The unfortunate crash of Air France flight AF 447 Airbus 330-200 on 01 June 2009 into the Atlantic has once again underlined the importance of the ‘black box’ in investigating the causes of an aircraft accident. The ‘cockpit voice recorder’ and the ‘flight data recorder’ together constitute the black – box, considered the most critical piece of equipment in terms of providing vital clues to the causes of the accident. Recovery of the black box subsequent to a mishap is done by means of the Underwater Locator Beacon (ULB), a device fitted to the black box. The ULB sends out an ultrasonic pulse that can be detected by sonar and acoustical locating equipment. Unless the black box is recovered, one cannot progress further, and the analysis of experts/professionals runs the risk of being incomplete or off the mark. The ULB has a design life of 30 days and therefore, recovery of this black box has to be carried out on a war-footing, inclement weather conditions notwithstanding.
The accident investigation agency doubts whether the black box flight recorder of AF447 could be ever recovered under miles of water! When black box is such an important system, why is it not designed to become available immediately after accident? Why is it bolted to the aircraft structure? When the aircraft fuselage lies at the bottom, along with the black box, how can one quickly unravel the mystery of the crash? There is therefore an urgent necessity to re-visit the black- box design.
Design Challenge with stringent specs
Painted bright orange to make it more visible in an aircraft’s wreckage, the black-box is a tough metal-and-plastic case fitted in the rear of the aircraft-the area most likely to survive a crash. The case has two shells of stainless steel with a heat-protective material between the shells. The case must be able to withstand a temperature of 2,000°F (1,100°C) for 30 minutes. The flight data recorder provides information about air speed, direction, altitude, acceleration, engine thrust, and rudder and spoiler positions from sensors that are located around the aircraft. There are 88 parameters required as a minimum under current U.S. federal regulations as against 29 required until 2002. The recorder must be able to withstand an acceleration of 3400 g (33 km/s²) for 6.5 milliseconds. This is roughly equivalent to an impact velocity of 270 knots and a deceleration or crushing distance of 450 cm. Additionally, there are requirements for penetration resistance, static crush, high and low temperature fires, deep sea pressure, sea water immersion, and fluid immersion. This is in itself a tall order and a challenge for the designer.
The real challenge now!
What is required is a black-box, which floats and does not go down with the aircraft, so that it can be recovered in no time!
That is to say, a black-box must be self ejecting and positively buoyant. Ejection can be made possible with depth sensors or by taking advantage of kinetic energy at impact to separate itself from the aircraft chassis. On reaching the surface, it can be provided with a standard GPS for localization. Today it is possible to develop such self ejecting and position indicating systems for recovery of underwater systems such as Autonomous Underwater Vehicle, underwater weapons, etc.
Certain unique facilities are available to test the black- box to a shock level of 3500 g, or even more,for 6.5 msec duration. Specifically designed ‘shock resonance tube’ can be fabricated to develop such high shock levels.
Carbon composites reinforced unidirectionally with carbon fibres have a flexural strength of 8400 to 9800 kg per square cm at 1500oC (in Helium). Densities range from 1.4 to 1.64 g/ml, and are considerably lighter than other materials in use currently. To make the black box a floating system on ejection, there are ‘balloon recovery’ systems as floatation devices available. The balloon recovery system can be made to withstand high temperatures, a must for aircraft applications, by cloaking it with intumescent paints/flame retardant additive coating.
Developments in technology also throw up new and exciting possibilities in the design of the black – box. Some form of cockpit video recorder can also be incorporated into the system. The mp4 player, the current rage of music fans the world over could become part of the flight data recording software. If suitable software were to be used, then mp4s would be capable of recording over 500 hours of flight as against present 30 minutes. The black box can also be made to transmit and receive real time data continuously like cell phones. This would enable saving valuable resources for the investigators, and also aid transmission of recorded data wherever needed. Backup can also be built in with the provision of a similar box inside the aircraft itself.
A self-ejecting black-box equipped with balloon recovery can be retrieved fast, hold large amount of data, and throw light quickly on what causes aircraft mishaps.
The author is the Director, Naval Science & Technology Laboratory,