Page 37 - Jan 20 TNT

Not-So-Evenly-Distributed
In 45-some thousand hours of flying time, my North- west/Delta/Duke instructor friend has experienced 20 engine failures. Some of them were in GA and some were at the airlines. Some were in single-engine airplanes and some were in twins. Some were in his own, some in that of an employer or a client. Some piston, some jet. And of all the failures, the one this past summer was the first that ended in an off-airport landing. His engine seized during day-VMC, and fortunately, the airplane had enough energy to make it to a soybean field just short of the air- port. His 20 engine failures in 45,000 hours caused me to make a quick comparison of his flying time to engine failure ratio and my own.
In 25,000 hours, I’ve had four engine failures and all of them ended with on-airport landings with no damage. One was a recip and three were jets. The First Officers that I fly with have 10,000 to 15,000 hours, and most have had one or two engine failures, but many have had none. While I am a proponent of anthropomorphism and intuition, I’m not so much inclined to endorse unicorns, four-leaf clovers or fate. So, what the heck is going on with the engine failures? From the above piston engine vs. jet statistics, we know that recips, like the board game Mouse Trap, are a Rube Goldberg jumble of moving parts much more vulnerable to failure than a jet. So, let’s take a look at some recip components from a “Mike Busch” (a well-known engine guru) perspective to learn more. The turbine folks can skip to the end of the story, but you may gain some empathy (and sympathy) for us recip schmucks by reading on.
Bottom End
The bottom end components of our piston aircraft en- gines – crankcase, crankshaft, camshaft, bearings, gears, oil pump, etc. – are very robust. They normally have a useful life that is many multiples of the TBO.
Top End
The top end components – pistons, cylinders, valves, etc. – are considerably less robust than the bottom end. It is not unusual for top end components to fail prior to TBO. However, most of these failures can be prevented by regular inspections and use of a digital engine moni- tor. Most top end failures are random (there’s that word again) and do not correlate with TSMOH (time since major overhaul).
Crankshafts
Lycoming did a study that showed their crankshafts often remain in service for more than 14,000 hours (that’s seven-plus TBOs) and 50 years – no problem here.
Camshafts and Lifters
Cam and lifter spalling is the number one reason that engines fail to make TBO, and it’s common in the owner- operator fleet (i.e., T&T readers) where aircraft tend to
fly irregularly and sometimes sit for weeks at a time. Tiny corrosion pits can lead to rapid destruction (spalling) of the surfaces, sending metal flakes, or even chunks, into the oil filter and beyond. The good news is that this problem has been mostly negated by a friend of mine (Gary Bongard) who has a patent on carbide tipped lifters. He recently completed the arduous process of getting approvals from The Man and has begun manufacturing. I have his lifters in both motors on the Duke.
Bearings
Bearing failure is responsible for a significant number of catastrophic engine failures. Bearings fail prematurely for three reasons: They become contaminated with metal from some other failure (i.e., lifters), or they become oil-starved when oil pressure is lost; or main bearings become oil-starved because they shift in their crankcase supports to the point where their oil supply holes become misaligned. Contamination failures can be prevented by using a full-flow oil filter and inspecting the filter for metal at every oil change.
Connecting Rods
Connecting rods usually have a long useful life and are not normally replaced at overhaul. Many rod failures are caused by improper tightening of the rod cap bolts during engine assembly. Failures can also be caused by the rod bearings, usually due to oil starvation.
Valves
It is quite common for exhaust valves and valve guides to develop problems well short of TBO. Failures are less common nowadays because problems can usually be detected by monitoring EGT’s on digital engine moni- tors. Even if a valve fails completely, the result is usually only partial power loss. I will add two caveats: There is an increasing failure rate of valves in some engines and one of our readers (good job and thanks for the story Pete) recently had a valve fail, which cascaded through
Ruined cam and lifters from my Duke replaced with carbide tipped lifters and a new cam.
January 2020 / TWIN & TURBINE • 35