New Emergency Locator Technologies
By Bob Chambers and Bryan Chambers
Congress mandated emergency locator technology (ELT) carriage as an aid to search and rescue operations in an amendment to the 1970 Occupational Safety and Health Act. The equipment was hailed at the time as the "Mae West of the airways," referring, of course, to the World War II life vest named after the legendary actress.
One of the primary reasons that Congress included the ELT in the legislation is that a group of senators went down near Portage, AK, in an aircraft and were never found.
The FAA followed up with regulations that required the installation of ELTs in virtually all general aviation aircraft by June 30, 1974. This was the basis of technical standard order (TSO) C91 and it caused a flurry of activity throughout the aviation community.
The original ELT units came to market in a short time frame and that caused many problems that would air themselves over the next 10 years. Among them was false activations, bad mounting brackets, "G" switches that did not operate in actual crashes, and units that failed for a number of reasons including corrosion or just bad workmanship.
The FAA knew that a problem existed and in 1986 had created TSO C91a to rectify the problems that existed in the field. Major areas that were addressed in the new TSO were as follows:
1. A newer style "G" switch (delta velocity) and specification that emulated an actual crash condition.
2. A remote switch/monitor that indicated either aurally or visually that the unit is "ON" or "OFF" and allowed for manual reset or activation from the cockpit.
3. A tighter frequency specification.
4. Rigorous environmental testing.
a. Vibration: 10 "G"s 5Hz to 2,000 Hz
b. Shock: 500 "G"s 4 milliseconds
c. Crash: 100 "G"s 23 milliseconds
d. Penetration: _" x 1.0" bar weighted by 55 lbs. from 6"
e. Crush: 1,000 lbs.
f. Temp: -20C to +55C
g. Water: spray & immersion
Newer plastics, better battery technology, and electronics have helped to improve the reliability of the newer ELTs built to TSO C91a.
The FAA mandated the implementation of TSO C91a June 21, 1995. At that point, all manufacturing of the older TSO C91 ELTs ceased and only the newer TSO C91a units could be manufactured. All new installations after this date must install TSO C91a units, and all units that fail under the older TSO C91 and cannot be repaired must be replaced by a TSO C91a unit.
All ELTs that meet TSO C91 and TSO C91a have an inherent problem in that their accuracy is no better than +/- 20 km. There is no definition or ID for a transmitter, only a warbling tone that could be bouncing off a mountain range or out of a valley. They do work and have saved many airmen and mariners from disaster, but many parts of the world are not covered.
The world marketplace took a different direction in 1991 by addressing the newer technology of the COSPAS/SARSAT Search and Rescue System. This system uses a digital 5 watt signal at 406.025 Mhz that carries data essential to the search and rescue system identifying the aircraft in distress. This data included items such as country of registration, the ELT manufacture, and the ELT serial number.
It is capable of locating a transmitting beacon on the face of the earth to within 3 km and will marry up with a database which will give all the particulars — aircraft type, owner's address, name, telephone number, etc. From all of this information, we can pinpoint the accident and even call the downed aircraft owner's telephone number prior to the search. All 406 Mhz transmissions are considered emergencies and are acted upon immediately. TSO C126 became effective from the FAA Dec. 23, 1992. There is no requirement from the FAA to carry this ELT in North America but countries all over the world are mandating this system because of its reliability and accuracy.
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