The crash of a de Havilland Canada DHC-3 charter aircraft in Alaska five years ago on 9 August is remembered mainly for causing the death of US Senator Ted Stevens.
But the fatal accident soon focused the attention of safety investigators, aviation lawmakers and Stevens’ fellow legislators on the failure of the emergency locator transmitter (ELT) to function properly, which delayed the rescue of five survivors – including then-EADS North America chief executive and former NASA administrator Sean O’Keefe – for several hours.
Now a joint effort by three US agencies – NASA, the National Oceanographic and Atmospheric Association (NOAA), and the air force – are trying to establish the criteria for more reliable aircraft ELT technology, which could inform a new specification being drafted by two standards organisations – RTCA in the US and EUROCAE in Europe.
ELTs represent “a known weakness, we feel, in the system” says Chad Stinson, NASA’s project manager for the ELTSAR effort.
To understand why ELTs so often fail in general aviation crashes, NASA acquired three Cessna 172s for a special series of tests. Leveraging an A-frame steel structure originally built to test the Apollo lunar lander, NASA is purposely crashing the three Skyhawks, with each packed with multiple ELTs.
A crash test on 29 July destroyed a 1958-vintage Cessna 172, featuring the classic straight tail beloved by Skyhawk enthusiasts. It was loaded with five active ELT systems, raised by the A-frame gantry to a height of 100ft and then released for a nose-first plunge to the ground.
The data from the crash – detailing precise crash loads and how each of the ELTs performed – will help NASA validate a software model that could be used to run hundreds of crash simulations.
In a rare lapse of aviation’s tightly-regulated safety culture, even the most modern ELTs often fail to perform in a “survivable” crash. Precise numbers are difficult to pin down, but a 2012 NASA study – commissioned by Congress in the wake of the Stevens crash – estimates the onboard ELTs fail to alert rescuers in 50-60% of survivable accidents, Stinson says.
The ELTs fail even when the installation conforms to the US Federal Aviation Administration’s (FAA) latest standard, as highlighted in the crash that killed Stevens and three others.
The ELT device includes a beacon connected by cables to a satellite antenna. Both the antenna and the beacon survived the impact that killed Stevens. The ELT installation met the FAA’s standard at the time, but the Velcro strap attachment failed to contain the beacon, severing the cords connecting the device to the antenna.
In such cases, the existing ELT technology would be effective if the installation was better designed. “Manufacturers can meet all the requirements using the technology and components they already have,” Stinson says. “They just need to put more engineering into it.”
In other cases, the location where the ELT is installed in the aircraft is the source of the problem. In the 29 July crash test of the Cessna 172, NASA put two of the ELTs in locations required by the FAA, while the other three were installed in spots that NASA believes could be more effective to test its assumptions.
There are also cases in which a post-crash fire destroys the cable connecting the beacon to the antenna. NASA’s research tested commercially-available, thermally-coated cables that can survive exposure to flames for more than 20min. Despite only costing $1 per foot, such cables are rarely found in ELT devices.
“People think if you’re improving something it has to cost more. In this case, a very cheap cable will do the job. But not all vendors are using it because they don’t yet have specifications on what cable you are supposed to use,” says Lisa Mazzuca, NASA’s mission manager for search and rescue.
NASA plans to submit a package of design recommendations in the second quarter next year to the RTCA committee developing the new standard. The RTCA is expected to complete its work in 2017. Then it’s up to the FAA to decide whether to allow the general aviation community to use the new standard as an option, or mandate that the industry convert to the updated ELT specification within a certain time period.
Source: FlightGlobal.com
