How Black Boxes Actually Work

The Flight Data Recorder (FDR) records the aircraft's parameters — altitude, airspeed, heading, engine settings, control surface positions, autopilot mode, warnings, and on modern aircraft well over a thousand separate values, many sampled several times per second. This is what lets investigators reconstruct exactly what the aircraft was doing.

The Cockpit Voice Recorder (CVR) records the audio environment of the flight deck: crew conversation, radio transmissions, and the ambient sounds picked up by an area microphone — switch clicks, alarms, engine noise. This is what lets investigators reconstruct what the crew understood, which is frequently not the same thing.

Both are installed in the tail, the part of an aircraft statistically most likely to survive an impact.

Recorder crash protection is certified against a brutal set of tests: impact shock at extremely high g, static crush, penetration by a weighted spike, a fuel fire at around 1,100 °C, and prolonged immersion in seawater under deep-ocean pressure. The recording medium is solid-state memory wrapped in thermal insulation inside a steel or titanium shell.

Air France 447's recorders spent two years at about 3,900 metres and their data was read successfully afterwards. That case is routinely cited because it is the strongest single demonstration of what the design withstands.

Each recorder carries an Underwater Locator Beacon, which activates on contact with water and emits an acoustic ping. The historic standard was around 30 days of transmission, which turned out to be the weak point: in both AF447 and MH370, narrowing the search area took longer than the beacon lasted.

Requirements were tightened afterwards. Beacon duration was extended to 90 days for flight recorders, and aircraft flying long over-water routes were required to carry an additional low-frequency beacon on the airframe itself, which propagates much further through water than the original higher-frequency pinger.

If the unit is intact, the memory module is removed, cleaned — often gently washed and dried if it has been in seawater — and read on equipment specific to the recorder model. If the module is damaged, the memory chips can be extracted and read directly. Laboratories capable of this work exist in a small number of countries, and it is common for a state without one to send recorders to the BEA in France, the NTSB in the United States, the AAIB in the United Kingdom, or the TSB in Canada.

The obvious question after every ocean search is why the data is not simply transmitted continuously. The obstacles are bandwidth and cost: an FDR captures thousands of parameters many times per second across tens of thousands of daily flights, and satellite capacity to move all of it in real time is expensive for a benefit that materialises in a very small number of accidents.

The compromise adopted after MH370 is triggered rather than continuous: aircraft must report position at short intervals in normal flight, and autonomous distress tracking transmits position when the aircraft enters an abnormal state — so the search area is narrow even if the recorders are never streamed at all.