What Is a Black Box? The Recorders That Explain Crashes

The two units do different jobs and both are required on commercial aircraft:

• •The flight data recorder logs parameters from sensors throughout the aircraft, a minimum of eighty-eight required by regulation and often well over a thousand in practice, including altitude, airspeed, heading, control surface positions, engine settings, autopilot modes and switch positions, at sampling rates of several times a second
• •The cockpit voice recorder captures crew conversation, radio transmissions, and the ambient sound of the flight deck through an area microphone, which picks up alarms, switch clicks, engine noise and impacts
• •Both retain at least twenty-five hours of data under current rules for new aircraft, extended from the two hours that applied for decades and that repeatedly proved too short
• •The crash-survivable memory unit is the only part that matters structurally: a stack of solid-state memory boards inside a steel or titanium shell packed with high-temperature insulation
• •An underwater locator beacon attached to the unit emits an acoustic ping for at least thirty days, extended to ninety on some units after a search took two years
• •They are mounted in the tail, the part of an airframe statistically most likely to survive an impact

The certification requirements are extreme and are tested physically rather than modelled. A recorder is fired from an air cannon into an aluminium target at an impact of 3,400 times gravity, held for milliseconds. It is crushed under five thousand pounds of static load for five minutes, and a steel pin is dropped onto it from three metres to test penetration. It is placed in a furnace at 1,100 degrees for an hour and at 260 degrees for ten hours, to simulate both a fuel fire and a prolonged smouldering one. It is immersed at a pressure equivalent to six thousand metres of seawater for thirty days and soaked in aviation fluids. Only the memory module is required to survive, and the solid-state boards inside have no moving parts, which is why they hold up far better than the magnetic tape used until the 1990s. The orange colour with reflective tape is for finding them, and the name is unexplained: the devices have never been black, and the phrase may derive from wartime electronics slang or from early recorders being kept in light-proof housings for their photographic film.

The recorders are the central tool of a particular kind of investigation, established by international convention and carried out by agencies whose sole purpose is determining cause and preventing recurrence. The crucial legal feature is the separation of that investigation from any criminal or civil proceeding: the findings are not admissible in most jurisdictions for assigning blame, crew statements are protected, and cockpit voice recordings are subject to strict handling rules and are usually not released publicly, with transcripts of the relevant portion published instead. The reasoning is that people who fear prosecution do not report mistakes, and aviation safety depends on a continuous flow of reports about things that nearly went wrong. That principle is under pressure in several countries where prosecutors have sought recordings, and the profession regards each such attempt as a threat to the reporting culture the system rests on.

The list of changes that followed recorder analysis is the argument for the whole apparatus. Data from crashes in the 1970s and 1980s established the pattern of controlled flight into terrain, in which a functioning aircraft is flown into the ground by a crew unaware of their position, which led directly to ground proximity warning systems now mandatory worldwide. Voice recordings from the 1977 collision at Tenerife and the 1978 fuel exhaustion crash at Portland showed junior crew members failing to challenge captains, which produced crew resource management training, now standard across aviation and adopted in surgery and other fields. Recorder data has identified design faults including cargo door latches, rudder actuator reversals and pitot tube icing, each of which produced fleet-wide modifications. The loss of Air France 447 in 2009 was explained only when the recorders were recovered from four thousand metres of the Atlantic two years later, and the loss of Malaysia Airlines 370 in 2014 remains unexplained precisely because they never were, which is the strongest argument for the streaming and deployable recorders now being introduced.

An airliner carries two orange recorders, one logging over a thousand flight parameters several times a second and one capturing flight deck audio, both required to hold at least twenty-five hours and both built around a solid-state memory module tested at 3,400 times gravity, 1,100 degrees for an hour and thirty days under deep seawater. The investigations that use them are legally separated from blame so that crews report freely, and their analysis produced ground proximity warnings, crew resource management training and numerous design corrections.