aguilo
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Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 5:09 pm

Noting new carriers like PrivatAir and PremiumAir that are flying 737 and A319 Business Jets from the Eastern US to Germany and Switzerland - I was wondering: Are there any limits on the amount of time you can run a jet engine?

Does the engine need "a break" or "cool down" session at some point, or can you run these things forever as long as you have the fuel supply?
 
airxliban
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 5:24 pm

is there a limit on the amount of time you can run a car engine?

if you enough oil and fuel and whatever else fluids necessary, then technically you can.

same with jet engines i would think, although i could be wrong.
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Starlionblue
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 5:27 pm

AF1 can be refueled by air, but the running time is limited to about 72 hours until you need to lubricate the engines.

In longer terms, you have to replace stuff every now and then.


But this is way longer than you would need for A to B travel since the Earth isn't that big.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
aguilo
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 5:34 pm

I didn't know that jet engines ever needed to be lubricated?
 
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Starlionblue
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 5:42 pm

'fraid so, but that's a question for tech ops.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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PW100
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 9:37 pm

Turbine engines could go on for serioulsy extended periods of time. It very much depends on the engine model. Turbine engines like the PW100 turboprop series are designed for short hop flights, usually less than 1 hour, although on some aircraft [F50 MPA, 2x PW127B engines], they can do missions of over 10 hrs. In normal airliner use, these engine can do upto 4000-8000 flights without any shop maintenance, only the normal line maintenance checks required. I have seen PW118B engines that ran for 16,000 hrs/20,000 flights with only one Hot Section shop visit!

Large turbofan engines like CF6 are more designed for long range flights, which usually have a duration of 10 - 15 hrs per flight. I believe these engines can be run for 10,000 - 20,000 hrs on wing [or about 1500 - 2500 flights]. GE [also Rollce-Royce] built land based engine based on their big turbofan turbomachinery. These engines are used in electricity gerating power plants, gas pumping stations, ships etc. and can be run continueously for over 20,000 hrs [there are 8670 hrs in one year - 2004 btw has 8694 hrs . . . ].

Keep in mind that max power output determines the life of an turbine engine. De-rating an engine by 10-15% will double engine life. Or in other words, the last 10-15% of the engine power range is responsible for 50-75% of engine wear. Reducing the amount of time the engine runs at this level [like long range cruise], will seriously increase engine life. If the engine lubrications systems are slightly modified, most aircraft turbine engines can be run for over 20,000 hrs continueos operation at reduced power level.

Once a turbine engine has been shut down, usually it needs to cool down before restarting, depending on power levels prior to shut down. Cooling down can be done at ground idle power setting. Turbine engines generally don't like to be shut down straight from take-off power. They also require warming up before slamming to take-off power.
Hope this helps.

PW100

PS. Indeed this is more for tech/ops.
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broke
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RE: Jet Engines: Do They Ever Need To "rest"?

Mon Apr 12, 2004 10:08 pm

In the early days of high fuel prices, several US domestic DC-10-10 operators began the practice of taxiing out on the #1 and #3 engines and starting up the #2 once they were close to getting take-off instructions.

Not long after this practice began, a series of C sump failures, on #2 engines only, occurred. The C sump (from what I understand) is in the area of the rear most bearing in the engine at the back of the low pressure turbine. In some cases, the whole aft bearing support would depart the airplane.

The investigation determined that the C sump was being thermally shocked by the rapid temperature rise from start up to take-off power. The outboard engines were given time to warm up more gradually during the taxi from the terminal to the end of the runway and did not have the problem.

The airlines went back to 3 engine taxis and the problem ended.

So jet engines do need some TLC in order to reach the extra-ordinary times that they are capable of.
 
air2gxs
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RE: Jet Engines: Do They Ever Need To "rest"?

Tue Apr 13, 2004 6:57 am

The limiting factor on a turbine, discounting fuel and component failure, is oil. ETOPs approved engines use a very little amount of oil and in theory can run until they wear out. Older engines tend to use more oil and thus are limited by that consumption. It is not uncommon to add 6 or 8 qts of oil to a JT9 that completed a six or 8 hr leg. Whereas, if you added that much to a CF6 or PW4000, you'd ground the aircraft until you found the cause.

The standard we use for cool-down is 5 mins at idle. Usually more than enough when taxiing back from run-up. As for starts, we only accelerate the throttle when oil temp is in the green band (old airplanes) or stabilized (new fangled EICAS and ECAM type aircraft).
 
IMissPiedmont
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RE: Jet Engines: Do They Ever Need To "rest"?

Tue Apr 13, 2004 2:06 pm

There are turbine engines that have been running pretty much nonstop for 50 years in the agricultural and power generation fields.
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DC-10Tech
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RE: Jet Engines: Do They Ever Need To "rest"?

Tue Apr 13, 2004 2:55 pm

Actually, on the DC-10/MD-11/KC-10, the two engine taxi is preferred, in fact usually required by MX. I haven't seen any airlines do this for revenue hops, but in the AF, our KC-10's always taxied out on wing engines. The main reason for this being the tendacy of the #2 to stir up a lot of FOD, damage buildings/windows/cars, etc.

Our AF crews would taxi out on two then start #2 once clear of the parking ramp, observing the 5 min warmup time before accelerating to takeoff power.
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galaxy5
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RE: Jet Engines: Do They Ever Need To "rest"?

Thu Apr 15, 2004 5:46 am

A jet engine is like any other mechanical thing, eventually things are gonna wear out and quit, the idea is to properly maintain the engine and monitor it for these failures before a catastrophic failure occurs. SOAP (Spectrographic Oil Analysis Program) sample is one why to monitor engine health by seeing what kinds of metals are floating around in the oil. Also engine health an be determined by the TIT or EGT that is being attained by the engine at specific atmospheric conditions. Usually as an engine get long on hours, it tends to start losing airflow due to larger air spaces between moving parts like rotors and stators, this will cause cooling air to not be as available for the engine ( basically robing air from the engine) and will show up as increasing temperatures during operations. Thats about the time engines will be replaced.
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canoecarrier
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RE: Jet Engines: Do They Ever Need To "rest"?

Thu Apr 15, 2004 8:08 pm

I can't speak to whether or not you can fly a P&W or Garrett engine for 100 hours straight, but I can say that there is some protocal as to when you shut an engine off and when you turn it back on.

I used to work J31's and Metroliners frequently, with them you had to "cool" the engine back down by spinning the props when the engine shut down. With piston engines you had to worry about "shock cooling" an engine when it came down from altitude. You didn't want to throttle back to idle and drop 10,000 feet, drop the pax off and climb back to altitude. I'm sure there's an A&P out there than can explain the effects better than I.
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411A
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RE: Jet Engines: Do They Ever Need To "rest"?

Thu Apr 15, 2004 10:06 pm

A small bit of trivia about jet engines, and their reliability.

in 1965, Pratt&Whitney started a JT3D-1, which was mounted on a test stand, and after a brief time at idle thrust, accelerated the engine first to takeoff thrust, then climb thrust, and after 20 minutes, to max cruise thrust.
After running for one hour at cruise thrust, all of the oil was removed (via special plumbing), and the engine continued to run for 20 hours...when the fuel was shut off at the end of this period, the engine rotors seized on spooldown.

Try this with your auto engine someday....
 
A/c train
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RE: Jet Engines: Do They Ever Need To "rest"?

Fri Apr 16, 2004 1:12 am

VSV actuators need lubing and fan blades!! filters need changing and oil needs changing, cant run forever without maintenance, engine routine every A check, in other words, no.
 
musang
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RE: Jet Engines: Do They Ever Need To "rest"?

Fri Apr 16, 2004 5:19 am

411A - I heard something similar about a PT-6 in a C206 Caravan. The story goes that it lost all the oil in the cruise, and the pilot reduced to best endurance power while he considered his options. After a while he concluded the engine couldn't be totally out of oil, so relaxed a bit, left the power where it was, diverted, and the engine seized when he reduced power for the approach.

It was in fact totally empty.

Anyone know the technical explanation for this (if true) and the JT-3 incident?

Regards - Musang
 
DeltaGuy
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RE: Jet Engines: Do They Ever Need To "rest"?

Fri Apr 16, 2004 8:56 am

If you're supposed to let a jet engine cool down, then DL is in big trouble  Big grin. Last weekend, our B767ER came CDG-ATL (what, a 9 hour flight? not sure), sat on the ground for an hr or less, then it's engines were on again for another 5 hours for out ATL-LAX run, then it was back up again an hour and some change later for the LAX-ATL run...these jets hardly ever sleep. Amazing to think of the technology that allows these beasts to keep going, day in and day out.

DeltaGuy
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IMissPiedmont
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RE: Jet Engines: Do They Ever Need To "rest"?

Fri Apr 16, 2004 11:50 am

"Try this with your auto engine someday...." OK

In 1981 my friends and I pulled a 302 from a 1968 Mustang with about 150000 miles on it, put it on a test stand without any oil or coolant and ran it for about 2 hours before it stopped running. The next night we figured it was dead but the darn thing started right up and ran for another hour before finally dying for good. Course all this was at idle. And, when we pulled it apart, everything was trashed. But it was fun.
The day you stop learning is the day you should die.

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