Monday, March 28, 2011

What Hapens to ELTs During a Crash

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At this point in this series (which started here) I was going to ask you to apply your imagination to what can happen to an ELT during a crash. I thought this would be necessary because hard data on ELT crash survival has been difficult to come by. However, in going through Transportation Safety Board of Canada (TSBC) Aviation Reports over the past decade I've discovered that these reports have included steadily increasing content on the status of the ELT. The debate over ELT failure rates has been going on almost as long as ELTs have been mandated. In a NASA report published in 1990 the National Transportation Safety Board (NTSB) using 1983 through 1987 data found that in 75% of accidents the ELTs did not operate, the Air Force Rescue Coordination Center AFRCC using 1984 through 1987 data found that in 77.9%  of accidents the ELT did not operate. The NTSB further found that 88% of the failures were accident related.

Of the available TSBC Aviation Reports for the years 2008 and 2009 thirty six discuss the state and operation of the ELT. In nineteen (53%) of those reports the ELT could not provide alerting or search guidance because it was destroyed in the crash, destroyed by the post crash fire, submerged in water, unserviceable, or did not activate. Of the remaining seventeen accidents, in nine (25% of the total, 53% of the remaining), the ELT system was compromised by the ELT being separated from the antenna or antenna cable, or the antenna was damaged or destroyed. Of those nine reports, there were indications of a detectable or usable signal in four. Four of the reports indicate the ELT was capable of transmitting on 406MHz (TSO-C126), fourteen indicate the ELT was not able to transmit on 406MHz (TSO-C91 or C91a).

Of the four ELTs specifically identified as TSO-C126, one operated normally, one was submerged in water, and two of the ELTs had compromised antenna systems. Of these two, one was detectable by Low Earth Orbit (LEO) Search and Rescue Satellites (SARSATs) which were able to produce a position. The other was not detected by SARSAT or aircraft overflying the site, though there is no discussion in the report of the equipment, or operating procedures used by aircraft. Surprisingly similar to the numbers given in the 1990 report considering the TSO-C126 design was supposed to improve survivability. The Canadian Mission Control Centre (CMCC) is quoted in TSBC report A09Q0181:
...because the satellites are positioned so high above the Earth, if the antenna is damaged or blanketed by wreckage after the accident, the coded message cannot be captured. However, theLEO satellite can capture the coded message on 406MHz and fix the position of the signal, even if the antenna is damaged or poorly oriented. LEO satellites can sometimes receive a signal and identify its location, as was the case in this situation. [sic]

The corollary is of course that sometimes in these circumstances the LEO SARSAT can not receive the signal, as in TSBC report A09Q0111. So what does this tell us? In 1990 the best estimate was that in about 25% of air accidents the ELT was able to get a signal out to searchers. TSBC data for 2008 and 2009 confirm this except that in about 50% of accidents the ELT is not able to get a signal out, in about 25% it is, the remaining 25% is in a bit of a grey area. The ELT system has been compromised, but in in half of those there was evidence of a potentially usable signal. Would different techniques have allowed SAR crews to be able to detect and home a signal? My personal experience is that a compromised antenna system is often not a factor in airborne electronic searches when the receiving radio equipment is operated in accordance with best practices for the receipt of weak signals. For example, ELTs accidentally activated while being shipped to service depots do not have antennas attached. They are routinely located by SAR resources. I will have to keep looking.

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