For rescue forces in search of a downed airliner, signals from an emergency locator transmitter (ELT) can help save lives. For air accident investigators, memories in flight data and cockpit voice recorders (FDRs and CVRs) help reconstruct tragic events.
Yet while ELTs, FDRs and CVRs on today’s commercial transports are designed to survive the impact and fire of a crash, protected solid-state memories have been lost. Moreover, there are no guarantees that locator beacons will work and that recorder memories will be recovered from the deep ocean.
The answer to these possible problems may be the deployable flight incident recorder (DFIR). Separating from the airframe in a crash, the DFIR can save lives, time and money. It provides rescue teams with an efficient means of finding survivors, and can provide air safety officials with answers more quickly.
The latest-generation DFIRs made by DRS Flight Safety and Communications for U.S. Air Force transport aircraft offer a comparable device, which is ready for civil certification and integrated avionics systems, and aimed at designers of new commercial aircraft.
A Flush Mount
DRS’ beacons and DFIRs have flown on military jets, turboprops and helicopters for more than 30 years. The EH-101 Cormorant and other military and civil helicopters fly with a lightweight EAS-3000 combination beacon/recorder. The U.S. Navy’s F-18 supersonic fighter uses a high-performance DFIR, and the U.S. Air Force recently ordered the next-generation DFIR 2100 for installation on up to 35 Boeing RC-135 electronic reconnaissance aircraft derived from the commercial Boeing 707.
Like the deployable recorders flying on Boeing E-3s, E-4s and other large military aircraft, the DFIR 2100 with its integrated ELT will be faired flush into the RC-135’s vertical fin. It imposes no drag penalty and weighs less than a suite with discrete FDR, CVR and ELT boxes.
In a crash on land, an automatically released airfoil carries the locating transmitter and solid-state recorder memory 100 feet (30.5 meters) or more to clear a possible fireball. At sea, the deployed locator and memory float indefinitely. They transmit satellite-compatible signals for recovery.
Based on recommendations from the International Civil Aviation Organization (ICAO) and National Transportation Safety Board (NTSB), the Federal Aviation Administration (FAA) plans to require all commercial transports built after Jan. 1, 2003, to carry two combination CVR/FDRs, rather than separate voice-only and data-only recorders. To improve the survivability of the digital memory, one "combi" must be located close to the cockpit and the other near the tail. In a dual-recorder suite, a deployable package in the tail could maximize not only the survivability, but the recoverability of precious data.
Finding the Boxes
Current FAA requirements call for recorder data media to survive accelerations to 3,400 G for 6.5 milliseconds and temperatures to 1,100ï¿½ C for 30 minutes. Crash survivable memory units generally are protected by titanium boxes 1/8-inch (3.18-mm) thick, and solid-state memory is replacing fragile tape.
Tough as they are, CVRs and FDRs on commercial aircraft are not invulnerable. Digital flight data recorders were destroyed, for example, when a Lauda Air Boeing 767 crashed near Bangkok in 1991, and when an Air Inter Airbus A320 crashed at Mount Sainte Odile in 1992.
Even when they survive a water crash with their acoustic "pingers" intact and attached, fixed CVRs and FDRs can be lost at sea. Despite relatively shallow water, data recovery from the ocean floor in recent crashes has taken four, five or more days.
The Alaska Air MD-90 that crashed off Los Angeles in February sank in 640 feet (195 meters) of water. The Egyptair 767 that plunged into the sea in 1999 required an elaborate recovery effort at 250 feet (76 meters), and recorders from the Swissair MD-11 that crashed off Nova Scotia in 1998 came to rest in 180 feet (55 meters) of water. All three tragedies happened just minutes from flying beyond the continental shelf to where the ocean floor plunges thousands of feet.
While conventional recorders are designed to withstand the pressure at 20,000 feet (6,100 meters) and their pingers are detectable down to 14,000 feet (4,270 meters), CVRs and FDRs lost at extreme depths can be difficult or economically impractical to locate and recover.
Recorders from the Air India Boeing 747 that crashed in deep water took months to bring to the surface. The sonic locator on conventional "black boxes" is designed to operate for 30 days. An extended search could lose a recorder on the muddy ocean floor forever.
The same crash forces and circumstances that threaten recorded data also can silence emergency locator transmitters. ELTs attached to the airframe can be crushed by impact, buried in collapsing wreckage, or burned by sustained fire on land. At sea, the 121.5 and 406 MHz radio signals from submerged ELTs do not penetrate water.
NTSB data shows crash victims have a 60% chance of survival if rescued within eight hours of the accident. Their chances fall to just 10% after 48 hours, so an ELT’s survivability is important. True, a deployable beacon can drift in water, but it will instantly broadcast Global Positioning System (GPS) signals to COSPAS/SARSAT satellites.
Since 1967, deployable recorders, including the DFIRS on the F-18, have logged a perfect data recovery record. The new DFIR 2100 stores up to 25 hours of flight data covering 256 parameters per second, plus two hours of sound on each of four audio channels.
Fly off and Survive
Today’s deployable recorders are commonly triggered by frangible switches located on the aircraft nose, wingtips and stabilizers, and by hydrostatic switches under the tail. As a frangible switch crushes on impact or a hydrostatic pressure switch sinks under 3 feet (0.9 meters) of water, the closed electrical circuit releases a spring catch to extend the airfoil into the slipstream. The 5-pound (2.27-kg) airfoil flies free of the aircraft with the locator beacon and memory chips, while DFIR processing electronics remain in the aircraft structure.
While the F-18 system uses a small pyrotechnic charge to ensure deployment in a high-speed crash, the DFIR 2100 for the Boeing RC-135 and other large subsonic aircraft uses an electromechanical release. Deployment time is less than 50 milliseconds, regardless of crash attitude and airspeed.
Shrinking flash-chip memory has made it possible to build a 13-by-18-by-3-inch (33-by-45.7-by-7.6-cm) deployable airfoil that meets civil crash survivability requirements. In one incident, data was successfully recovered from an F-18 DFIRS despite a tail-first impact that gave the airfoil no chance to deploy.
Ride the Bus
Like earlier deployable systems, the new DFIR on the RC-135–set for initial delivery late this year–uses an aircraft management unit (AMU) to collect data from distributed sensors and systems. AMUs now interface with discrete ARINC 717 inputs and with ARINC 429 or Mil-Std-1553 databuses. For future aircraft, DFIRs could tie directly into an integrated avionics computer and take processed data from instrument displays and aircraft subsystems.
The remote frangible and hydrostatic switches of today’s deployable recorders are frequent maintenance items. Future DFIRs for new commercial aircraft can be triggered by accelerometers in the recorder itself. The integrated, software-controlled deployment sensors can simplify airframe integration and minimize maintenance requirements. Installed, the DFIR 2100 system with its airfoil and optional data acquisition and control units weighs just 14 pounds (6.35 kg).
Deployable flight incident recorders separate data from the full fury of a crash, and they provide the most reliable way to summon help and answer questions after the worst has happened.
Rob Austin is a senior systems engineer at DRS Flight Safety and Communications (www.drs.com), Carleton Place, Ontario, Canada.