Turbine engine failures

[allseeingeye 0]

In regard to the recent C208 deadstick landing, I would never condemn a pilot for running off the end of the runway, especially in a Caravan. They are notoriusly efficient gliders with the prop feathered and the best of the best could easily float one right down the runway. In fact, it can be difficult to settle onto the runway with the prop governor/beta being slightly out of rig. Caravans are not STOL aircraft by a long shot although they are often treated as such due to the effective reverse thrust available.

What bothers me is the rate of turbine engine failures in the skydiving industry. Pratt & Whitney claims an in-flight shutdown rate of .3 per 100,000 flight hours. I seriously doubt if the skydiving world flys 100,000 hours per year in and of itself. I suspect that this number is derived from operators who participate in formal reporting which would most likely include only Part 135 and 121 operators.

Why are there so many failures? I have my own ideas but would like to know what the thoughts of others are on this subject. Just from reading these forums one can find one or more failures every year and those are just the ones which we hear about on this website. I know of several operators who have had more than one over varying lengths of time.

[Calvin19 0]

A few reasons, the engines are run 100%, then super cooled idle. Normally a major problem for piston engines, but when 100% for 15 min, then 0% for a fast descent, it wears on even turbine engines. Same thing for shut downs, 20 heat/cool cycles a day are bad for anything.

FURTHER, on twin turbine airplanes, there is an asymmetric wear on the motors from jump run, mainly prop governors. not as big of a deal though.

-SPACE-

[allseeingeye 0]

I can see some issue with shock cooling but not enough to cause the high failure rate. At most, it could shorten over all engine life and be a cause for a more frequent HSI interval. 100% of torque is usually used for takeoff and even then it is (or should) be flown within the temperature limitations. Some engines torque out before temping out and some are opposite. Regardless, neither limit should ever be exceeded. These limits are published by the mfg. and the engine should be able to take it if maintained properly otherwise. (HINT, HINT) In the descent, the compressor is still generating plenty of heat at flight idle of 48% N2 (I think this N2 number is accurate - correct me if I am wrong).

[grimmie 173]

There have been many Caravan engine failures. Tropic Air flies a fleet of 208s in Belize. They pile up the hours, with a lot of start ups and shut downs during 15-20 minute legs. They had a failure of a brand new engine, with about 120 hours and they had an older engine fail. Google "Caravan engine failures". a lot of info worldwide comes up about failures.
Some think the turbine blades are being damaged too often with "blade creep", corrosion.
Pratt is working with a company in Nevada, Metalast Inc., to improve the corrosion resistance on their turbines.
Most operators under 135,121 do the Pratt engine replacement program now on the Caravans.
Most Caravans are flown in pretty harsh environments around the globe.

What do you think is causing the break downs?

[JohnRich 4]

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on twin turbine airplanes, there is an asymmetric wear on the motors from jump run, mainly prop governors. not as big of a deal though.

Could you explain that further, please?

[totter 2]

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flight idle of 48% N2

51% - 53% is the proper speed for N1 at idle, also refered to as Ng (gas generator) since all the compressor components ( including CT Disk) rotate at the same speed. 46% - 48% is for Min Flow N1 speed. But Min Flow is mainly for for starting purposes and a maintenance setting that ensures that when the plane is at altitude and the pilot pulls the power back to idle the engine does not flame out.

To build on what grimmie has posted, on the Single Engine Turbine Otters we operate here in Southeast Alaska we have found will only go about 3000 hours before we need to replace the Pratt & Whitney CT Blades due to corrosion. What is refered to as Sulfidation. Sulfidation happens in all enviroments and can not be prevented, only slowed down in its formation. Some enviroments will excellerate its growth. It is a by producted of burning fuel.
Now I stated Pratt & Whitney blades because the OEM blades don't seem to hold up. One of our planes, which has had PMA blades for a while (around 2000 hours) shows no signs of sulfidation at this point. These are coated with Surmalloy J which is a process used after market in the reduction of sulfidation.
To Add:
We know none of the current particulars for this aircraft. I used to work on 81F when it was operated by its previous owner. At that time the installed engine had around 4000 total time. Its been 5 years since I've last seen it. Back when it was painted White, Blue and Silver.
Don't even know if its the same engine.

[totter 2]

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A few reasons, the engines are run 100%, then super cooled idle. Normally a major problem for piston engines, but when 100% for 15 min, then 0% for a fast descent, it wears on even turbine engines. Same thing for shut downs, 20 heat/cool cycles a day are bad for anything.

Please explain. In my 15 years of working on turbines I have never known this to be a factor.
If it was a factor, so much so as being a safety factor, then one would think that Pratt & Whitney would have, at least, issued a Service Information Letter pertaining to this subject to limit their liability.
As opposed to a pistion engine the hottest parts of a turbine are never in direct contact with cold air, even at idle in a descent.
100% power is not used for 15 minutes and then directly to 0%. If I were trying to exit from an aircraft and the pilot was using 100%, lets just say Tail Strike.
On jump run the engine/s are at a reduced power setting for several minutes (ballpark/sake of argument 3-4 minutes), then pull back to idle for descent. You have to remember that even at idle the engines are at, just over, 50% and still around 500 C on the temps.
As far as starts go the only really bad thing about them is that it costs the aircraft owner a percentage of the replacement cost for the life limited items every time the engine is started.
Yes, most of the wear and tear on a turbine is from starts, but you're only doing real damage if your starts are above the normal operating range red line.

Even here in Alaska where, at sea level, the temperature difference between the engine ITT and ambient can be 750 C. Shock cooling is just not an issue.

[allseeingeye 0]

Thank you Mr. Totter for an explanation on shock cooling and on correcting my erroneous statement about compressor RPM - Ng or N1. - It has been a while for me.

I am well aware of the problem with blade creep which appears to be more prevalent but not limited to PMA blades, I still think that blade creep and the resultant catostrophic failure when it goes unchecked is an anomaly on a properly operated and maintained engine rather than an epidemic. A properly maintained engine might be getting scoped on a regular basis and this problem could be noticed. Maybe not all of them are caught in time, but probably most of them are or we would be hearing a lot more about it.

I would also like to point out to those who are talking only about the C208, I am talking about failures of all PT6 powerplants on all skydiving jump ships. Think about those of which you personally have knowledge, and then think about those of which few people know, and then think of .3 per 100,000. I think the industry rate is somewhere between 5 and 10 times worse than this rate based on my opinion and some fuzzy math.

[riggerrob 558]

I wonder how much of the engine corrosion (in Belize) is caused by salt water?

When I used to work on Sea King helicopters, we had to (fresh water) rinse compressors at the end of every flying day. After "X" number of flying hours, we had to spray some potent detergent (B&B 3100) through them to loosen the soot and salt.

Keep in mind that Royal Canadian Navy Sea Kings spend a lot of time hovering at low altitudes, sucking in salt water and re-ingesting their own exhaust.
Part of the problem was accumulated salt distorting the aerodynamic profile of compressor blades, similar to the way that ice can stall an engine, simply by accumulating on inlets.

Hint: General Electric T58-8F engines leave a trail of soot everywhere they fly.

[totter 2]

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I wonder how much of the engine corrosion (in Belize) is caused by salt water?

I would say a large part of it is caused by salt. You don't have to be in direct contact with salt water or spray to collect deposits on the blades. Just being by the coast is enough due to the salt laden air.
This is one reason that corrosion is such a big issue in Florida, even if you only have a land plane, because you're never more than 50 miles from the coast.
Also, the salt content in the water increases the farther south you go. So the salt content is 10x greater in the Caribbean than here in Canada and Alaska.
The rest of the engine corrosion probably comes from pollutants in the air.

[kallend 1,623]

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The rest of the engine corrosion probably comes from pollutants in the air.

And fuel. Although the sulfur content of jet fuel is controlled, it is not zero.

...

The only sure way to survive a canopy collision is not to have one.

[totter 2]

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And fuel. Although the sulfur content of jet fuel is controlled, it is not zero.

There is still sulfur in kerosene based fuels.

The sulfur in the fuel, is of course, the root of sulfidations.
Its what is found in the air and enviroment that speed up the process, with salt and then chemicals being the worst. The more salt, or direct contact with salt water, and pollutants the quick sulfidation forms and grows.

[jumprun14K 0]

Operation of the engines beyond recommended overhaul period WITHOUT the benefit of mandatory accessory (think fuel pump, fuel control, prop governor) replacement and enhanced engine condition (Oil analysis AND vibration analysis) monitoring that is required by approved service life extension programs. The PT 6 will usually talk to you for a while before it fails. Perhaps skydive operators aren't listening.

My resume; A&P, I.A. ATP King Air Charter and skydive pilot 34 years in civilian aviation maintenance and flight operations.