Sunday, March 25, 2007

Emergency Beacons

Last week, I was set to begin a flight with an instrument rating candidate when we overheard an aircraft inform the ground controller that they could hear an emergency locator transmitter (ELT) on 121.5. We tuned our number two radio to 121.5 and we heard it, too. We immediately checked to be sure it wasn't the transmitter in our aircraft. It wasn't. Later, before we shutdown, we checked the emergency frequency again and we still heard an ELT.

Virtually all GA aircraft in the U.S. are required to have an ELT installed and functioning. Each ELT must undergo a functional inspection every 12 calendar months and the battery must be replaced after half of its useful life or after one hour of cumulative use. With some exceptions, virtually all aircraft I fly have an ELT. ELTs are designed to sense G-forces consistent with a crash and when they do, they start broadcasting a siren-like signal on 121.5 MHz. The presence of an ELT signal is detected in the contiguous 48 states by one or more search and rescue satellites in low earth orbit and is automatically forwarded to the United States Mission Control Center (USMCC) in Suitland, MD. ELTs can also be manually activated within the first 5 minutes at the top of the hour to test the unit, but you should limit the test to three sweeps of the emergency signal.

121.5 ELTs have several disadvantages, the biggest being that the beacon transmits an anonymous signal and that means SAR (search and rescue) teams must physically investigate each occurrence. Other disadvantages include false signals from non-ELT sources and the fact that early models often go off when they shouldn't. In fact, ELTs manufactured under the original TSO -C91 have a high rate of false alerts that consume a large amount of resources. Fewer than 2 in 1000 alerts are actual emergencies. That's why I train pilots to monitor 121.5 as part of their shutdown procedure to be sure their ELT has not been mistakenly activated by a firm landing or a defective crash sensor.

When a 121.5 MHz ELT is activated, either by mistake or as part of a crash sequence, it can take a couple of satellite passes to verify the general position of the signal. That process might take up to three hours. Newer and more expensive ELTs broadcast on 406 MHz, they broadcast the aircraft's identifier, and they can be located much more accurately by geostationary satellites. And the latest advance is an ELT that include its GPS location on 406 MHz, which can save precious time in a real accident scenario. Given all these advantages, it's easy to understand why all 121.5 MHz ELTs will be phased out in the U.S. and must be replaced with 406 MHz units by February 1, 2009.

As we were putting our aircraft away, an airport security vehicle came by and asked if we had checked our ELT. We told him it wasn't us and I decided to do some detective work of my own. Without specialized equipment, it can be difficult to determine which aircraft has an ELT that has been activated. Here's a simple procedure I use to identify which aircraft parked on the ramp has an activated ELT.

I retrieved the hand-held transceiver I keep in my trunk, tuned it to 121.5 MHz, unscrewed and removed the antenna, and turned off the squelch. Then I drove around the ramp waiting to hear the ELT signal start coming through the white noise. Soon I head the signal and got out to investigate. With the antenna removed, I had to be right next to the offending aircraft and that was how I knew this was the airplane.

I told the ramp crew the tail number of the offending aircraft, but they didn't seem too concerned. So I called airport security and told them the tail number, too. Just to be sure, I called the local Flight Service Statioon and gave them the information, too. The next day, I saw a group of Civil Air Patrol volunteers and a county sheriff come onto the ramp and approach the plane. Apparently the aircraft's owner couldn't be located and I assume the sheriff was there to help the CAP volunteers gain access to the aircraft so the ELT could be deactivated. I bet the pilot of that aircraft will check 121.5 before shutting down from now on.

Anytime you realize that your ELT has been activated by mistake, be sure to call Flight Service after you have deactivated it. They'll be happy to hear from you.


Eric said...

The 121.5 phaseout is unfortunate, especially considering our recent purchase of an aircraft with a now-basically-useless ELT. I did a quick search on Aircraft Spruce, and it seems like there are two (affordable) crash-activated 406 ELTs available for GA use, both from Artex - the ME406-HM ($1,126) and the G406-4 ($1,621).

The extra $500 seems to cover a NAV interface that can grab GPS data and include that in the broadcast. Of course, these are uninstalled costs; the total out-of-pocket would probably wind up being in the $1,500 to $2,100 range. All things considered, I suppose it's worth it for the peace of mind and 'what-if' scenarios, but it seems like this corner of the market could really benefit from a little competition. The monopoly Artex holds is likely preventing a lot of GA pilots from upgrading some extremely important safety equipment.

Doug said...

Thanks for this reminder. I've been meaning to annotate my checklist with this and keep forgetting.

Daniel said...

Interesting post, I enjoyed reading it. I was struck by the similarity in your procedure for finding the active ELT to that of using an Avalanche Tranceiver to find someone burried in an avalanche, or who has ended up upside down in a tree well while skiing and needs assistance getting out.

An avalanche tranceiver has the ability to limit how sensitive it is to the broadcast signal, and more advanced units have the ability to indicate the direction of the flux lines of the signal (not directly at the source itself. instead the signal moves outward in the same way as the magnetic field of a magnet

John said...


Maybe some enterprising engineers out there will put together a start-up and begin marketing a competing product. If anyone is interested, the TSO for certifying 406 ELTs can be found here.

billcroghan said...

Well done finding the offending aircraft. As a CAP volunteer with over 70 ELT finds, all but 2 being false alarms, I can appreciate your quick thinking. If the handheld radio, scanner or whatever you use can't get you close enough to determine if it's one of a bunch of aircarft, tune off frequency by 25 KHz, then 50KHZ and so on until you get right up to it. If you sweep the radio receiver past the antenna that's transmitting and have it quite a ways off the frequency it will definately let you know you have found it. look for rapid change. In case you don't have a handheld 121.5 receiver, you can do fairly well close up with a standard broadcast FM radio tuned to 100.1. That accounts for 4 of my finds, and a few others were done with nothing more than a $20 radio shack tuneable radio.
Lt. Col BIll Croghan, CAP

John said...


Can you refernce where the following is required?

"Given all these advantages, it's easy to understand why all 121.5 MHz ELTs will be phased out in the U.S. and must be replaced with 406 MHz units by February 1, 2009."

John said...


Thanks for the additional pointers on locating an active ELT. Your suggestion is an excellent one and I know it should work because an active ELT (false alarm) occurred several weeks ago at HWD and it was causing the ground controllers a lot of problems because their ground frequency is 121.4.


Here are several references (all PDF files) to the 406 ELT switch:

NOAA brochure

U.S. Dept of Commerce/NOAA press release

Public notice published in the U.S. Federal Register

Tomas said...

Years ago I was part of a SAR team that normally searched for people lost in the Souther California desert. For some reason we got a call from the FAA to help locate an ELT signal. I was called to do the air search.

I had never tried to locate an ELT from the air before, but I said I'd give it a try. I quickly found that if I did a 360 turn the signal would fade when the wing blocked the ELT signal from reaching the radio antenna. After I figured that out I did 360s from two locations and quickly triangulated the ELT signal to a small nearby airport.

Next time you hear an ELT you might give this a try.

John said...


Thank you for the references. From the Federal Register quoted below, the existing ELT's can continue to be used on aircraft and are not required to be changed to the 406 ELT, however they will not be detected by satellite after the Feb 2009 date. I don't think many aircraft owners, many who are CSOB's, myself included, will upgrade their ELT's until prices come down to a few hundred bucks or it is mandated by the FAA.

"The implication of this Cospas-Sarsat decsion is that users of ELTs, EPIRBs, and PLBs that operate on 121.5/243 MHz should eventually begin using beacons operating on 406 MHz if they
wish to continue having their beacons detected by satellites. United States registered civil aircraft may carry a 121.5 MHz ELT to satisfy the requirements described in CFR Title 14, part 91, section 207. At the present time, the United States does not mandate the carriage of 406 MHz ELTs. The carriage of 406 MHz ELTs is optional."

George said...

One respondent expressed concern over the cost of replacing the 121.5 ELT in his aircraft with a 406 system.
This week, during the afternoon, an aircraft owner reinstalled his 121.5 ELT in his floatplane while it was in his boat house/hanger at his rural home. Unfortunately the ELT was activated. The owner finished up, had supper and eventually when to bed.
The signal was picked up by a commercial overflight and reported. A light aircraft flew a search pattern until dusk but could not see anything. During the evening a C130 and a helicopter were dispatched. Two ground crews were also called in. During the night the C130 was able to resolve the search area to a small general area and made many low level passes, dropping over 20 flares to illuminate the area but they could not see the source of the beacon. The noise of the aircraft plus the flares woke the aircraft owner and at approximately 3am he turned off the beacon but did not notify any authority. The C130 had to return to base as it was low on fuel, the helicopter followed after the beacon stopped transmitting. The ground crews stood down until dawn.
The hydro wires acted as an antenna for the transmitter so the signal was equally strong anywhere in the square defined by the concession roads surrounding the owner’s property. In the morning another ground crew commenced a house by house search and finally located the source.
The respondent should consider the cost the aircraft owner in the above situation is facing;
- Light aircraft plus crew for four hours
- C130 plus crew for six hours
- Helicopter plus crew for four hours
- Three trucks plus ground crews for an average of eight hours per crew.

With a 406 ELT the aircraft owner would have got a phone call and likely would have simply been told to turn the beacon off.