JAM747
Topic Author
Posts: 524
Joined: Thu Feb 03, 2005 12:17 am

How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 9:30 am

How long do jet engines go before overhaul on average? Do engines on planes that do more landings and take offs wear out faster that engines on long haul jets? Can a jet aircraft go through its life eg. 20 yrs or more with the same engine? I figure it might be similar to a car where one that does alot of stop and go has it engine wear out faster due to more stress than one that does mostly highway.
 
TheJoe
Posts: 60
Joined: Thu Oct 19, 2006 1:02 am

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 10:44 am

That's a good question... I.I.R.C. the major rotating parts of an engine are life limited. I know for sure that this is in terms of hours of engine operation but I'm not too sure about cycles. So an engine gets pulled off wing when the hours done by the life limited parts gets very close to full.

Quoting JAM747 (Thread starter):
Can a jet aircraft go through its life eg. 20 yrs or more with the same engine?

Engines get pulled for all kinds of reasons. For example, a new aeroplane might have two almost zero hour engines on it. The airline might want to stagger the life of the engines so they don't come up for replacement at the same time, so they will pull a reasonably young engine off and replace it with an older one or vice versa well before the engine is due for overhaul. In my experience, an engine will not stay with the aeroplane for it's life. You may have bird strikes which damages more that just the fan or you may notice that the E.G.T. margin is getting a little low which suggests the hot section is starting to wear out. An engine change is quite a routine job.

Quoting JAM747 (Thread starter):
How long do jet engines go before overhaul on average?

So it all depends on the engine I guess. Some are built out of better parts which makes a big difference! When the old parts are repaired, if a bad job is done and inferior parts are used you'll notice that the E.G.T. margin is lower on test than that of an engine that has been overhauled really well. The E.G.T. margin is one of the major contributing factors in making the decision to pull an engine. So if an engine is released from test with a low margin, it probably won't last a huge amount of time in service! For those that are not familiar with the term "E.G.T." it stands for "Exhaust Gas Temperature". An engine may produce 27,000 pounds of thrust at a certain E.G.T. at max power. Over time as the engine wears out, the E.G.T. will slowly creep up for the same amount of power produced which suggests that engine is starting to get a little tired.

Another example, you may find holes in the combustor that engineering say is serviceable, but repeat boroscope inspections are required. The rest of the engine may be serviceable, but it would be time to pull the engine if the holes burnt out of limits.

But to answer your question, I think the engine that has the highest amount of hours is an RB211-524 on a 757. Something like 9 years and 45,000 hours so the rumour goes. I'm not sure about that so someone correct me if I'm wrong or add the specific details!

Cheers!
 
Molykote
Posts: 1237
Joined: Tue Aug 23, 2005 8:21 pm

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 10:50 am

Quoting JAM747 (Thread starter):
How long do jet engines go before overhaul on average?

This varies depending on design decisions made my the manufacturer.

I have no experience with a PW2000 but heard that early examples were staying on wing only half the time of the RB211 competition (although the PW was a bit more fuel efficient).

Exact times and cycles will depend on a given operator's usage.

Quoting JAM747 (Thread starter):
Do engines on planes that do more landings and take offs wear out faster that engines on long haul jets?

Yes. This is called a "flight cycle" and is typically the best measure of "age" or wear for both airframes and engines.

Engines are subject to the highest thrust settings during takeoff and high cycle engines will stay on wing for a lower number of service hours than long haul engines. Higher thrust settings mean higher temperatures, pressures, and stresses.

I am not aware of pure age being a removal reason for any engine that I've worked on but some preservation measures have to be taken for an engine that sits unused for more than a month or so.

Quoting JAM747 (Thread starter):
Can a jet aircraft go through its life eg. 20 yrs or more with the same engine?

I've seen an RB211 (Rolls-Royce) hit 37,000 hours with a pretty agressive service schedule. This particular engine was on wing for over 10 years. As far as I am aware, the "life limited" parts are controlled by cycle accumulation and not by chronological age (although perhaps some deteriorated seals or similar could drive a lightly used and old engine out of service early). In general, I see no reason why an engine couldn't last 20 years with light usage.

My end of the engine business is AOG support. As I consequence, I know only a small amount about routine overhaul procedures, so forgive my lack of knowledge.

Quoting JAM747 (Thread starter):
I figure it might be similar to a car where one that does alot of stop and go has it engine wear out faster due to more stress than one that does mostly highway.

Given the drastic differences between the engines, the basic concept is the same, yes.

If you'd like me to elaborate on anything let me know. I'll cut this short as I tend to get long winded.
Speedtape - The aspirin of aviation!
 
Molykote
Posts: 1237
Joined: Tue Aug 23, 2005 8:21 pm

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 10:53 am

Quoting TheJoe (Reply 1):
But to answer your question, I think the engine that has the highest amount of hours is an RB211-524 on a 757. Something like 9 years and 45,000 hours so the rumour goes. I'm not sure about that so someone correct me if I'm wrong or add the specific details!

Cheers!

I looked this up for a discussion in another thread: Rolls Royce Engines - Unreliable Or Over-worked? (by Noelg Feb 19 2007 in Tech Ops)

Quoting myself:


Ok, I did some research:

According to my (2005) copy of "The Jet Engine" by Rolls-Royce, one example of an RB211 set a world record of 42,000 hours on wing in the year 2000. (Page 30) - Truly amazing. We now know that the record is at least 42,000 hours (given the date of 2000)

This is a fantastic book and I'd recommend it to anyone. (ISBN: 0 902121 35)
I paid about $100 for mine directly from Rolls-Royce.
Speedtape - The aspirin of aviation!
 
TheJoe
Posts: 60
Joined: Thu Oct 19, 2006 1:02 am

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 2:12 pm

Quoting Molykote (Reply 3):
This is a fantastic book and I'd recommend it to anyone. (ISBN: 0 902121 35)
I paid about $100 for mine directly from Rolls-Royce.

Yes, I have a copy too. Fantastic book! But mine was printed in 1992...
 
Tristarsteve
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Joined: Tue Nov 22, 2005 11:04 pm

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 6:28 pm

The word overhaul is difficult to quantify.
All jet engines have life limited parts. Most rotating parts have a cycle limitation, typically 20000 cycles for a disc. When this limit is reached the engine must be removed and reworked.
But as other posters have said, the RB211-535 and the CFM56 are renowned for long time on wing. 10 years on the wing is not unknown.
But when an engine is removed from the wing, it is not automatically overhauled. The engine is boroscoped and inspected, and the engine records are checked, then a group of engineers sits down and decides what to do. They might decide to change the HP module and release the engine. Decisions are made depending on the condition of the engine, and the cyclic life remaining. If the engine has a lot of cycles remaining, then a hot section module replacement might get it back in service, but if the whole engine is coming close to cyclic time limits, they may decide to replace all the modules and release the engine with more time remaining. It is all down to cost. The cost of module overhaul is much greater than the cost of changing the engine, so it is usually worth releasing an engine from the shop with a short life, in order to use up the life and not throw it away with an overhaul.
It is rare to overhaul an engine and release it as new.
 
MarkC
Posts: 238
Joined: Tue Apr 25, 2006 8:10 am

RE: How Long Before Overhaul For Jet Engines?

Sun Feb 25, 2007 11:32 pm

For the large PW engines that I am familiar with, a good average is 3,000 cycles. Its all based on cycles. This could be 4,000 hours with some, and 24,000 hours with others. The record-holder is much longer than that, but, just as with that high time Rolls engine, it takes a good environment, and some luck to achieve.

As above, engines operated at reduced thrust will last longer. Life limits for major rotating parts are less for certain high thrust versions. Again, all based on cycles. There are one or two operators who are coming up on life limits on 777's that have less than 20,000 hours because they use them on such short hops (not even ETOPS approved).

An engine will never stay with the same plane. As soon as the engine needs overhaul, a replacement engine is installed to minimise AOG time. Nobody is going to let a plane sit for 60 days while the engine is overhauled.

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