Thursday, January 1, 2004
Emergency Locator Transmitters
How they work and why things are about to change
It took 33 years of flying before I finally needed an ELT. It happened in 1998 during a 9,000-nm delivery flight of a Piper Lance from Santa Monica, Calif., to Nairobi, Kenya. On the final hop of the trip, four hours out, coming up on the Ethiopia/Kenya border, the EGT bar graph readout on the Lycoming's number-two cylinder dropped off the bottom of the scale, the engine went rough, then smoothed out, then went rough again and finally quit altogether.
The dead-stick emergency landing went okay, and no one was hurt, but the aircraft owner and I were stranded in the 110-degree desert 70 nm from the nearest town, Mandera, Kenya. We had two ELTs with us, the plane's built-in, G-activated Ameriking system (which failed to activate) and my manual, portable ACR 406 EPIRB unit capable of bouncing a signal off the satellites.
Long story short, we were located in a few hours, trucked to a U.S. Save The Children compound in the village of Dolo, Ethiopia, rescued four days later and air-evacuated to Addis Ababa. Four days after that, when I returned to the U.S., I had a renewed interest in and respect for ELTs.
Like so many other aspects of aviation technology, the basic concept of ELTs is a fallout from the military. Used by the Air Force in various forms to locate downed airmen throughout the Vietnam War, ELTs were mandated in every U.S. registered aircraft in 1970, and those original systems were little more than radio beacons, designed to emit a continuous oscillating tone on 121.5 and 243 MHz following manual activation or a mid-level five-G shock. In theory, this would allow searchers to home in on the signal by triangulation.
Sadly, theory was more optimistic than reality. Though ELT signals were available to overflying satellites, the ELT's signal was analog, and the transmitter, satellite and receiving Search And Rescue (SAR) station all needed to be within range of each other. Original ELTs had notoriously short battery lives, usually not more than a few hours, and rescuers often were frustrated at losing the signal just as they began to home in on a downed aircraft's position.
Another major problem with ELTs was an alarming incidence of false alarms. The Air Force Rescue Coordination Center (AFRCC) in Langley, Va., estimates that as many as 97% of the 95,000 ELT alerts received since 1986 have been unrelated to any emergency, a result of inadvertent G activation or other equipment problems. In other words, there were over 92,000 false alarms. That's 5,800 a year or about 16 a day.
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