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The ELT, Your Safety Net

Iím sure if I wrote an article about a new color MFD, portable GPS or a new mode of operation for your autopilot, Iíd get your attention in a second. But mention an ELT and its instant turn-off for most aircraft owners. Lets fact it, the ELT doesnít make the aircraft fly any faster, cheaper, aid in navigation or add a sexy new display to the panel. At best the ELT may save your life or the lives of your loved ones, thatís as good as it gets with an ELT; so I guess one could say it has little benefit for the pilot and crew. In my twenty some years of selling general aviation avionics, ELT in the Tail Iíve found it quite easy to sell DMEís, MFDís and autopilots. But selling an aircraft owner an ELT is in the same league as getting the "X" to voluntarily lower the spousal support payment. Most general aviation aircraft owners just arenít interested in safety items; unfortunate but true.

Recently I had a gentleman send me an e-mail that had some questions about ELT TSOís. He had the understanding that if he upgraded to another type of ELT, the newly installed ELT must meet TSO-91a but he found he could still purchase the older TSO-91 series ELTís new. Hereís a little history on ELT TSOís (Technical Standard Order). A TSO is nothing more than a standard set by the FAA in which a product must meet. The first TSO I could find on ELTís was TSO-C91 written back in 1971. Throughout the years the FAA found that the ELT often failed as a result of a crash, thus no signal was given off. In many cases the FAA found the inertia switch (commonly called the G Switch) failed to activate. ELTís under TSO-C91 were prone to failure and didnít work the one time you needed it. Prior to June 1995, there wasnít even a requirement to test the ELT, only replace the batteries if expired. Around June 1995, the FAA created FAR 91.207 which tells the avionics shop and A&Pís just what must be tested to comply with FAR 91.207.

Lets discuss what FAR 91.207 says. Within each 12 calendar months FAR 91.207 must be complied with.

∑ The inspector must verify the ELT is placed as far in the tail of the aircraft as "practicable".
∑ The batteries must be replaced if the transmitter has been on for more than one cumulative hour or if the battery date has expired (such as the Narco ELT 10). Many of the newer ELTís that meet TSO-C91a use standard "D" type batteries. These batteries must be replaced when 50% of their dated life has expired. In other words, it the battery says life is good until Feb 2004, you would have to change them in Feb 2002. No biggie, these are standard "D" batteries, just use the old batteries in your flashlight until they die.

∑ Proper installation, mounting brackets, coax and antenna.
∑ Corrosion of the batteries or battery compartment
∑ Operation of the controls (on/off/arm/test), inertia switch
∑ The presence of a sufficient signal radiated from the antenna

These are items that FAR 91.207 requires to be tested, Iíd recommend having your local avionics shop verify the power and frequency output of the ELT . The TSO-C91/91a class ELT transmits on 121.50 Mhz and 243.00 Mhz at 100mW of power on a good day. When this FAR first came into life we found that around 60% of the TSO-C91 ELTís failed to operate due to a defective inertia switch.Inside, the Actual ELT Transmitter Imagine, carrying an ELT around for decades and the one time you need it, itís out to lunch.

Lets say your present ELT fails, must you upgrade to the newer TSO-C91a models? If you can find an exact part number replacement then you are allowed to replace your defective ELT with that like part number, new or used. Used TSO-C91 ELTs often can be found, such as the Narco ELT-10, EDO Aire, Larago and Sharc 7ís. In fact, many of the TSO-C91 ELTís are still being manufactured today for direct replacement of the older ELTís. If your old ELT has failed and you canít find a Serviceable or new exact part number, then you must upgrade to the new TSO-C91a model. Any new installation requires the use of the newer TSO ELT as called out in FAR 91.207.

Whatís the difference between the old TSO-C91 and C91a  you ask? Actually they have more differences than things in common. The TSO-C91a ELT has a beefier mounting rack that keeps the it in place during a crash. The case of the ELT is much stronger and durable. Older ELTís had a problem of the transmitter drifting off frequency, the newer ELTís have a better transmitter that is more likely to stay on frequency, thus making it easier to pin-point the aircraft if a crash happens. The biggest update is the inertia (G) switch which had been completely redesigned. As I mentioned above, we found about a 60% failure rate directly related to the inertia switch in the older TSO ELTís.  False alarms caused by the newer ELTís is less than 5% as compared to over 90% caused by the TSO-C91 models. The newer TSO ELTís require small annunciator/control panel to be installed somewhere in the cabin. This panel provides a method for the pilot to turn on the ELT if desired, to test the ELT and provide a visual indication via a blinking light when the ELT is activated. These little panels do take some time to install because thereís a wire that goes from the ELT in the rear of the aircraft to the front of the aircraft, thus part of the interior must be removed.

Most ELT systems are a matched set. In other words, the ELT transmitter, coax and antenna have been designed together and no manfacturerís parts should be used. In other words, use all the parts provided, donít mix and match ELT parts, you may void the TSO by doing so. Thereís going to be some "engineer" that will call wanting the specs on the typical 121.5Mhz beacon ELTís so Iíll list them now. Oh, its been estimated that world-wide, thereís around 600,000 121.5 Mhz beacons in use today. TSO-C91/91a ELTís are scheduled to phased out Feb 1, 2009 to be replaced by the 406 Mhz TSO-126 ELT, a far superior unit. Your ELT can be removed from the aircraft up to 90 days if a placard is placed on the instrument panel that states, "ELT not installed". You'll also need to make an entry in the aircraft records stating the date the ELT was removed along with make, model, serial number and reason for removal.  I wouldn't recommend flying the aircraft very far without an ELT.  

RG Signal

∑ Transmit power 50-100mW PERP, I normally see around 100mW
∑ Transmission life: Normally 48 hours
∑ Transmit Frequency 121.50 & 243.00 Mhz +/- 6 khz
∑ Polarization: Linear


∑ Sweep rate: 2-4 Hz
∑ Range: 300-1600 Hz (swept at least 700 Hz)
∑ Modulation: AM
∑ Percent of Modulation: >85%
∑ Duty Cycle: 40%

Just how does the ELT as a system work? The nominal system configuration is four satellites,

How the System Works
two Cospas and two Sarsatís. Russia supplies two Cospas satellites placed in near-polar orbits at 1000 km up and is equipped with SAR instrumentation at 121.5 Mhz and 406 Mhz, Russia does not monitor 243. Mhz. We Americans supply two NOAA meteorological satellites placed in sun-synchronous, near-polar orbits up around 850 km. Each satellite makes a complete orbit around every 105 minutes, traveling at a velocity of 7 km/sec. The satellite views a "swath" of earth of approximately 600 km wide as it circles the globe, giving instantaneous "field of view" about the size of a continent. When viewed from earth, the satellite crosses the sky in about fifteen minutes, depending on the maximum elevation angle of that pass. The GEOSAR Satellite Constellation is composed of two satellites provide by us Americans referred to as GOES East and GOES West and one satellite provided our friends from India. These satellites provide the continuous earth coverage.

Lets say youíve had a bad day; the family flying chariot just went down and the ELT goes off and works properly. What happens next? One thing to keep in mind is the satellites provide coverage to only one-third of the world, hopefully you will crash in a satellite covered area. Two passes of the satellites are required in order to figure out your approximate location. Two passes could take up to 3.5 hours (with your luck), which is a long time when you are hurt. With any luck at all the best you can hope for is they can pinpoint your position within 144 square miles, often the distance is over 400 square miles because the ELT transmitter may be off frequency. Mission control now sends out the Civil Air Patrol, cops or whoever to find you. Not knowing your exact location could take from hours to days, depending on the terrain. Take a look at the table below.

The old TSO-C91/91 systems still leave a lot to be desired but they are better than nothing. Upgrading from an old ELT to the C91a models cost about as much as two aviation hamburgers (1 hamburger = $100.00),LUT Map so thereís really no reason to be flying around with a questionable ELT. Whenís the last time you tested your ELT for proper operation? Iíd recommend before your next flight you take a peek in the tail of your aircraft and make sure the ELT is properly mounted, the coax is connected from the ELT to the antenna and the switch (if equipped) is placed in the "ARM" position. Next time you are by the avionics shop, have the boys check out the ELT transmitter for power and the proper frequency. That three pound orange box may save your life some dayÖ

Notice the superior coverage and performance of a TSO-C126 ELT which transmits on 406 Mhz. Itís obvious from the chart below that 406 Mhz is better, why arenít more folks using it now days? We will dive into the 406 Mhz ELT in the future. To listen to the tone of an ELT, click here.


121.5 MHz

406 MHz


12 miles

2 miles





0.1 Watt

5 Watts





2 Hours



Two Passes

Single Pass



100m Accuracy