Holey Safety Net
ELTs are there to aid rescuers, but false alarms are compounded by the fact that ELTs usually don't work when they should.
This article appeared in the January 2002 edition of Aviation Safety and is reprinted here by permission.
Aircraft components are tested and retested. Manufacturers are subjected to inspection, airplanes are inspected, mechanics are inspected. The goal: Nothing mechanical should go wrong without plenty of warning.
There is one part that bucks that trend. When called on, it works only 12 percent of the time. About 97 percent of the time it's on, it's not supposed to be.
That part, paradoxically, is the emergency locator transmitter.
Designed to aid in locating downed aircraft and rescuing surviving occupants, the ELT has only occasionally lived up to its lofty goal. Although a newer design promises to help, ELTs are likely to remain what skeptics consider regulation run amok and what supporters call a last-ditch survival tool when the other options are about gone.
In theory, the concept is simple. When an airplane crashes, a sensor detects it and transmits a signal that is picked up by another aircraft or a satellite. The aircraft or satellite relays the information to a command center, and search-and-rescue teams are dispatched to the crash site. Executing that plan, however, is filled with pitfalls.
There are three main problems with making ELT rescues work: having the unit work as intended, actually reading the signal within a reasonable time, and finding the downed aircraft based on the signal.
The Black Box
If general aviation has a black box, the ELT is it – and not just because it's painted orange like those well-known airline boxes. The ELT adds nothing to the performance or operation of the airplane except potential post-crash assistance. No other part of the airplane, except perhaps the emergency parachutes worn by aerobatic pilots and carried by new Cirrus airplanes, is intended for use only when the flight has already gone impossibly bad.
Most ELTs contain a cockpit switch that selects the unit on, off or armed, a sensor or g-switch that detects a crash, and a transmitter for broadcasting a distress signal on 121.5 MHz/243 MHz or 406 MHz. Some include portable antennas that can be used if the crash damages the antenna installed on the airplane.
The older designs, which broadcast on 121.5 and usually 243, provide only an identifiable sweep tone that is picked up by aircraft or satellites. Once alerted to look – usually by satellites – searchers can use direction-finding equipment to home in on the location, though it's often a time-consuming and dangerous practice.
The 406 MHz model, which was introduced to aircraft in 1994, broadcasts a digital signal that identifies the registered airplane as being down. The satellite coverage is also different, which may reduce the problem with false alarms that plagues the earlier design.
ELTs are not tied into ship's power. They contain either rechargeable batteries or dry cell batteries that must be inspected yearly and replaced long before they're dead. Unfortunately, this is far from a fail-safe procedure, and many airplanes routinely fly with dead ELT batteries.
Regulations require the unit to be placed as far aft in the airplane as possible to insulate it from crash forces. Even so, the history of ELTs is replete with malfunctions.
Most NTSB accident reports do not mention whether the ELT activated or not, although in the investigator's standard report there is a spot to note whether it functioned and whether it was useful in locating the wreckage. In most accident reports, the field is blank.
Nevertheless, there have been several cases in which the aircraft occupants survived the crash, the ELT failed to work, and the survivors died while awaiting rescue. There has been a much larger number of reports in which the ELT failed to function, but in those cases either the occupants all died in the crash or rescuers were summoned to the scene by witnesses.
The failures fall into predictable categories. The antenna or the ELT can be destroyed in the crash. The antenna can be buried in the wreckage such that the metal fuselage or wings block the signal. Signals can be blocked by terrain.
Although ELTs are supposed to be checked periodically and inspected as part of the annual inspection, officials found at a recent aviation event in Alaska that less than half of the ELTs worked properly. Most of the problems were traced to dead or corroded batteries.
On the other end of the equation, the National Oceanic and Atmospheric Administration tracks those accidents in which an ELT signal was key to rescuing survivors. Since July 1999, the agency has recorded 13 accidents involving 19 survivors in the United States that owe their outcome to ELT detection. Of those, all but two accidents happened in Alaska. Worldwide, the system rescued 85 people in 60 ELT-located aviation accidents.
The Space Connection
The system was originally designed to pass signals to nearby aircraft, but satellites were soon pressed into service. Today, four low-orbit U.S. satellites and three low-orbit Russian satellites listen for signals, and the newer 406 MHz ELTs can be detected by two operational and two standby geostationary satellites.
For 121.5 MHz ELTs, the satellites rely on measuring the Doppler shift as they pass overhead. After several passes, computers can process the signals to the point where the location can be determined within about 12-15 miles, but the process typically takes 18 hours.
For a classic example of how wrong it can go, consider the plight of a pilot in a Cessna 182 flying VFR over the Washington mountains in 1995. The pilot, who was in training to fly for the Civil Air Patrol, filed a flight plan but did not contact controllers during the flight and apparently neglected to switch his transponder out of standby mode. When the airplane became overdue, a search began.
Authorities received sporadic ELT transmissions, but they were not sufficient to pinpoint the location of the crash. Three days and five hours later, the crash was located in a mountainous wilderness area.
The pilot had survived the crash with only minor injuries. Despite the fact that he was a CAP survival skills instructor, he was dressed in dress clothes, a lightweight flight suit and a flight jacket. The aircraft carried no survival equipment and the pilot had left his personal survival kit in his car.
Evidence at the scene indicated the pilot had tried to hike out on foot but returned to the airplane. The airplane battery was dead, perhaps from attempts to use the aircraft radios and a cellular phone. When the ELT was examined, the cable connecting the unit to the antenna was found to be improperly installed, leading the cable to kink and pull out of the transmitter. In addition, a tuning crystal in the unit had become intermittent.
Bad weather and hostile terrain further slowed search operations, despite the use of more than 50 aircraft flying more than 200 sorties and nearly 200 ground personnel on 15 vehicles.
The pilot built a shelter and wrote notes. Eventually, however, he died of hypothermia.
The lack of reliable ELT signals is compounded by a plethora of false alarms. Because of the g-switches built into them as crash sensors, ELTs can fire off after hard landings, handling on the ground, even turbulence. The NOAA, which operates the Search and Rescue Satellite-Aided Tracking (SARSAT) system, estimates that about 97 percent of all ELT signals are bogus.
In an effort to reduce the problems with the devices and speed up rescue times, the government has moved toward digital ELTs that transmit on 406 MHz.
The 406, especially when aided by a GPS location added to the signal, cuts the response time to an average of 45 minutes and pinpoints the location to within one to three miles. Modern design means they are less likely to produce false alarms, and if they do fire off inadvertently the increased precision often means the airplane can be located using telephone calls rather than search parties.
NOAA estimates that requiring 406 MHz ELTs in place of the 600,000 121.5 MHz ELTs worldwide could save 90 to 130 additional lives annually, reduce false alerts by at least 95 percent, reduce the risks to search parties, and save searchers about $7.1 million a year.
The improvements in the 406 MHz units result from the fact that they were designed specifically for detection by satellites, rather than the other way around. The signal lends itself to Doppler location better. In addition, the geostationary satellites, which can see much larger areas, can only process the 406 MHz signals.
The satellites also have a higher processing capacity with the newer versions. The 406 MHz design transmits a half-second, five-watt burst every 50 seconds. The geostationary satellites can track 90 signals simultaneously, where the older satellites listening for a weaker 121.5 MHz signal get bombarded with simultaneous continuous tones from multiple sources, as well as voice traffic.
Search and Rescue
The problem of finding downed airplanes is complicated by the fact that ELTs frequently don't work. In addition, many times bad weather is what forces the airplanes down, and search crews are hampered by that same weather.
Studies show that people who survive the crash have an overall survival rate of 60 percent if they are rescued within eight hours. The odds drop until, after two days, the survival rate is less than 10 percent.
That underlines the need to find downed airplanes, and find them fast. In some cases, the best technology isn't enough because the equipment isn't on board.
A Learjet 35A crashed on Christmas Eve, 1994, while flying an instrument approach into Lebanon, N.H. Because federal regulations made jets exempt from the ELT rules, search teams were forced to use radar data and eyeballs to find the wreck site. Despite the fact that the search continued for days, the wreckage was not found until a forester happened upon the site nearly three years later.
The tragedy led lawmakers to slip a provision into the Aviation Investment & Reform Act, which was signed into law in April 2000, that required a removal of the jet exemption from the FARs.
One of the reasons for the relatively low number of claimed ELT "saves" is that most accident locations are identified by radar locations or witnesses long before the ELT signal is received and processed. A review of ELT saves shows a much larger number of boat occupants and lost hunters/campers being rescued through emergency beacons, most likely because they're not in radio or radar contact and they're more likely to be in places where trouble goes unseen by witnesses.
The large number of boat rescues stems primarily from mechanical breakdowns, which are more likely in an industry that does not have the mandatory maintenance procedures aviation has. In addition, boat accidents tend to be engine failures and slow sinkings, which do not carry the threat of impact-related fatalities.
With all of the problems with 121.5 MHz units and the cost of maintaining two detection systems, SARSAT operators have determined they will stop processing the 121.5 MHz signals in early 2009. Current market economics put the cost of a 121.5 MHz ELT at less than $500, while a 406 MHz unit costs about $2,500.
While the cost of the modern unit is likely to come down somewhat as more people buy them, the price difference is not likely to be erased by the time the older ELTs become obsolete.
Many operators balk at the cost of replacing ELTs with more expensive versions when their usefulness is questionable. They advocate allowing pilots to install or carry them, but argue that making them mandatory does no good.
Policy questions aside, it's clear that pilots who want to maximize their chances of survival in case of an accident should not put much stock in a ELT-aided rescue. Better strategies include flying IFR or using flight following, and planning routes so your time over wilderness area is minimized.
Reporting emergencies, including reading your GPS coordinates to controllers on the way down, can make a world of difference in helping rescuers zero in on you. You may also want to carry at least a minimal survival kit of blankets, water, and flares or other signaling devices.
The fact that the vast majority of U.S. airplane accidents are not located through ELT signals points to the fact that emergency locator transmitters may be a solution searching for a problem. Follow the regs regarding ELT installation, testing and maintenance. But don't bet the farm help is on the way just because the ELT is there.