Those FAA accident reports are quite sobering, pointing the finger almost entirely at engine problems, inadequately prepared pilots, or just lack of attention to detail (and then there's the stupidity).
One that caught my attention (and doesn't fit any of the above descriptions) is this one regarding a departed propeller. > The FAA inspector reported that the propeller is attached to a > flange, which in turn is secured to the Volkswagen engine crankshaft > by means of a bolt and cotter pin. The pin was found sheared and the > bolt backed out. Evidence of full thread engagement at some time was > noted on the bolt. The inspector stated that 5.7 hours of ground run > and taxi tests had been completed by the pilot prior to this first > flight. > > Probable Cause The failure of the propeller attach bolt retaining > cotter pin, and the pilot/builder's inadequate preflight inspection > of the aircraft prior to the attempted flight. If this hub was anything like my GP hub setup, the determined cause is backwards...the hub slipped on the crank taper, and THEN the cotter pin sheared, not the other way around. That little cotter pin couldn't possibly keep the hub from spinning on the crank, although somebody might think that it would. That cotter pin is simply an indicator that the hub has slipped, and that the taper connection needs immediate attention. Why would the hub slip? It's as simple as an engine backfire. When I was having problems with my Compufire ignition just before Chino, I didn't have enough spark to start the engine when the starter was running, but when the ignition was switched off, the coil field would collapse and a spark would fire on some cylinder that was charged and ready to go, and spin the engine with a bang and a dramatic shock to the crank. The prop/hub mass makes it reluctant to move, so the hub slips on the crank, unscrews the bolt, and shears the cotter pin in the process. The cotter pin will shear right off with that kind of load applied to it. Next time the engine starts, the prop comes off, and if you're lucky, it lands on the ground nearby, rather than killing somebody or something. In my case, it just killed itself on the concrete...$400 down the drain, and killed the spinner as well. I think that bolt should have left-hand threads, be larger diameter, and be torqued higher than the manual calls for...like Revmaster does their prop hub connection. Yes, there is also a steel key between crank and hub, but the keyways are surprisingly shallow, the key is pretty thin, and it does not always offer enough resistance to keep the hub from simply "overriding" the key. The key is also not designed to prevent rotation...it's just there to keep your timing mark in the same place every time you rebuild the engine. And when you remove the hub (or it removes itself) and discover the key has fallen into the engine (because it was facing down when the hub was slid off), you get to tear the engine down to retrieve it! The moral of this story is be super careful not to misfire the engine, and if you do, take a look at that cotter pin and ensure that it's not twisted or sheared. Also, torque that bolt to the high end of the given range or higher, and secure it with Loctite 620 (although that still wasn't enough in my case). I'm not trying to start an AD here, but am advising those with this hub connection to pay particularly close attention to detail during installation. If you have a bunch of hours on it, it's probably going to take a lot of tons on the press to get it off, and would probably run without the bolt even installed! Contrast this hub connection to what you get on even a stock Corvair crank...six high-strength 3/8" fine-threaded bolts (formerly holding the flywheel on) that practically guarantee the hub isn't going to slip or come off the airplane, and removing six bolts and the hub is easily removed. It couldn't be much simpler or more trouble free... Mark Langford ML at N56ML.com http://www.n56ml.com
