The two black boxes

The 747's accident pages in this section are largely written from recorder evidence - the altitude, the Mach number, the engine readings at the moment things went wrong. Those numbers come from two recorders that most people know only as the black box, and which are actually quite different machines doing quite different jobs.

Two recorders, not one

The cockpit voice recorder captures the aural environment of the flight deck: radio transmissions, conversations between the flight crew, and the sounds of the aircraft itself. Its cockpit area microphone is normally mounted on the overhead panel between the two pilots.

That ambient sound channel is what makes the CVR worth more than a transcript. An investigator can often establish a great deal from sounds alone - engine noise changing, a stall warning, the extension or retraction of the landing gear, various clicks and pops. From those, parameters like engine speed and system state can be inferred, along with when each event occurred.

The flight data recorder does the numbers instead. By regulation, newly manufactured aircraft must monitor at least eighty-eight important parameters, including time, altitude, airspeed, heading and aircraft attitude. Modern recorders go far beyond that. The range includes anything from flap position to autopilot mode to smoke alarms, and a capable installation records well over a thousand characteristics.

Airlines typically also fit a quick access recorder, which is not a legally required item but a company-owned equivalent used for routine monitoring, training and trend analysis. The data is the same kind of data, kept for a different purpose.

The 747s span both recorder generations

The technology changed across the 747's fifty-year production run, and the regulations followed. The earliest 747s recorded on quarter-inch magnetic tape; later aircraft use digital technology and memory. The FAA has been phasing tape out through both mandates and voluntary effort, and the current approval guidance points to modern solid-state recorders meeting EUROCAE ED-112A crash-protection standards. Tape needed an hourly replacement and a calendar check on its thermal protection capsule; solid-state recorders need neither.

What the recorders have to survive

This is the part that determines where they are fitted. A recorder that survived only the initial impact would be of little use, so the survivability requirements are severe and identical for both recorders:

	Flight data recorder	Cockpit voice recorder
Recording time	25 hours continuous	30 minutes, or 2 hours for solid-state digital
Channels or parameters	18 to 1,000+	4
Impact tolerance	3,400 g at 6.5 ms	3,400 g at 6.5 ms
Fire resistance	1,100 °C for 30 minutes	1,100 °C for 30 minutes
Water pressure	Submerged to 20,000 ft	Submerged to 20,000 ft
Underwater locator	37.5 kHz, battery shelf life 6 years or more, 30-day operation on activation	Same

Because of all of that, the recorders are installed in the most survivable part of the aircraft - the tail. The underwater locator beacon, with its decades of shelf life, is what allows them to be found when the aircraft comes down in water.

Two rules that exist specifically to defeat one failure

The certification requirements contain a pair of conditions that are easy to skim past and are really the whole point of fitting two recorders:

• No single electrical failure external to the recorder may disable both the CVR and the FDR.
• They must be in separate containers.

Together those mean a single wiring fault, a single circuit breaker or a single bus problem cannot silence both boxes. The regulation also allows a combination unit, but only with a separation requirement attached - and where two combination units are fitted instead, the unit satisfying the CVR requirement may be located near the cockpit, while the FDR requirement still demands a second, separate container.

This is the same design philosophy as the rest of the 747, seen from a different angle. Redundancy only works if the redundant paths are genuinely independent.

What happens to the data afterwards

After an accident, both recorders are removed from the site and taken to the NTSB. A CVR committee - the Safety Board, the FAA, the operator, the airframe and engine manufacturers and the pilots' union - listens to the recording and produces a written transcript. Air traffic control recordings with their time codes are used to line that transcript up against real time, and sound analysis software gives finer timing on critical events.

The handling of the audio is deliberately constrained. Congress requires that no part of a cockpit voice recording be released, because of what the crew said in the last minutes of those flights. The transcript of the pertinent portions is what reaches the public, usually at a public hearing.

Related

• EICAS - the system whose messages the recorders capture
• Autopilot and flight guidance - a recorded parameter that explains a great many accident sequences
• Accidents involving the Boeing 747
• 747 systems