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.