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.
September 6, 1998
|About the Author ...
Tom Rogers is Avionics Editor for AVweb.
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
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"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
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!
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
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