Page 20 - Volume 13 Number 8
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A Power Turbine (PT) guide vane on a PT6 power section; the engine is about to be reassembled.
Even I sometimes feel these engines are a Pandora’s Box and as soon as we split that Cflange, all heck breaks loose. In reality, however, that is not the case.
Inside these engines the various parts begin to crack and/or erode on a microscopic basis. Over time, noticeable anomalies develop. Usually they are well within limits when first noticed, but eventually these anomalies exceed allowable limits and the part must be repaired or replaced. As simplistic as this may sound, any problems found and corrected during a Hot Section Inspection begin in this manner. My apologies to PT6 fanatics everywhere – I realize this is problem for you.
The 1,000-hour HSI
“If it ain’t broke, don’t fix it” is one of my most basic principles, but I will tell you a story that flies squarely in the face of that concept. Going back to 1978 or so, when the King Air 200s first came out, there was no Service Bulletin 3360. Engines were overhauled at 3,000 hours and HSIs were done in between at 1,500 hours. I was running the Beechcraft shop in Van Nuys, California, and King Airs reigned supreme.
In those days there were 1,000hour inspection items on the airframe and 1,000cycle items on the gear. Most of my customers did these simultaneously and had to leave their aircraft in the shop for more than a few days. I convinced a few of them (particularly those for whom downtime was a big problem) to let me perform hot sections at this 1,000hour interval, concurrent with all this other maintenance. This meant they didn’t have to come back in 500 hours just for the HSIs. They could go 1,000 hours, then they came back for another HSI at 2,000 and an overhaul was at 3,000 hours.
These 1,000hour HSIs were minimal in expense because the cracks and erosion were caught very early and were easily remedied. When we compared the HSI costs, we found in every case that the aggregate cost of two HSIs at 1,000hour intervals was less than the cost of one at the 1,500hour interval. Not only did these owners save downtime, they saved money. In fact, Pratt & Whitney even adopted a 1,000hour HSI schedule for the PT6A41s, for a while, based on the work we were doing in Van Nuys.
There is a little more to this whole picture, obviously. For example, back then we didn’t perform nozzle or borescope inspections every 400 hours; these engines weren’t touched between HSIs. Ah, those were the days. It was a different time and a much different economy. But you see my point: catching things early really paid off.
Splitting the Engine
When a PT6 is split at the Cflange, the power section is pulled away from the hot section, with temp harness and probes still attached. Portions of the power section are inspected onsite, and if no anomalies are found, the
A power section shown hanging from a forklift after splitting the PT6 at the C-flange.
18 • KING AIR MAGAZINE
AUGUST 2019
