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.
The 406 Mhz system can also be coupled with a NAV interface which puts GPS coordinates in the data stream for the search and rescue operations. This brings the accuracy of the 406/Nav system down to 100 meters worldwide, which can be picked up by the geostationary satellite within one minute for positive identification and position on the earth.
The same system is used worldwide by the marine industry on the 406 Mhz frequency which works with the same satellites and search and rescue teams as the aircraft industry.
HOW DOES AN ELT WORK?
There are three different classifications of ELTs that are present today. The first, and the oldest, is the FAA, TSO C91 beacons. These beacons represent the oldest technology in ELTs and are being replaced by the new TSO C91a beacons. All general aviation aircraft that are operating within the United States are required to carry a TSO C91 ELT.
The difference between the two TSOs is that the latter can withstand much more severe abuse in a crash. Also, it utilizes a remote switch in the cockpit that enables the pilot to activate or reset the ELT. Any aircraft that currently has a TSO C91 ELT installed, must replace it with a TSO C91a beacon if it is unrepairable.
The last type of ELT available today is the TSO C126 beacons. Again, these units must withstand even more stringent abuse than the latter beacon. The main difference between this technology is the fact that these units operate on three frequencies rather than two. The third frequency (406.025 MHz) has a much higher accuracy from space than the other two, and the 406 transmitter is much more stable and transmits a digital signal at 5 watts up to the satellites. Currently no aircraft operating within the United States is required to carry a TSO C126 ELT, but many countries are mandating the use of this product worldwide.
Many original ELT installations are inadequate as far as unit location and surface rigidity are concerned. Just because the old ELT was located in a particular position does not mean the new ELT should be located there.
Statistics show that the tail section of an airplane is least likely to be damaged during a crash and, therefore, provides a good mounting environment for the ELT unit.
Accessibility of the unit is an important factor in the location of the ELT. Mount the unit as far aft as practical but where it can be easily retrieved for maintenance.
The mounting surface must be extremely rigid; therefore, mounting the ELT directly to the aircraft skin is unacceptable.
The mount location must be able to support 100 "G"s of force in any direction with no appreciable distortion. It must also be able to withstand a 350-pound force in any direction without tearing or breaking the aircraft structure. This is derived at by 100 "G" force multiplied by the weight of the ELT. In this case, the ELT weighs 3.5 pounds and a 350-pound force must be used.
The FAA guidelines for mounting an ELT are in RTCA DO-183 Section 3.1.8.
MAINTENANCE OF ELT s
United States To insure continued reliability of the ELT, it must be inspected for damage and wear which could be caused by age, exposed elements, vibrations, etc. Even the best designed equipment, if not properly maintained and cared for, will eventually fail.
The units must be inspected at least once a year unless required more frequently by FARs (e.g. 100-hour inspections). FAR Parts 91.207, 91.409, and 43 Appendix D make detailed ELT inspections mandatory.
How detailed should your inspection be? FAR 43, Appendix D(I) states, in part, that each person performing an annual or 100-hour inspection shall inspect the following components of the ELT.
1. ELT unit and mount — for improper installation and insecure mounting.
2. Wiring and conduits — for improper routing, insecure mounting, and obvious defects.
3. Bonding and shielding — for improper installation and poor condition.
4. Antenna, including trailing antenna — for poor condition, insecure mounting, and improper operation.
To ensure continued reliability, the ELT must be performance tested within the 12-month period preceding installation in an aircraft and within 12-month intervals thereafter.
For Canadian installations, all maintenance shall be performed in accordance with DOT Engineering and Inspection Manual, Part II, Chapter III, Section 3.12.7.
How detailed should your inspection be? The same reference quoted above states five essential tests:
1. The measured peak power after 3 minutes of operation.
2. The measured frequency after three minutes of operation.
3. The audio modulation, which shall be recognizable as a typical ELT signal.
4. The measured current draw in the "OFF" (ARM) position and in the "ON" position as specified by the ELT manufacturer.
5. The automatic activation system.
To comply with the above quoted FAA and Canadian DOT regulations, the maintenance procedures in the following graphic insert should be followed.