ELTs of Tomorrow Today

Traditional emergency locator transmitter technology is old, the infrastructure creaky, and parts of it are going away. Whats this mean to you?


Since 1971, when the FAA mandated the use of ELTs, theyve been installed on approximately 170,000 aircraft. The results, frankly, have not been great. The Air Force Rescue Coordination Center (AFRCC) processes roughly 6000 transmissions from aircraft ELTs per year. Whats more, 400 will be shut off prior to being located, leaving the SAR team nothing to do but go home. Fewer than 100 of the approximately 6000 yearly signals are actual distress calls. But the bell rings and the rescue starts; there’s no other option. Those false alarms cost about $3.5 million in federal, state and Civil Air Patrol volunteer resources.

Initially, it was thought that the signal would be picked up by other aircraft. That turned out to be not very effective, because those aircraft overhead must have a radio tuned to 121.5 MHz and, with increasing use of flight following combined with the expense of radios, we in the private pilot community are just not devoting the resources. With that in mind, the Cospas-Sarsat satellites we now rely on were implemented to reduce this dependence upon overflying aircraft. Keep this in mind, as well get back to it later.

The stats are not encouraging if you’ve gone flying with the idea that a 121.5 MHz ELT will be your salvation. Set aside the false alarms and look at the search-and-rescue (SAR) deployments initiated because an aircraft was overdue. In 1988 this occurred 191 times. The distressed aircraft was found 107 times, but in only 11 cases did the ELT aid in the location. Of those 191 deployments there were 16 missions where the ELT did not function, and the length of time to locate the aircraft was greater than 72 hours.

Failure Modes

The cause of the non-functioning ELT has to do with the design of the triggering mechanism. Its a little device known as a ∆V (Delta-Vee) switch. Delta is engineer-speak for change, and V is velocity, so ∆V simply means a change in velocity. Whats not in there is the time constraint (T); make a hard landing and the switch should tolerate up to 7.5 G; crash into the brush where you slow at 2.5 G for several seconds and its supposed to trigger.

Add to this that the switch works in only one direction. If you install the ELT sideways, it wont work at all. In fact, it should be within 10 of the longitudinal axis of the aircraft, and you have to crash such that you exert that ∆VT factor within that 10 or it wont work. Do a 4 G turn, and nothing happens because the force is directed perpendicular to the axis of the switch.

Assuming the switch has made contact, the next factor in this pas de deux is simply that the signal is both weak, a measly 0.1 watt, and non-directional; the 0.1 watt radiates off in all directions. The receiving satellite is able to pinpoint the source within only 10 to 15 miles, and yet per the FAA …not having an ELT signal in an accident reduces chances of survival by 43%. Clearly, the ELTs are not perfect, but they’re better than nothing.

Sorting the Solutions

Improvements are being brought to the system, but more importantly, the satellites wont be listening for 121.5 MHz after February 1,  2009. Don’t throw away your 121.5 MHz ELT yet. You still need it for three reasons: The FAA rules still require it, ground stations and overflying aircraft will still be online, and the rescue folks need that signal once they get in the vicinity.

The next-generation ELTs will have many improvements. They will send out a signal unique to the owner, which will allow the SAR team to simply call the owner or another contact for verification of the signal. The frequency will also be changed to something unlikely to be confused with a stadium scoreboards signal leakage, the power level will be such that it can be spotted by a new group of satellites, and GPS will be optional. Enter 406 MHz ELTs.

Problem Solved?

Implementation for non-maritime use in Canada began in 1994, and with 90% of the 406 MHz ELTs registered, some 70% of the false alerts are resolved by radio or phone call. The location of the other 30% was known to within 2 to 3 miles for the non-GPS-equipped unit, and to within 100 yards for those with GPS. With this accuracy and the ability to deploy at first signal, the search times were reduced by an average of 6 hours.
This new system started more than 20 years ago when the maritime industries jumped at the chance of having a system to locate them at sea. The United States and Canada, France and the U.S.S.R. cooperated in putting up two groups of satellites specifically for this purpose. One group of satellites is in geosynchronous orbit, and the other is in polar. More on why we need two groups later.

Other than never being in a situation that needs it, the ideal scenario is that you hit the remote switch as you go down, triggering the ELT before impact. Every 50 seconds, the 406 MHz ELT sends out a 0.4-second burst of information that includes the latitude and longitude from the GPS. The geosynchronous satellite picks up that signal and relays it to AFRCC, which makes a phone call. No response or he went flying means SAR teams deploy, knowing location within 100 square meters.

Without the GPS option, rescuers saddle up with the knowledge that the signal is somewhere in a 65-square-mile box. But as they head out, the polar orbiting satellites passing over every 15 minutes are able to triangulate the position to within 5 square miles. Now they fly around listening for the 121.5 MHz signal until the plane is found. Thats the main reason the FAA is keeping 121.5 MHz, and you should keep it healthy.
The next scenario is that you’re using a 121.5 MHz ELT and a handheld personal locator beacon (PLB). This is an ELT for hikers. You have to get out of the aircraft if its aluminum, open the antenna, and hit the transmit button. If you crash after February 1, 2009, and you cant deploy the PLB, you’ll have to hope an overflying aircraft has a radio tuned to 121.5 MHz.

More Capability, Cost

You can buy a 121.5 MHz ELT for less than $200, which uses inexpensive C-size batteries that you’ll have to replace at each annual. A 406-MHz unit will take about a $1000 bite, which includes a lithium battery that will last six years. The good news is that prices are predicted to come down to half of that within a year; ACK Avionics plans to have a unit for about $600 by the time you read this.

Price is driven by several factors. If you are replacing your Ameri-King 121.5 with an Ameri-King 406 MHz, its a simple, minimal-cost swap. Several manufacturers are offering this sort of convertibility, so it is potentially a huge money saver. Be sure to ask.

What you don’t want to do is install a second ELT. The 406 MHz devices also broadcast on 121.5 MHz. If your new one and the old one both trigger, the resulting 121.5 MHz signal will be distorted, making the SAR job tougher than need be.


If your airplane is finished and you don’t want to do all that rework, there’s a fine fourth option. You kept your 121.5 MHz ELT, so you’re legal. You can augment it with a PLB for less than $400, and now you’re covered. Not perfectly, but its an affordable upgrade that takes care of some of the problem.
PLBs are 406 MHz units that were initially legal only for hikers in Alaska. When the test program demonstrated that the performance was there, PLBs were approved for the rest of us. While this seems an attractive idea on first blush, consider that the problem of a non-traceable false alarm is still there if you rely on your original ELT. Because a PLB will not automatically activate, you have to be out of your metal airplane, deploy the antenna, and flip the switch. Forget setting it off while in an engine-out emergency. You’re flying the airplane, remember? Think of it as an insurance policy. Limit yourself to low coverage and you get a lower price.

Testing your 121.5 MHz unit is pretty easy. Wait until the top of the hour, tune your radio to that frequency, push the test button briefly and hear that wheeeep, wheeeep sound. Or you could put a simple AM radio tuned to any frequency within 6 inches of the ELT antenna, and you should hear that distinctive noise.

A 406 MHz device transmits a digitally encoded signal of your registration, so there’s nothing to hear. You simply look for a flashing LED and buzzing of the unit itself. The buzzing is quite loud, too. At 80 dB a false alarm can be heard through a closed hangar door.

During a test that digitally encoded signal has had two of its digits swapped. The satellite sees this, knows you’re testing and ignores you. The 121.5 side of the box is doing what it always has done, so remember to do this at 5 minutes before the hour.

There’s a limit to the number of times you can test the unit. The on-board computer will debit you for 30 seconds of use each time, and when you reach one hour, an LED flashes to tell you to install a new battery.

Category and Class

Weve already spoken of the three major categories: EPIRB, PLB and ELT. They’re all the same basic device in that they transmit a unique call for assistance on the 406 MHz wavelength. Lets summarize them and then look at the sub-categories.

EPIRB (emergency position indicating radio beacon). This is the nautical version, so it has to float and resist water intrusion to a depth of 1 meter, but it wont have the ∆V switch. Its not used on aircraft.

PLB (personal locator beacon). This version is lightweight for hikers. It transmits your registration, along with a P to indicate that SAR should look for a human-sized target, not a boat or aircraft. You have to be out of the aircraft and trigger it manually.

ELT (emergency locator transmitter). Its for aviation use, so it has the ∆V switch and limited impact and fire resistance.

Within those three categories, there are designators:
AF (auto fixed). This means its triggered automatically and is bolted to the airframe.

AP (auto portable). Its triggered automatically and can be quickly removed for portability.

P (portable). You have to manually activate it, and you can easily take it with you.
H (helicopter). This has the omni-directional G switch, not the ∆V type. Its also AF rated.

S (survival). This has been tested for fire and is waterproof to a depth of 1 meter.

The designators can be mixed and matched. Flying your helicopter in Alaska? Then AF/H/S is for you.

There’s also the issue of programming them. There are four programming protocols: tail number, serial number, aircraft designator and ICAO. The only time you need to worry about this is if you spot one on the Internet thats too good a bargain to pass up. Be sure that its programmed for either the tail or the serial number.

If you buy it from a reputable dealer, you need not give this another thought as it will be programmed before shipment. But you still need to register it. This requires completion of a simple, one-page form that lists the typical name, phone number, aircraft color and type, and includes instructions on where to send the form. If you sell the airplane later, the new owner will need to fill it out again so that SAR can go looking for him, not you.

What to Buy

In one sense, the decision about what to buy is simple. The FAA requires all multi-seat aircraft to have an ELT. You can continue to use an inexpensive 121.5 MHz unit and hope you get heard. Or you can opt for a more expensive 406 MHz ELT and have assurance that your distress calls are more likely to be heard. The market is still responding to the essential phase-out of 121.5 ELTs and prices for the 406 units are expected to come down as the category fills out with more new product. Today, its probably a bit early to spend big on a 406, but keep one in mind if you’re still in the building process. By the time you’re ready to fly, the category might look substantially different. 

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Bob Fritz
KITPLANES readers will remember Bob Fritz (1947-2011) for his acclaimed Home Machinist series, but his accomplishments go well beyond that long-running feature. Following a stint in the U.S. Navy, Bob put his degree in mechanical engineering to use and was a tireless advocate for effective and consistent quality control. He brought that discipline to his work for KITPLANES. An avid diver and motorcyclist, Bob's love of flying was a surprise to no one.


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