ELT Basics

In recent years, the FAA has made big changes to both the specifications and regulations covering Emergency Locator Transmitters. Now, your ELT must be tested annually and, if it flunks, it may have to be replaced with a new unit that meets all the latest requirements. The new ELTs are much better than the old ones, but installation can get complicated and costly. AVweb's avionics guru explains all these changes and what they mean to you.


Avionics"ELT" stands forEmergency Locator Transmitter. It’s the little "blackbox" (actually, most are orange) mounted in the tailcone and connected to a flexiblewhip antenna. The purpose of the ELT is to transmit a distress signal if the aircraftimpacts something with high "G" force. This little jewel is probably the leastappreciated 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 both121.5 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 muchbut is usually plenty to be picked up by search and rescue satellites which usetriangulation to pinpoint the location of the transmitter, and to be homed onto by searchaircraft.

Until recently, the only ELT maintenance required by the FAA was to replace the batteryevery two or three years. No functional checks were required, so you never really knew ifthe ELT worked or not until you really needed it. But that’s changed. Now FAR 91.217requires a test of the G-switch and the mounting and antenna of your ELT once a year. Thisis a really good thing, but it has uncovered some really bad news. At our shop, we’vefound that nearly two-thirds of the older ELTs we check fail! They simply will not alarmwhen they should. You carry this unit around for years, and the one time you really needit, 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 offalse alarms, causing many wasted hours for the Civil Air Patrol and others searchinghangers and tiedowns for aircraft that might have just touched down a trifle hard. Andthere have been plenty of cases in which the aircraft did hit a mountain or a tree but theELT did not go off.

Another problem with the old C-91 ELTs is mounting. Often during a collision with asolid object, the ELT will break loose from the airframe and never activate. Yet anotherproblem is that the older ELTs didn’t have to meet a tight frequency standard, and some ofthem 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 forELTs: TSO C-91a. The new C-91a are required to have a remote panel-mounted switch (whichallows the pilot to manually activate the ELT) and a panel-mounted light or horn to alertthe 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-dutyairframe 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 whichis covered under the old C-91 TSO in your aircraft and for whatever reason, it flunks itsannual FAR 91.217 check-up and cannot be fixed. Unless you can find another (used) NarcoELT-10, you must install a new-style ELT that complies with TSO C-91a. You can not installanother C-91 ELT. Keep in mind that when you install a new C-91a ELT, you’ll have to addwiring for the new panel-mounted swtich and annunciator required by the new TSO. Often youare looking at eight hours of labor or more for the installation, plus giving up someprecious 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, theELT110-4, which lists for $500.00. For a little extra, they even have a system that willattach to your GPS/Loran and will broadcast your tail number and your fix via synthesizedvoice. Imagine, you run into a solid object, your ELT now transmits your tail number andlocation. This could change hours of searching to just minutes. This would really be niceif 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 TSOC-91a unit, you’ll face installation labor that could easily cost as much or more than theELT itself.

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