Turbine Failures

[ArrowFlyer86]

I've recently invested a bit of time reading about turbines.

And one thing I'm curious about is where they tend to fail.

Not necessarily while in service (I know they have remarkably high reliability), but during an inspection when something is found that disqualifies it from continuing service. What is it that they find? What are the common culprits? Aside from some abrupt problem that presents itself as an issue, what are symptoms that the turbine engine is worn or not behaving properly?

I've heard some about catastrophic failures, like where a fan or compressor blade breaks off and causes cascading damage through the successive components, but I'm curious about the slower more insidious types of failure that occur over a longer period (of good operation) which you only uncover during inspections. What are some of the root causes? Specifically curious about turboprop and commercial jet engines, not crazy military stuff that might be used + abused more (just a guess).

 

[Mikey52]

Deteriorating turbine blades.

 

[ArrowFlyer86]

Deteriorating turbine blades.

Curious how does that manifest itself? Corrosion? Deformation? Cracks? Discoloration?

 

[Bell206]

What are some of the root causes? Specifically curious about turboprop and commercial jet engines,

While specific engine types tend to have different causes, if you are only looking for a very general cause across the board it would have to be foreign object damage (FOD), turbine blade and stator erosion, and making metal tend to be the top 3 reasons turbine engines are taken out of service during normal inspection compliance.

 

[AV8R_87]

Blade tip clearances reduce. You can actually have them rub on the case when things are cold.
Blades erode from oxidation/minute FOD (sand and such). Cooling passages get clogged.
Cracks can start to form in the blades, and you usually have inspection schedules that try to catch them (based on the number of cycles) before they turn into a Sioux City UAL 232 kind of event.
Uneven cooling can cause shafts to bend slightly, causing even more blade rub against the case. Usually caused by improper cooldown on engine shutdown.

 

[ArrowFlyer86]

While different specific engine types tend to have different causes, if you are only looking for a very general cause across the board it would have to be foreign object damage (FOD), turbine blade and stator erosion, and making metal tend to be the top 3 reasons turbine engines are taken out of service during normal inspection compliance.

I'm curious at a high level if these causes vary much between a turboprop (say a PT6A on a PC12 or a C208) versus a big honking GE engine on an airliner?
And also wondering is the erosion caused primarily by the high stress atmosphere that they operate in, or is there much room to have that accelerated by a dumb/ignorant operator?

 

[Bell206]

I'm curious at a high level if these causes vary much between a turboprop (say a PT6A on a PC12 or a C208) versus a big honking GE engine on an airliner?

Yes, due to the design of the engine/aircraft and the operating environment. For example, with FOD, on a PT6 series the compressor inlet is annular which allows increased FOD protection on a PC-12 or 208. But the same PT6 in a helicopter is exposed to more opportunities for FOD damage due to the installation requirements and the environment helicopters tend to operate in.

On turbojet and turbofan engines, the compressor inlet is wide open to anything that can get sucked in. But the environment they operate in tend to be “cleaner” and the number of FOD events tends to be lower. If that makes sense.

And also wondering is the erosion caused primarily by the high stress atmosphere that they operate in, or is there much room to have that accelerated by a dumb/ignorant operator?

Erosion can manifest in different ways. It can be as simple as ingesting copious amounts of an abrasive like sand through the compressor which erodes those blades profile, to the erosion caused by high temp cycling in the power turbine side. For example, in turbines that are manually controlled vs a FADEC, repetitive hot-starts can erode the stators/vanes in the power turbine rather quickly. So a prudent operator will train his people to keep hot-starts to a minimum. However, helicopters tended to be more hot-start susceptible than say an older King Air, but I can speak from experience it is possible to hot-start a PT6 in an airplane too.

 

[Velocity173]

Yes, due to the design of the engine/aircraft and the operating environment. For example, with FOD, on a PT6 series the compressor inlet is annular which allows increased FOD protection on a PC-12 or 208. But the same PT6 in a helicopter is exposed to more opportunities for FOD damage due to the installation requirements and the environment helicopters tend to operate in.

On turbojet and turbofan engines, the compressor inlet is wide open to anything that can get sucked in. But the environment they operate in tend to be “cleaner” and the number of FOD events tends to be lower. If that makes sense.


Erosion can manifest in different ways. It can be as simple as ingesting copious amounts of an abrasive like sand through the compressor which erodes those blades profile, to the erosion caused by high temp cycling in the power turbine side. For example, in turbines that are manually controlled vs a FADEC, repetitive hot-starts can erode the stators/vanes in the power turbine rather quickly. So a prudent operator will train his people to keep hot-starts to a minimum. However, helicopters tended to be more hot-start susceptible than say an older King Air, but I can speak from experience it is possible to hot-start a PT6 in an airplane too.

Sand destroyed our engines in Afghanistan. No failures that I know of but we got compressor stalls on almost a daily basis during start up.

 

[RussR]

or is there much room to have that accelerated by a dumb/ignorant operator?

Modern turbine engines are really quite simple to operate. As long as you are staying within the temperature and other limits, there's not a whole lot you can do to hurt them. There's no shock cooling, ROP, LOP, "red box", over square or any of that. It just runs.

Now, flying through ice or sand or taxiing over FOD, sure that will get you. But compared to pistons, turbines are easy.

 

[AV8R_87]

As long as you are staying within the temperature and other limits,

The FADEC does a great job at keeping things in limits, despite the pilot's best attempts at smoking the engine.

 

[Bell206]

The FADEC does a great job at keeping things in limits, despite the pilot's best attempts at smoking the engine.

Except keep in mind FADEC won't provide total protection during certain RPM ranges especially if attempting a start below the recommended minimum battery voltage or in some cases above the maximum TOT/ITT start temps.

 

[Timbeck2]

Other than FOD taking them out, most items on turbines are time critical, meaning for example, a fuel control can be in service for X amount of cycles then it has to be replaced. Every component has this and the times aren’t the same. As was said before the clearance between the outer tip of the blades and the case wears down over time. This can be compensated by making adjustments to other components such as temperature amplifiers but they can only be adjusted so far until the engine has to be overhauled. Things crack, wear, etc. this I why they are inspected at regular intervals via boroscope and other methods

 

[kmacht]

A lot depends on what you define as a failure as well as what type of operating environment the engine has been running in. There are design failures (I.e a component in the engine experiences a harmonic response that it wasn’t designed around and fails), there are FOD failures (I.e a rock gets ingested during takeoff and damages compressor blades), and then there are wear failures (I.e operating an a routinely sandy/dusty environment causes the turbine blade cooling holes to block up which requires they be replaced/overhauled before their scheduled interval).

At a high level, parts that spin such as rotors and blades tend to get at lot of publicity and attention when they fail because a part spinning at thousands of rpms tends to cause massive collateral damage when they fail and are ingested through the rest of the engine. More often it is external components that fail (oil pumps, gear boxes, starters, sensors, tubes, brackets, etc). These can cause an engine shutdown but are usually localized to the failed component or subsystem the component is in.

Each manufacturer has a specific list of hours or cycles that a component can operate to before it either needs to be inspected or replaced. The limits for each parts are different but the overhaul shops with the guidance of the manufacturer tries to group them so the engines stay on wing as long as possible. That is why you have things like hot section intervals where all the turbine components get inspected and overhauled at the same time but the cold section compressor hardware doesn’t get touched during that shop visit since most of its components are good for twice or more than the turbine hardware is.

 

[Mikey52]

I

 

[flyingcheesehead]

Not necessarily while in service (I know they have remarkably high reliability), but during an inspection when something is found that disqualifies it from continuing service. What is it that they find?

So, I gained a LOT of respect for the reliability of the PT-6 when this happened.

We had an airplane go in for routine scheduled maintenance. When they stuck a borescope into the engine for a visual inspection of the blades, it was just trashed . And the inlet screen looked like something got trapped inside and was trying to get out, with a tiny hammer. I don't think we ever found out what it was or how it got into the engine, but something bigger than a pea and smaller than a marble and hard enough to cause big indentations in the leading edges of the blades got in there and just beat the crap out of it.

We couldn't tell. No loss in performance whatsoever, no weird temperature issues - We even looked at all the engine data back to the previous inspection in detail and couldn't even figure out when it might have happened. The dang thing just said "it's only a flesh wound" and kept working until it was caught on that inspection. It may have been running like that for up to 200 hours.

Needless to say, I'll happily fly behind a PT-6 any day of the week.

Erosion can manifest in different ways. It can be as simple as ingesting copious amounts of an abrasive like sand through the compressor which erodes those blades profile, to the erosion caused by high temp cycling in the power turbine side.

Ice crystals can be quite abrasive too. This is why ice vanes and inertial separators exist.

For example, in turbines that are manually controlled vs a FADEC, repetitive hot-starts can erode the stators/vanes in the power turbine rather quickly. So a prudent operator will train his people to keep hot-starts to a minimum.

What are you calling a "hot start" here? To me, a hot start on a turbine means you exceeded the temperature limitations on the engine during start, usually by giving it fuel too early.

What we call a "hot start" on a piston engine, we would call a "warm start" on a turbine, where you're beginning the starting process with an engine that was running fairly recently. A warm start can easily lead to a hot start, though.

Which are you referring to above?

 

[Jim K]

FOD damage due to the installation requirements and the environment helicopters tend to operate in.

We worked a rollover accident a couple years ago, and had Airlife come down to transport one of the patients. The helicopter (an EC-130, I think) landed in a harvested corn field, and I vividly remember the cloud of leaves being kicked up and getting sucked back down through the rotor. I was thinking about those leaves getting sucked into the engine. Are helicopter engine intakes usually protected in any way?

 

[Bell206]

What are you calling a "hot start" here?

Any start that requires a maintenance action which is usually determined by temperature and duration. In the mx manuals is usually referred to as a Hot Start or Overtemperature Inspection depending on OEM. As I recall, your PT6 has 3 levels off hot start inspection depending on temp/duration. There is no equivalent inspection in the piston world that I'm aware of.

usually by giving it fuel too early.

In my experience, the main cause is typically a battery or electrical issue that will not allow the engine to spool up fast enough or obtain the RPM to sustain combustion. While early fuel can cause it that is more a technique issue than a system issue like a low battery.

Are helicopter engine intakes usually protected in any way?

Depends. Most OEMs use a course screen of some sort to keep big things out of the inlet, but there are various types or particle separators or inlet barrier filters that prevent small stuff like sand, etc from being sucked in. However, those tend to be optional equipment and only installed when needed. Most EMS helicopters have inlet barrier filters installed because of where they can operate.

 

[nrpetersen]

Does turbine disc gyroscopic-induced deflection of rotating components ever cause an interference a problem? Given the large component diameters and very high speeds it would seem to be a major design limitation.

 

[Velocity173]

Except keep in mind FADEC won't provide total protection during certain RPM ranges especially if attempting a start below the recommended minimum battery voltage or in some cases above the maximum TOT/ITT start temps.

Compressor stall as well. One thing we watched closely was TGT during compressor stall on startup because the ECU / DEC didn’t protect for over temp in that condition.

The 407 FADEC doesn’t say if it’ll protect for that but I know there’s a caution for high temp (150 C) or low batt (24 V) starts. I’ve started well above 150 C, especially in the winter with no issues though. Wouldn’t try that at high altitude.

 

[kmacht]

Does turbine disc gyroscopic-induced deflection of rotating components ever cause an interference a problem? Given the large component diameters and very high speeds it would seem to be a major design limitation.

Without going into too deep of detail since I work for a major engine manufacturer, the short answer is that failures do happen but the forces that rotating component experience are modeled as part of the design process. Creep is a concern with the forces and temperatures the components see but life limits are usually set well below the point where it would cause a failure.

 

[Bell206]

Does turbine disc gyroscopic-induced deflection of rotating components ever cause an interference a problem? Given the large component diameters and very high speeds it would seem to be a major design limitation.

No clue. But it wasn't something pushed at the field level. Heat tended to be the more common culprit on blade interference with some engines requiring repetitive checks on clearances or tip rub. However I don't quite follow your comment on "large diameters" as most turbine wheels aren't what I would consider "large diamemter". Perhaps you're referring to the large diameter bypass fans on large aircraft? Except they don't really turn at very high speed relatively speaking when compared to turbine wheel speeds.

The 407 FADEC doesn’t say if it’ll protect for that but I know there’s a caution for high temp (150 C) or low batt (24 V) starts. I’ve started well above 150 C, especially in the winter with no issues though.

407gx? The older models had no cautions on volts/temps so we had to install volt meters to monitor voltage and had to set a company level minimum value. And with the older EECUs, if you were above say 200 deg and let the ECU handle the light off, sometimes it would happen at <150 and sometimes not. Was a real pain when doing multiple runs to balance a T/R or something and you forgot to dry motor 1st to lower the TOT before you started. Seemed when the reversionary governor mod came out that issue went away.

 

[Velocity173]

No clue. But it wasn't something pushed at the field level. Heat tended to be the more common culprit on blade interference with some engines requiring repetitive checks on clearances or tip rub. However I don't quite follow your comment on "large diameters" as most turbine wheels aren't what I would consider "large diamemter". Perhaps you're referring to the large diameter bypass fans on large aircraft? Except they don't really turn at very high speed relatively speaking when compared to turbine wheel speeds.


407gx? The older models had no cautions on volts/temps so we had to install volt meters to monitor voltage and had to set a company level minimum value. And with the older EECUs, if you were above say 200 deg and let the ECU handle the light off, sometimes it would happen at <150 and sometimes not. Was a real pain when doing multiple runs to balance a T/R or something and you forgot to dry motor 1st to lower the TOT before you started. Seemed when the reversionary governor mod came out that issue went away.

It’s a legacy (2010). RR250 C47-B. I don’t have my RFM in front of me but our company checklist just shows it as a note. Thought it was a caution on the 150 C but the caution is on the 24 V and for the 1 minute warm up after sitting for more than 15 minutes.

I suppose if the FADEC is working properly it SHOULD abort the start if started above 150 C and hits the MGT limit (843 C) depending on DA parameters. I’ve also started with only 18 volts before and she cut the engine at 10.3 volts as advertised. Not something you’re supposed to do but I watched MGT carefully.

 

[AV8R_87]

Most modern FADECs won't add fuel if they see temps above 150°C, instead will motor the engine until it cools down enough.
Also on the topic of smoking engines - the S-61 had a tendency to do so, and it wasn't uncommon to see a switch on the collective that would shut down half the fuel injectors to control start temperatures. The pilot would watch the temps and manually click the switch to keep things under control. Fun times.

 

[Sac Arrow]

It has long been known that the M1 Abrams tanks, which have turbine engines, do not like sand or desert environments. When the Soviets designed the T80, they basically wanted an M1 spec tank, which was basically a T72 with a turbine engine.

In a promotional video (I'm too lazy to find it on Youtube but it's there) they claimed to have solved the sand-clogging problem with the Abrams' turbine engines by incorporating a vibratory shaker that allows sand to pass through the turbine blades without clogging it. (WTF???)

They never made many turbine powered T80's, and the later versions of the T80, and in the T90 the have gone back to diesel engines. I think they realized they were going broke paying for fuel and replacement turbines.

Also hilarious is that they had the worlds fastest tank, and that the T80 could reach the 50 mph mark. The early production M1's could achieve 70 mph on a flat road, but they were later speed limited to 45 mph because they were breaking tracks and suspension components too easily.

 

[flyingcheesehead]

Any start that requires a maintenance action which is usually determined by temperature and duration. In the mx manuals is usually referred to as a Hot Start or Overtemperature Inspection depending on OEM. As I recall, your PT6 has 3 levels off hot start inspection depending on temp/duration. There is no equivalent inspection in the piston world that I'm aware of.

OK. So a legit hot start... I don't know what inspection intervals were because I never hot started one. I did abort one start that would have been a hot start if it had been allowed to continue though.

In my experience, the main cause is typically a battery or electrical issue that will not allow the engine to spool up fast enough or obtain the RPM to sustain combustion. While early fuel can cause it that is more a technique issue than a system issue like a low battery.

For us, it was mostly that a warm start could easily turn into a hot start on the older birds. The newer ones had a redesigned inlet that let a lot more air in for the start and they were pretty easy not to hot start - Motor until 150C ITT and then light it and you'd be fine.

Mainly, it was early fuel by about a half hour, if you catch my drift. And while a low battery will do it, you just need to spin it up enough that it won't. If it still does, then the battery is too low to fly with anyway IMO. We wouldn't even attempt a battery start below 24.5V.

For example, in turbines that are manually controlled vs a FADEC, repetitive hot-starts can erode the stators/vanes in the power turbine rather quickly. So a prudent operator will train his people to keep hot-starts to a minimum.

We were trained to NEVER allow a hot start. I do remember in training that the instructor said this on the subject: "You can theoretically go up to 1000ºC for up to five seconds during a start. But if you're counting one one thousand, two one thousand, three one thousand, etc while you're cranking you're doing it wrong. You might as well be counting $100,000, $200,000, $300,000..."

Your point about a "prudent operator" is a good one though. Given what I've seen from certain corners of the industry I would expect there are those who aren't training people well, and those who are telling them to go, go, go, engine temps be damned.

 

[AV8R_87]

You might as well be counting $100,000, $200,000, $300,000...

Proper words of wisdom

 

[Bell206]

You might as well be counting $100,000, $200,000, $300,000...

Ha. In reality it should be counting A, B, and C. Level A is fixed with a logbook entry. Level B is fixed with a Hot Section Inspection. Level C is remove the engine.

those who are telling them to go, go, go, engine temps be damned.

And for the same reasons you experienced in your trashed engine above. The amount of internal damage a PT6 can handle and still perform is unbelievable. Even the level of damage that I can sign off as good after a borescope inspection is amazing in the big picture. Have seen a number of abused PT6s that couldn't pass a power check to save its life, but could still fly away with a city block in tow and stay within limits. Unfortunately people take advantage of a good thing to make more money.