I was involved in a discussion about engine failure after takeoff, mostly relevant to King Airs. However, this led me to do some searching in the NTSB database, as I was curious if my intuition that way more PT6-powered turboprops seem to have engine failures on takeoff versus 331s (intuition was correct, although not adjusted for more hours flown in the PT6 fleet).
I did, however, come across one interesting MU-2 accident that those of us flying Commanders should keep in mind.
A refresher: when we all learned to fly multiengine airplanes, we probably learned "identify, verify, feather" in the event of an engine failure just after takeoff. Identify, often using "dead foot, dead engine"; verify-- by pulling the suspected dead engine's throttle back, and then feather.
Once we transitioned into Turbo Commanders, we should have been taught that "verify" does not mean pulling the affected engine's power lever back-- instead, we should verify using engine instruments, leave the power lever where it is, and go ahead and feather.
The explanation for this, at least where I trained, was "beta followup." Specifically, when a 331 fails, the NTS system will automatically drive the prop almost to feather with no pilot intervention. If you pull the power lever back to idle, the underspend governor is going to sense low rpm and try to send oil to the prop hub to drive the prop out of feather, against the NTS system. At least, I hope I explained that correctly.
Well, here is an MU-2 accident, where something very similar happened (and they crashed):
https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20060630X00859&AKey=1&RType=HTML&IType=FA
Based on an analysis of evidence from the wreckage and technical data from the airframe and engine manufacturers, a likely scenario for the accident sequence is as follows: Shortly after takeoff, and after being instructed to change frequencies, the pilot may have perceived a loss of power in the right engine and an associated rise in rpm. The right propeller then went into a feathered position about 3 seconds later. The pilot then reduced the right engine power lever, contrary to the AFM procedure. At this point, the fuel flow decreased, leading to a decrease in power section rpm. The propeller governor then sensed an under-speed condition. As a result, oil was routed to the propeller by the propeller governor, causing the propeller to come out of feather toward a flat pitch (increased drag) position. The pilot may not have been aware that the propeller came out of feather. As a result of the increased drag condition on the right side of the airplane, the airplane yawed and rolled to the right and entered a spin. In an attempt to control the airplane, the pilot reduced power on the opposite (left) engine. However, at this point, the airplane was not at a sufficient altitude to recover.
The investigation revealed that a TPE331 engine gearbox uncoupling event is an unusual engine failure that results in substantially different engine indications to a pilot in comparison to a typical flameout event in which the NTS system in operable.
According to the engine manufacturer, there have been five incidents of similar TPE331 ring gear support cracks during about 29 million engine hours of service history. All of the cracks originated at the high-speed pinion cutout detail. Three of these incidents were shop findings, one incident resulted in an in-flight shutdown at altitude followed by a safe landing, and the other incident was this accident.
Something we should all think about, on each takeoff!
