Air Force acoustic emission crack-growth flight test

"The general said: you're not flying one of my aircraft with a crack." Study had to move to Australia because Australian generals knew their planes were full of cracks. Used to make the point that aircraft fly with cracks; we manage them, we don't pretend they aren't there.

In general the codes treat it as a sharp crack because they don't know — they take the most conservative approach. If you know your flaw is porosity, you can tolerate five times the size flaw, because porosity is a blunt notch, not a crack. Most codes will say no cracks allowed, period. If you really know you have a statically loaded structure like a building, there are cracks in everything. Back in the 1980s the Air Force wanted to run an acoustic emission study on airplanes with cracks in them, to see if they could detect cracks using acoustic emission — when a crack grows it sends out a little burst of noise and you pick it up on a sensor. The general said: you're not flying one of my aircraft with a crack. So they had to go to Australia, because the Australian generals were smarter — they knew their planes were full of cracks. The planes you ride on, that you go to Logan Airport and get on, are full of cracks, folks. But we know how big a crack we can tolerate.

Anecdote illustrating that all flying aircraft contain cracks. An Air Force general refused to sign off on flying a US plane with a deliberately-introduced flaw, so the test had to be conducted on an Australian aircraft.

There's a story about the Air Force wanting to do an acoustic emission test. They were going to put an acoustic emission monitor on an aircraft, and they were going to introduce a flaw, a crack, into one of the structures, and they wanted to see if the acoustic emission could pick this up as part of a research project. Since they were going to introduce a crack that would make the plane unairworthy, they had to get a general to sign off that they could fly an airplane with a crack in it, and he refused. I'm not going to fly any airplane with a crack in it. Excuse me, General, but every one of them out there has a crack, and you just don't know where they are. This one at least we know where it is and we're going to monitor it. They had to fly it on an Australian jet, because this Air Force general — not gonna fly airplanes with cracks. We have cracks all through planes and submarines and other places. Steel is very forgiving material. So they didn't have to derate it.

General refused to authorize flying an instrumented aircraft with a known crack to monitor its growth by acoustic emission, despite Tom's point that all in-service aircraft already contain undetected cracks.

Most things don't allow you to leave cracks. I'm sure the Westinghouse Bettis thing, someone did fracture mechanics and all kinds of studies and determined it was okay to leave small cracks. I told you the story about the Air Force wanting to do a test on acoustic emission, and they wanted to fly an airplane with a crack in it and follow the crack. The Air Force general says, you can't fly a plane with a crack in it. Well, every plane in the world has cracks in it. Don't tell Mr. General that every one of his planes has cracks — he just doesn't know where they are, okay. But we know how fast cracks grow, and people have done the fracture mechanics. All kinds of structures have cracks in them.