BEEF SUPREME
Well-Known Member
While Patey has the work ethic and demeanor of a cocaine squirrel, I love the work that he does. Despite not being a formally trained engineer he’s got a great street-smart for engineering design, especially mechanism/kinematics, and his designs for the cub double-slotted leading edge slats and the Turbulence nose gear are very cool. His recent videos on the build photos for Turbulence are very impressive and show the level of structural retrofit they went through on the airframe beyond simply bolting a PT-6 to a Lancair Legacy as many others in the experimental community would do.
While I doubt I’ll change many minds, here are some key points from the video since a lot of you will write this off as experimental stuff and not sit through 40 min:
- The engine was a PT-6A-42 (850 shp, used in C208 Caravan, King Air B200, etc). From Pratt & Whitney: https://www.prattwhitney.com/en/blogs/pt6-nation/2017/07/20/interview-with-pilot-mike-patey
- This engine was the second engine. His partner had accidentally hit started engine #1, so they pulled that engine and sent it to P&W for overhaul. They bought this engine used but directly from P&W and only had 5 hours on the airplane before the failure occurred. The airplane had just come out of annual where it was worked on by an A&P specializing in turbine engines and he didn’t think that person did anything wrong.
- There was no warning in engine instrument trend data leading up to the accident. ITTs were in the mid-700s, and he pulled Garmin’s data cartridge and plotted engine data for all previous flights and saw no trends indicating a looming failure. There was a slight vibration a split-second before the failure.
- The engine failure itself was catastrophic rather than gradual. There was a loud bang which was an explosion in the power turbine section and sent bits of molten turbine into the leading edges of the wings and horizontal stabilizer, and smoke in the cockpit. He said there was a sudden stoppage of the engine during the explosion which quickly stopped the prop and blew the igniter out of the bottom of the combustion chamber.
- He borescoped the engine near the end of the video and shows that the power turbine PT2 disc is nearly gone, while the PT1 disc behind it is totally intact. Since air flows backwards towards the nose in a PT6 I believe PT2 is the last turbine the exhaust hits before exiting the engine. So what causes the last power turbine in the engine to spontaneously explode if temperatures were all operated within normal limits? Potentially material science metallurgy issues with the original billet the turbine was machined from, resulting in lower than published material allowables, or flaws and inclusions resulting in early fatigue crack propagation starters and lower than expected fatigue life (see titanium fan blade failure in southwest 737 that killed the passenger sitting behind the wing for another example of this). Patey thinks that a King Air pilot hot-started the engine one or more times and never reported it in its previous life on a B200, because the logs were clean. This seems like as reasonable a hypothesis as any to me especially given the recent corporate aviation anecdotes on this board.
I believe PT2 is the furthest left power turbine. You guys who work on and fly them correct me if I’m wrong.
- They’re sending the engine back to Pratt, who is going to do a tear down and forensic analysis - so this probably isn’t going to be chalked up to “dumb experimental guys effed it up.” But if it is, I’m glad they’re committed to getting to the bottom of it rather than sweeping it under the rug.
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This is all really good stuff. The only thing I can add is that I have about 3 years of experience flying a King Air 200. Hot starts were never an issue and I don't remember being concerned about hot engine temperature numbers at all. Compared to a TBM700 where I was constantly very concerned about hot temperatures during start, taxi and take off.