet you now can answer this question.