Hot Section Inspections Revisited

Hot Section Inspections Revisited

Recently I was consulted about a PT6 undergoing a Hot Section Inspection (HSI). It began benignly enough; the hot section itself was in decent shape. But things took a left turn when a broken diffuser tube was found. This meant the rest of the engine (the “cold” section, if you will) had to be removed and inspected. The teardown of the compressor section revealed eight bad compressor blades (not to be confused with the CT blades on the turbine wheel). The engine shop tab is over $100K and the job is not yet complete.

Ballpark Estimates

King Air owners and operators need to know what it costs to maintain the aircraft, but when I’m asked for a ballpark figure on the HSI, I balk at the question – especially with someone new to King Airs. I hate to discourage them with the worst-case scenario, nor do I want to mislead anyone with a lowball estimate. When I had my shop, my wife often fielded this question. She explained that once the hot section is opened up, all sorts of things might be found that must be addressed. Her answer was “anywhere from $15,000 to $150,000, per side.” That’s a really big ballpark!

King Air owners who aren’t turbine engine experts can have a hard time wrapping their minds around the idiosyncrasies that can crop up during the HSI. And it’s especially frustrating when, prior to the inspection, the engine(s) seemed to be running great.

Sometimes you get a big zig when you expected a little zag. I remember the time I had two model 200s in my shop going through hot sections at the same time. The outcome for each was the opposite of what one would have logically expected.

King Air A versus King Air B

King Air A was a B200 with -42s (and Service Bulletin 3360 completed on each). It had around 5,500 total hours on the engines and 1,800 hours since overhaul. The tab from the engine shop was around $43,000 per side (this was about eight years ago). That didn’t include items like the R&R assist by my shop, freight charges and miscellaneous expenses.

King Air B was a straight 200 with -41s, both of which were on the MORE (Maintenance On Reliable Engines) program. One engine had roughly 9,800 total hours and the other had around 11,000 total hours. Both engines had run approximately 4,800 hours since overhaul. The last hot sections were 1,500 hours ago when these engines were first put on the MORE program. In accordance with the program, they were due another HSI.

One might assume that the older engines on King Air B, with 4,800 hours since overhaul, would have hot sections far more expensive than King Air A. Surprisingly, the for one engine on King Air B, the hot section was a mere $18,000 – nearly best-case scenario. The other engine was less cooperative and the engine shop’s tab came to $58,000.

Compressor blades from the compressor “cold” section of the engine. Some of the blades are good (shown by green line) and some are worn (shown by red line).

What Goes Wrong?

It is very troubling to pilots and owner-operators when they’ve seen excellent engine performance indications in the cockpit, but at the HSI they get hit with astronomic bills. Understandably, many feel they are at the mercy of the engine shops. Even I sometimes feel these engines are a Pandora’s Box and as soon as we split that C-flange, 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 1,000-hour inspection items on the airframe and 1,000-cycle 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 1,000-hour 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.

A Compressor Turbine (CT) guide vane inside a burner can. Its step-like characteristics reflect off the cylinder wall around it, making the segments almost impossible to see.

These 1,000-hour 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 1,000-hour intervals was less than the cost of one at the 1,500-hour interval. Not only did these owners save downtime, they saved money. In fact, Pratt & Whitney even adopted a 1,000-hour HSI schedule for the PT6A-41s, 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 C-flange, the power section is pulled away from the hot section, with temp harness and probes still attached. Portions of the power section are inspected on-site, and if no anomalies are found, the power section stays behind. The hot section components, along with fuel nozzles, are sent to the engine shop for inspection, repair and/or replacement. The power section (male) and the hot section (female) fit together like a hand in a glove. Great care is taken at reassembly.

Segments and T-Wheels

A Power Turbine (PT) guide vane on a PT6 power section; the engine is about to be reassembled.

Segments, which surround the T-wheel (turbine wheel), should always be changed at the HSI because the T-wheel is removed and trued (brought back to its original shape). If the old segments are left in, the clearances between them and the newly trued wheel are increased; this, in turn, causes the engine to run hotter and possibly not make performance specs.

Pratt had an issue a while back with warping segments rubbing against the T-wheel and reducing the outside diameter of the T-wheel too much. The T-wheels had to be re-bladed.

There are 58 CT blades on each wheel and they are expensive. These days they can run as high as $1,900 per blade. I’ll let you do the math on that one.

Sulfidation is a type of corrosion on the T-wheel blades found in engines that operate regularly in heavily polluted or salt-laden air. Turbine compressor washes help keep sulfidation to a minimum. Heavy sulfidation (Stage 3) will condemn the T-wheel blades.

Half of the newer two-piece style burner cans
(aka combustion liner).


Burner Cans

Burner cans (aka, combustion liners) always seem to crack, especially the newer two-piece construction. Some cracks are perfectly acceptable, but others need welding or new pieces installed (such as cooling rings). Burner cans are typically sent out for repairs which are always expensive and take extra time; but to exchange a burner can in the interest of saving downtime is an expensive proposition.

CT Guide Vanes

Compressor turbine (CT) guide vanes are subjected to the highest temperatures inside the engine. They are assaulted by flame pattern and corrosive particulate matter such as smog or salt-laden air. This abusive environment results in cracks, erosion and warping. Guide vanes have had lots of improvements in material and design over the years so they are better now than they were years ago. Nevertheless, since they take the brunt of the wear and tear inside the engine, they develop problems that must be addressed. Prepare for sticker shock as $40-$50,000 is not unusual.

All guide vanes have a class expressed as a numerical designat­ion. The class indicates how the guide vane flows. Replacement guide vanes, whether exchange or new, must be the same class, or the engine will not perform correctly. I cannot emphasize to you how critical it is that the engine shop correctly determine the class of vane being removed and replace it with the exact same class.

A power section shown hanging from a forklift after splitting the PT6 at the C-flange.

PT Guide Vanes

Power turbine guide vanes do not take as much abuse as their CT counterparts, but they still can crack or warp. Although lo­cated in the power section, they are inspected at the HSI. If they pass, they are not removed. If they don’t pass, they are removed for evaluation, repair or replacement. If replaced, the class is kept exactly the same as mentioned.

PMA Parts, Exchange Parts

Fortunately, there are now PMA parts available which can reduce the exorbitant costs in some areas. Not all engine shops will use PMA parts. Any warranty being extended by Pratt & Whitney will not cover PMA parts. At minimum, the engine shop should make every effort to find exchange parts where possible.

Preventative Measures

The best piece of advice I can give is to keep up with the fuel nozzle and borescope inspections at the recommended 400-hour interval. These will sometimes (not always) reveal a developing problem. Usually this would be something like minor sulfidation that does not need immediate attention but can be monitored and fixed at the next HSI. Occasionally the borescope finds something that needs to be addressed right away.

A T-wheel (turbine wheel) installed in a guide vane, inside the burner can. These are the infamous CT blades that cost up to $1,900 each, and there are 58 of them.

Certainly, if you operate regularly within 100 miles of the coast or in and around big cities with frequency, insist on turbine compressor washes.

My final recommendation is to keep good records. Trend mon­itoring on your engines could reveal a slow, subtle change that you wouldn’t notice until you start comparing the figures from last week to last year. Further, trend monitoring comes in very handy when the engines go back together and you are trying to compare post-inspection performance with the way they ran beforehand. If you have it in writing, there is no guesswork.

My main message is that HSIs can be very unpredictable. Engines that have been running perfectly fine before the HSI can take a substantial bite out of your budget. And, as mentioned in the opening paragraph, the HSI could lead to issues elsewhere in the engine that must be addressed.

While I hope that all of your inspections go smoothly with no expensive snags, let’s not forget the whole purpose of these inspections – to find and fix a developing problem before anything catastrophic happens.

Have confidence in your engines and keep enjoying your King Air.

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