How exactly do airplane black boxes work?

“If black boxes are indestructible, why don’t they make the whole plane out of that stuff?”

This quip has been tossed around ever since the advent of the unbreakable aviation recorder. It does, however, prompt the following questions: what exactly are black boxes made of and how do they work? 

In light of the recent Malaysian Airlines Flight 370 tragedy, we know several things: the black box is crucial to the investigation and increasingly difficult to find. It is necessary to first clarify that black boxes, or flight recorders, are actually painted bright orange to make them easier to locate. Black boxes are also technically two distinct pieces of equipment: a flight data recorder and a cockpit voice recorder. Planes are fitted with both.

The flight data recorder stores any electronic information sent to electronic systems within the aircraft. This includes a variety of flight parameters such as location, altitude, airspeed, engine condition, time of day, etc. The cockpit voice recorder records the last two hours of conversation between the flight crew and air traffic controllers along with any ambient background noise — which can be crucial to understanding the context of a crash. For a flight that is longer than two hours, the cockpit voice recorder tapes over the earliest data to continuously capture the last two hours of information, and all of this data must be protected. 

To achieve the black box’s indestructible status, companies design the equipment to withstand 3400 G’s of force (forces that are 3400 times the force of gravity) which is roughly the equivalent to an impact velocity of 310 mph. To put this in perspective, commercial planes at cruising altitude travel at around 600 mph. Black boxes are also designed to withstand temperatures of up to 2000 degrees Fahrenheit (which is around the same temperature as molten lava) for one hour of time.

These specifications are made possible via a distinct infrastructure. The inner memory chips are first wrapped in a thin layer of aluminum. This layer is then surrounded by a one inch thick encasing of heavy-duty insulation. Lastly, the whole package is surrounded by a layer of corrosion-resistant stainless steel or titanium. Surprisingly, black boxes have been known to outperform their standards. Recovery difficulties are historically attributed to lost boxes rather than destroyed boxes.

To aid investigators searching for an unknown crash site, aviation recorders are also mounted with an underwater locator beacon (ULB). This canister is activated via contact with water, which causes it to emit an ultrasonic pulse once per second for about thirty days. Thereafter, the lithium battery that powers the frequency emission generally dies out.

In the case of Flight MH370 and the eerily similar Air France flight 447 five years ago, the crash location was in the ocean and unknown, using up crucial time in the recovery process. The range of the beacon is about 15 miles so search efforts outside of this perimeter can turn up empty. The more time passes without discovery, the more time that wind and ocean currents have to move debris away from the original crash site. Even after the location of the black box is determined, retrieving it off the ocean floor presents a variety of novel problems.  

In the case of Air France flight 447, the black boxes were not recovered until two years after the accident. With any luck, investigators will recover the Malaysian flight black boxes in less time than that, although current efforts involving a U.S. Navy Bluefin-21 sub scanning 51 square miles of ocean floor, about 15,000 feet below the surface, have turned up nothing. Officials have discussed the possibility of expanding the search area to help solve the mystery of the missing black box.