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Expected Engine Life

Fri Nov 01, 2002 12:47 am

I tried searching the forums but couldn't find this anywhere.

Does anyone know the expected engine life for typical, modern airplane engines? How does this life compare to the expected airframe life?

Also, how has this improved over the years, and what are the most common reasons for engines to fail prematurely?

Thanks for any info.
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RE: Expected Engine Life

Fri Nov 01, 2002 10:24 pm

A turbine engine has many parts that have what are known as hard times. These parts (mostly rotating parts like discs, hubs, and shafts) have limits that are expressed in hours and cycles (a cycle is essentially one flight, but as with everything else, it isn't that simple). Once a life limited part (LLP) hits either limit, it has to be scrapped. There is a lot of shannagins that go with these parts.
The data tag for an engine is usually on one of its major structural members (a case, for instance) and as long as that data tag is kept properly, everything else in that engine may be replaced or exchanged. The tag and its associated case will collect the accumulative hours, cycles, and calendar time and that will be the times recorded for that engine serial number.
An airplane will have many parts replaced or swapped during its life, also; but its basic structure will remain intact.
So the life of an engine and the life of an airplane are very difficult to compare with each other.
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RE: Expected Engine Life

Sat Nov 02, 2002 12:27 am

I want to say they where getting 600 hours out of the engines on the Mig-29.
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RE: Expected Engine Life

Sat Nov 02, 2002 4:05 am

The answer to this question is very complex. As Broke stated, all turbine engines have life limited parts which are generally disks, hubs, and sometimes shafts. The manufacturers try to design the engine so that the published lifes for these parts within an engine are generally about the same so the operators don't have to pull an engine because one part has a life limit of say 5,000 hours while the remaining parts, which might have limits of 8,000 hours, still have 3,000 hours of life, which are referred to as stub times. Airlines generally do not like to install stub time parts as it means they have to pull their engine back out of service ahead of schedule. The design life limits are really driven by where an engine is used. Turbine engines that are used in Part 91 operations generally have published life limits that are in the 6 to 8,000 hour range. Turbine engines that are used in Part 121 operations typically have published lives around 30,000 hours, or more if the part is cycle limited and used on an engine that is typically on long haul airplanes. The difference between the Part 91 and 121 operators is how fast the airlines run up the time on their airplanes. Many airlines will fly their airplanes between 2,000 and 3,000 hours each year and they would be up in arms with the manufacturers (more than they already are) if they had to scrap out their very expensive rotating parts every 2 or 3 years. The published life limits do not mean that an engine goes into service and remains on week until it reaches the life limits of its parts. Some operators remove engines at prescribed intervals for inspection and overhaul, or for unscheduled repairs due to FOD, birdstrike, or internal failure or damage. When an operator pulls an engine varies from operator to operator based on the reliability history of an engine in general and also at that operator, and the maintenance schedule of the airplane. (An example would be if an operator did a C-check at 12,000 hours, they would try to coincide the engine removals with that check rather than have to bring the airplane back into the hangar to do an engine change.) Other operators monitor the engines' performance and remove an engine once it has deteriorated to a certain point in what is known as on condition maintenance (OCM). Most airlines are using a combination of both OCM to monitor the engine for any premature deterioration and will pull it to prevent a failure but also have a soft time limit on when they will pull an engine for inspection or overhaul. It is not uncommon for operators to fly 10,000 to 15,000 hours between scheduled removals. Some operators such as those based in Hawaii will not build up that many hours because they fly such short routes that their removal schedule is based instead on cycles and they may get 5,000 to 8,000 cycles out of their engines between scheduled removals. Occasionally, and I mean occasionally, some engines just go and go and go. There was a European operator that ran a pair of JT8D-15A engines on a 727 out to the 30,000 hour life limit. They had removed the third engine at around 25,000 hours just to see what it looked like and decided to keep the others in service so long as they maintained good EGT and RPM margins. There was a CF6-80C2 on a 767 operated by a US carrier that was in service for 30,000 hours before it had to be removed for a scheduled insepction. And there is an RB211-535 that is on a European operated 757 that the last time I heard had something like 36,000+ hours and was still going strong. Engines designed in the West (Europe and North America) have much greater reliability and service lives than engines designed elsewhere. When the CIS was certifying the JT8D and JT9D engines for use in that country, their airworthiness officials requested information on the typical lives of these engines. After seeing how high the numbers were, the requested and were provided with documentation to confirm those numbers. They then stated that the average overhaul life of a Soviet-built engine on an Aeroflot jetliner was less than a 1,000 hours!
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RE: Expected Engine Life

Sat Nov 02, 2002 4:51 am

Soviet military aircraft and engine design lives were based on the theory that a combat airplane would only survive about 100 hours of combat flying. So they had very low time between overhauls (TBO) and low design lives. When they decided to get into the commercial transport business, they tried to use the hardware already in hand with its high repair and replacement costs. They tried to counteract this by almost giving the airplanes away.
The result is that only countries that could not afford airplanes from the western bloc would operate Soviet built transports initially and often, due to the reasons discussed above, would go to used Western bloc airplanes for their second generation fleets.
In order to compete with airplanes built in the West, Russian and Ukrainian design teams have had to develop products that can meet western certification standards and have design overhaul and total lives that are comparable.
Remember, even Western designed military airplanes are not generally designed to meet the commercial requirements for overhaul and design lives.
The longevity experienced by the B-52 and the KC-135 in the U.S. Air Force has only been successful because of the relatively low utilization of the airframes, when compared with commercial transport operators, and with significant structural and system upgrades.

RE: Expected Engine Life

Sat Nov 02, 2002 6:17 am

While Soviet-built engines had bad TBO, their airframes were excellent... got to fly a little Yak-40 in Africa a few years ago, would be a nice plane if had Western engines and avionics... did a takeoff and a landing, was a very nice aircraft, half cockpit components were from USA or Western Europe...
There is a JT9D-7J (a spare in a hangar here) that has 32,000 hours since new...
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RE: Expected Engine Life

Sat Nov 02, 2002 12:28 pm

I'll throw in my .02 here. There was an America West 757 that had an engine with about 35000 hours on wing in 1996. I'm sure that this record has been broken since. I'm not sure if the RB-211 Expratt refers to is on wing, if so I apologize.
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RE: Expected Engine Life

Sat Nov 02, 2002 7:55 pm

Improvement of engine life depends also what climate they operate in and if its an old design or not. I saw an example of the rotor-blade of the Ka-26 the orginal metal ones had a life-span of 600-800hrs and the newer composite rotor-blades had a life-span of 5000hrs!
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RE: Expected Engine Life

Sat Nov 02, 2002 10:23 pm

Sorry, I missed the second part of your question concerning the reasons for premature removals.
If an engine is well built and balanced and is one that has reached maturity in that a lot of the initial bugs have been worked out previously, it will be removed due to high Exhaust Gas Turbine (EGT) temperature. The reason being that as an engine ages, the first stage turbine nozzle guide vanes (1st NGV's) are continually thermally cycled and they will begin to develop what is known as a bow.
A normal 1st NGV leading edge is straight, a bow is a condition where the leading edge begins to bend or warp in the middle so the leading edge begins to form an arc. As this process continues, the efficiency of the first stage turbine (in multi-rotor turbines, usually the high pressure turbine) begins to decrease and it takes more fuel to get the same performance and this is indicated by an increase in fuel flow and and increase in EGT. In cooler weather, this increase in EGT can be tolerated, but in warm weather or with the change of seasons; you finally reach the point where you reach red line on EGT and the crew has to reduce power to keep from really frying the turbine.
This whole process will take a considerable amount of engine running time and can be extended even further by using reduced power take-offs when operating at low gross take-off weights.
I won't go on right here about other types of failures or this piece will be too long. Ask, if you want to know more.
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RE: Expected Engine Life

Sat Nov 02, 2002 11:51 pm

Thanks to everyone, this was exactly the type of information I was looking for..

Broke: No need to go into more detail, I was just looking for the usual point of failure, a.k.a. the part that demands engine replacement first.
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RE: Expected Engine Life

Tue Nov 05, 2002 10:55 pm

An RB211-535E4 powering an Icelandair 757-200 was operated for 40.531 flying hours and 11.090 cycles during a 9 year period without removal.Total distance was over 18 million miles. The engine was removed on 1st March 2000 and refurbished before re-installation. According to Rolls Royce, this is a world record.

See: Rolls-Royce: Civil aerospace - RB211-535

Icelandair only recieved one 757 in 1991, TF-FIJ, so I presume that's the aircraft we are talking about.

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