Aloha Airlines Flight 243

Used to introduce aluminum pressure-vessel fatigue. Tom emphasizes the high cycle count (~60,000) accumulated in only ~40,000 hours due to short inter-island flight legs, and uses the survival of the keel beam to introduce structural redundancy. Treated here as corrosion-fatigue.

We were talking about aluminum, and I want to finish up by giving you some idea of the scale of aluminum production, compare and contrast it with steel, which is forty times the scale in terms of amount of material made. I also want to talk about some of the corrosion or fatigue problems. Anybody know what this is a picture of?

Compared to the Comet as low-cycle-fatigue precedent. 37,000 cycles, 40,000 flight hours equivalent to 250,000 hours of normal Boeing service due to short inter-island flights.

Another one is the Comet aircraft, in the middle of the 1950s. Anybody hear about the Comet?

Student: Yeah, low cycle fatigue.

It was low cycle fatigue, but the problem was, they were falling apart in the middle of the Atlantic, and no one could get the evidence to find out what was causing it. They just break up in the middle of the Atlantic Ocean, and there's no evidence. Finally Alan Wells, who was head of the British Welding Institute and a fracture mechanics guy, looked at it and realized the windows were square.

Student: They had a big water-filled tank, they pressurized and depressurized it for months, and they actually grew the fatigue cracks from there in one day.

Yeah. It's not all that different than Aloha Airlines, except Aloha was 37,000 cycles, because you were in Hawaii and it was all 15, 20, 30-minute flights from island to island. The plane had 40,000 hours, where a typical Boeing aircraft would go 100,000 hours. But in terms of ground-air-ground cycles — takeoffs and landings — it was 250,000 hours equivalent compared to everything else in terms of low cycle fatigue.

The keel-beam-and-rib-cage architecture of pressurized aluminum airliners. Forty-five-minute average flight = forty-five-minute fatigue cycle, plus saltwater corrosion = corrosion-fatigue failure of the upper skin. Used to teach pressurization cycles as fatigue cycles.

Does anybody remember the Aloha Airlines disaster? Not just a hole — the whole top two-thirds came off. A commercial airliner is actually called a ship, like an airship. It has a keel beam, just like a boat. You start with this great big aluminum beam at the bottom of the aircraft and build up from there. You build a structure, sort of like a backbone with a rib cage — lying on your back with your backbone on the table and the rib cage coming around — and you put an aluminum skin around it.