A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

If aircraft "black boxes" are indestructible, why can't the whole plane be made from the same material?

June 21, 1996

Dear Cecil:

If the "black boxes" used on aircraft to record cockpit conversation and flight data are so indestructible, why can't they make the whole airplane out of the same material?

Cecil replies:

They must get this question all the time at the National Transportation Safety Board. The guy I talked to didn't miss a beat with the answer: because the interstates aren't wide enough. His point, in case you're new to sarcasm, was that a plane built to black box standards would be so heavy you'd have to drive it rather than fly.

Unlike the rest of the aircraft, which is mostly made of light materials such as aluminum and plastic, the cockpit voice recorder and the flight data recorder are encased in stainless steel boxes roughly 10 inches by 10 inches by 5. The steel is maybe a quarter inch thick and the boxes are so heavy in consequence that the designers don't even bother enclosing the accompanying electronics, which in a crash generally meet the same fate as the passengers. As an added precaution, the boxes are lined with a liquid-filled foam bladder encased in plastic that's supposed to protect against the intense heat of a post-crash fire.

Over the years these precautions have proven pretty effective. Investigators got useful data off one recorder that had been immersed in the ocean for seven years. But in another crash in Thailand the recorder landed in a pool of flaming fuel and basically got cooked.

One thing the NTSB learned from experience: be careful where you put these things. Recorders used to be located near the point where the wings joined the fuselage, the theory being that this was the most heavily constructed part of the plane. Problem was, being heavily constructed, the parts of the plane falling on the recorders often crushed them. Now the recorders are put in the tail section so that, assuming your typical crashing plane goes in nose first, the forward part of the airframe absorbs most of the impact.

Sitting back there won't help you, though. When you mix stuff in the Cuisinart, you think it matters which end of the banana went in last?

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