Helicopter engine peashooter nitriding failure and lawsuit

Engine manufacturer's nitrided spline-shaft drive coupling cracking in service, causing helicopter crashes. The spec required hydrogen bake-out within one to four hours of nitriding; the process sheet started the clock when the metallurgical lab returned the coating-thickness reading two to three days later. Tom's case study in professional engineering duty: he reported the defective process through the attorney chain when he discovered it during litigation.

I will tell you about another company that makes engines, and they have something called a peashooter, which is nothing more than a drive coupling — a spline shaft that runs from the engine to the transmission of the helicopter. They were doing a nitrided heat treatment on these splines. You were asking about localized hardening; one way to harden the surface of a steel on these little spline gears is to nitride them. They had a spec, and they had four or five of these peashooters crack, brittle fractures. A couple of them caused the helicopter to crash. Wasn't a good day for the people in the helicopter.

Tom's ethical bind as a litigation expert. Manufacturer's QC procedure put parts in the dehydrogenation furnace within four hours of *the lab test results coming back*, not within four hours of the process — defeating the spec's purpose. Splines cracked, helicopters went down, people died. Tom notified through counsel; case tried in Lafayette, Louisiana; manufacturer ultimately fixed the process.

Three-quarter-inch high-speed transmission spline, nitrided for surface hardness. The 24-hour hydrogen bake-out was being performed *after* a three-day destructive case-thickness inspection, by which time delayed hydrogen cracking had already taken hold. Engine power loss in flight; people killed. ITAR-classified, so Tom (as US citizen) read the documents while the Canadian investigator could not. Tom invoked his professional duty to public safety to demand the manufacturer be notified outside the litigation timeline.

I had a case — this was a high-speed spline that goes from the engine to the transmission of a helicopter. A three-quarter-inch diameter high-strength steel tube. To prevent wear on the splines, just inserting a spline, male into female, this female spline, when they made it, they wanted to nitride the surface of the steel — make it very hard, very corrosion-resistant. They had a spec that after they took it out of the nitriding bath, they had to bake it for 24 hours, or four hours, to get rid of any hydrogen that might be there.

Tom begins the case — nitrided high-hardness steel tube, ~3/4-inch diameter, ~$5,000 part, splined coupling between helicopter engine and transmission. A Vancouver-based metallurgist is mentioned as co-investigator. Recording cuts off before Tom describes the failure mode.

That's one story. Another, a little more disconcerting, was a pea shooter. A pea shooter is the drive shaft coupling on a helicopter between the engine and the transmission. In a helicopter they flex, so you need to have something that can articulate a little bit between the engine and the transmission. They have a hardened steel tube with internal splines that fit in at each end. They call it a pea shooter because it's just a tube, but it's high hardness, it's nitrided. It's kind of a big pea shooter — like three-quarter-inch black iron pipe, except it's probably a five-thousand-dollar pipe, machined very precisely. They had a procedure, and there was another metallurgist — actually from Vancouver, Canada — working on this. And all —