How airplane black boxes survive crashes

Q: What materials are used in black boxes, and how do they prevent it from destruction?
Jeremy W., London, England

A: Manufacturers build black boxes (more formally, Flight Information Data Recorders and painted orange for visibility, not black) from three boxes: a hardened stainless steel or titanium box, an insulation box, and a thermal block. Producers put the boxes together like Russian dolls: one box is inside another box and that box is inside another box. They make it to last. It can survive a plane crash and function almost forever after that.

The brains of the box is the data storage unit, which is a magnetic tape or a memory board similar to the one in your computer. Technicians mount the memory board in a thick thermal block, which sits inside an insulating box, which sits inside a hardened steel or titanium armored capsule. Recent models measure about 5 by 6 by 9 inches (13 x 15 x 23 cm) — a little bigger than a six-pack of beer — and weigh nine pounds.

The armor protects the box from destruction by impact-up to 3,400 times the force of gravity. Honeywell tests theirs to 4,800 Gs. That's a force equivalent to stopping within 18 inches (46 cm) when going 400 miles per hour (640 km/h).

The capsule also prevents penetration, which workers test with a diabolical contraption. They fasten a quarter-inch diameter hardened steel rod to a 500-pound (230 kg) weight, and aim the weapon at the weakest point on the capsule.

The armor resists destruction by crushing, tested by applying 5,000 pounds of force (22,000 N) for five minutes.

The combination of armor, insulation, and thermal block protect the brains of the black box from flames and temperatures of 2,000 degrees Fahrenheit (1100 C) for 30 minutes-- the temperature and duration of a jet-fuel fire. It can also survive lying in the smouldering wreckage for ten hours, withstanding 500°F during that time.

Of course, the black box may end up at the bottom of the sea, so testers make sure it can survive in salt water for 30 days as deep as 20,000 feet (6100 m) and keep pinging so searchers can find it.

By the way, the term 'black box' describes either a mysterious set of circuitry whose workings the user doesn't understand or some device whose controls a user can't easily access. An airline's black box is the latter type.

"The internals of black box is invisible (appears black) for normal purposes and users (e.g. pilots) never care" to listen to what is normally only a routine recording of their flight, emails Carsten B. Poulsen, an engineer at MAN Diesel A/S in Copenhagen, Denmark. Even people who recover black boxes from plane crashes must work at gaining access to the information stored there. It's a black box.

(Answered August 15, 2001; updated Feb. 19, 2008)

Further Reading:

USATODAY.com: Flight 990's FDR black box recovered