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A cut-away view of a PT6 engine with an arrow pointing to the containment ring – a heavy band of metal that surrounds the PT disk(s) and protects against PT disk blade fragments being released.
found, the aircraft is authorized to fly 10 hours more before another check. If things are still copacetic on the follow-up check, then no further action is required. (One theory is that this lint may be manufacturing residue that was not thoroughly flushed away after the engine was manufactured or overhauled.)
Well, the crew’s hopes of “nothing major” were dashed when, in their words, “We could do chin-ups on the prop blade!” The gearbox had frozen solid with contamination.
A year or two after this incident, I was conducting recurrent training for the pilots of an early C90 based at Gillespie Field (KSEE,), near San Diego, California. We flew east to avoid the busy San Diego area, and while doing air work near Thermal, California, the chip detector on the left side came on. The weather was perfect, we were light with only about half fuel and three people (all pilots) on board, so we went ahead and shut the engine down. Being close to Gillespie, we decided to return to that airport where they had their own mechanic available. As we flew, having declared an emergency with ATC, I was pleased to see that occasionally the three- blade prop would turn slightly as we changed speed or configuration. “At least the gearbox isn’t frozen, unlike the Rock Springs case” I thought to myself. “Probably gonna be a little lint.”
In the hangar, the mechanic quickly pulled the forward cowling to gain access to the detector. He positioned a metal pan to catch the oil and removed the detector.
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As the oil fell into the pan, I swear we could hear metal hitting metal! Maybe not, but that oil was thick with metal contaminants. Have you heard the line about “Pieces being big enough to have part numbers on them?” That’s about what we had here! I’m sure glad we shut it down expeditiously!
By the way, both this engine and its partner on the other side were very close to the suggested 3,600-hour TBO, so both were sent out for overhauls.
The F90 model made its appearance in 1978. Like the other models being produced then, it also had the chip detector on the red, warning annunciator panel. As time passed, it became obvious that some chip light illuminations were indeed a precursor or indicator of major engine damage whereas other cases simply were lint-related.
At some point a meeting of Beechcraft engineers, pilots and lawyers was held and the issue was discussed in- depth. I was not in attendance, but I am guessing it may have gone something like this: “Should we really treat this basically equivalent to an engine fire, even when there may be no immediate problem? Aren’t we exposing our company to possible legal action if a pilot were to botch a single-engine approach and landing, when in fact the airplane didn’t need to be single-engine at all?”
Based on this type of concern, decisions were made to (1) change the annunciator in forthcoming models from red warning to yellow caution, and (2) insert a
AUGUST 2024