November 1, 1997
By Alton K. Marsh
There are two important things you need to know concerning the emergency locator transmitter (ELT) in your owned or rented aircraft, and neither of them is listed in the regulations.
The first is that your rescuers may not even start to look for you for hours. The reasons for that — explained below — have to do with the accuracy of 121.5 MHz ELTs, the most commonly used ELT costing as little as $200, and with the way search-and-rescue satellites (sarsats) process the signal. Possible solutions to that problem range from carrying a cellular telephone in your aircraft, useful in all but very remote areas where there are few cellular transmission towers, to buying a more accurate and powerful (and 14 times more expensive) current-generation 406 MHz ELT.
The second important fact, according to National Oceanic and Atmospheric Administration (NOAA) and Air Force statistics, is that there is a good chance your ELT will be destroyed in the crash and never send a signal in the first place. The majority of today's general aviation aircraft have older model ELTs built to less survivable standards. "ELTs are not built like airline 'black boxes,'" said Lt. Col. Stephen M. Roark, director of the Air Force Rescue Coordination Center at Langley Air Force Base, Virginia. "They are likely to be destroyed or fail to operate more than 75 percent of the time," he said. Sometimes the antenna is broken off in a crash, cables are severed, or the On-Off switch was bad before the flight began. At other times, the problem is a weak battery. No matter what the cause, the result is the same: no signal.
Both of those tidbits of rather important aeronautical knowledge came from a visit to the Air Force Rescue Coordination Center (AFRCC) at Langley Air Force Base, Virginia. The center serves as the single point of contact for air search missions for the contiguous 48 states; two other centers serve Hawaii and Alaska. In addition to aircraft search coordination, the center also aids in other types of emergencies, such as providing military assets to find children who have wandered away from home or rescuing injured mountain climbers. If there are no commercial air transport services available, the center can even arrange transportation of donor organs. Anyone with knowledge of an emergency, such as a pilot family member, can call the AFRCC emergency number at 800/851-3051. Think of it as dialing 911.
Among those not surprised by the ELTs' poor performance is Melissa Bailey, director of airspace for AOPA's Government and Technical Affairs Division. "When the ELT rule was first proposed by Sen. Peter Dominick (R-CO), it was opposed by the FAA [and] there wasn't sufficient time to ensure that ELTs were technically reliable," Bailey said. "But Congress prevailed and, as a result, the TSO 91 ELT was hastily developed and installed in general aviation aircraft. These ELTs have an abysmal 97-percent false alarm rate and only a 12-percent actual crash activation rate."
A new standard, TSO 91A, was issued on April 28, 1985, to address the earlier problems, but the majority of the general aviation fleet carries the earlier, flawed model.
Let's look at why you might lie in the wreckage, possibly injured, for hours before help arrives — even with a working ELT. If a pilot is lucky, there is a Search-and-rescue satellite (sarsat) overhead at the time of the crash. In such a case, the ELT signal is relayed in minutes from the aircraft to the satellite, then to the U.S. Mission Control Center operated by NOAA in Suitland, Maryland. Computers in Maryland determine whether the report is from a boat or an aircraft and pass the signal instantly, without human intervention, to either the Coast Guard or the AFRCC in Virginia.
So far, so good. But there are only seven polar-orbiting satellites equipped for receiving ELT signals. Ideally, they would array themselves equidistantly around the globe, but the satellites drift like racecars around a track. Sometimes they are separated, but at other times they are clumped together. If the whole pack of satellites departed the area prior to a crash, it would be hours before a signal is confirmed. The current average time before a rescue attempt begins is two hours, but it could take up to five hours.
For an emergency signal to be confirmed, two satellite passes — not one — are required. Two passes take about three hours. The Cospas-Sarsat system is able to calculate an ELT's location by using the Doppler effect between a rapidly moving satellite and a stationary ELT. The satellite precisely measures the frequency of the signal received from the ELT and plots this frequency with the corresponding time that it was received. This forms a Doppler curve with a measurable inflection point and slope. The inflection point on the curve corresponds to the time when the satellite reached its closest point of approach to the ELT. Relating this to the known location of the satellite at that time allows the USMCC to identify a line (perpendicular to the satellite ground track) along which the ELT is located. Then, using the slope of the curve , two equidistant positions along this line and on opposite sides of the satellite ground track are identified. The slope of the curve shows how close the satellite came to the ELT. If the slope of the curve is shallow, then the points are farther away. If the slope is steep then the points are closer together. To resolve the ambiguity between which point is the actual ELT location and which is a false image, data from a second satellite pass must be used. That pass will also yield two possible positions. One of these (the actual ELT location) will match up with one from the previous pass. The other two will be discarded. One of the advantages of a 406 MHz ELT is that it's frequency is extremely stable. This allows the USMCC to be so precise with their calculations that they can identify the actual 406 MHz ELT location from the false image 95% of the time on a single satellite pass. This can save crucial minutes or even hours in a rescue.
The next problem is to determine whether the signal was caused by an accident or by a pilot who perhaps had a hard landing that triggered the ELT. Sometimes a thunderstorm can rock an aircraft violently enough to trigger the ELT. The first action, in the absence of reports of a downed aircraft from the FAA or other sources, is to conduct a telephone search. Air traffic control facilities in the Minneapolis area are called to determine whether an ELT signal is heard. Telephone searches can use up another hour or two.
Hopefully, before the search begins, the pilot has tuned the aircraft radio to 121.5 MHz, discovered that the ELT was on, turned it off, and reported the incident to the nearest flight service station. There is no penalty filed against the pilot for an accidental ELT broadcast, but the staff at Langley does pay a penalty of sorts: It must continue the search, even if there is a high probability that the signal is accidental. If it can't locate the aircraft by telephone, the Civil Air Patrol may be called to find the ELT.
Langley personnel can't scramble the CAP every time the satellite reports a hit, since there are 30,000 such hits a year. That is why the Air Force personnel wait for two signals to merge. Sometimes the problem is manmade interference. Lighting equipment on theater marquees has been falsely reported by satellites as ELTs. Each year during the EAA Fly-in and Convention in Oshkosh, the CAP is called out more than 20 times to walk down the rows of parked aircraft at the airshow and find ELTs. All the incidents could have been prevented had the pilots listened to 121.5 after landing.
Let's say that it goes all the way to a CAP search and that the ELT worked properly on impact. Now the fate of the downed pilot depends on the accuracy of the ELT. If the aircraft is using a 121.5 MHz ELT, the search area will cover an area 10 - 20 nm in radius. The 406 MHz ELT cuts the radius of the search area ten-fold and the actual search area one-hundred-fold! It provides a search area 1 to 2 nm in radius. If it is a 406 MHz ELT, the search area shrinks to half that, and a code — like a dog tag — appears on the computer screen at Langley Air Force Base. If the 406 MHz ELT has been registered, the code will indicate the pilot's name, the type of aircraft, the pilot's address, and telephone number. Surprisingly, some 406 MHz ELT owners have not registered their beacons. It is as easy as signing on to the Internet ( http://usmcc.nesdis.noaa.gov).
The higher price of the 406 MHz ELT ($2,800 versus less than $200 for a 121.5 MHz unit) has resulted in poor sales of the 406 MHz units, although Delta Air Lines and corporate flight departments use them. Search and rescue personnel who must risk their lives or the lives of others clearly prefer that pilots buy the best technology available. AOPA has taken the position that pilots can buy the 406 MHz ELT if they desire, but the association opposes efforts to mandate the more expensive 406MHz units.
A new generation of ELTs, those broadcasting position information from the aircraft's GPS over 406 MHz, is nearly ready for market. The search area will be reduced to the size of a football field. All a rescue worker needs to do is land in the middle of the search area, step out of the helicopter, and scan 360 degrees. More than likely, he or she will be only 100 yards from the crash site. The cost of such units will be greater than that for present 406 MHz units. Software at NOAA's Mission Control Center will be converted in January 1998 to take advantage of GPS information. NOAA has a temporary software patch available now to use GPS-based ELTs, should they enter service prior to January.
From the standpoint of personnel at NOAA and the Air Force, the ideal goal is to speed the rescue as much as possible, but the goal depends on the users' willingness to pay the price for more accurate and survivable ELTs.
"121.5 MHz ELTs are just about worthless," said NOAA's Lt. Mark Moran. "Just about anything would be more effective."
E-mail the author at firstname.lastname@example.org.
ELTs were required by Congress after congressional leader Hale Boggs (D-LA) was lost Oct. 16, 1972, in an aircraft accident in southern Alaska.
The screen above shows how rescue personnel at Langley Air Force Base see your ELT once it has been activated. Satellites pick up the signals and report first hits with yellow lightning bolts. Once the Air Force takes some sort of action, the icon changes to red. New search missions are indicated by circles around the icon. Lightning bolts appearing to be gray or brown indicate signals that were not heard again after three satellite passes. Those cases are then closed.
Blue squares with airplanes mean that the Civil Air Patrol is conducting a search. Pilots can save the Air Force time by reporting false ELT signals. — AKM
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