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Jet Engines: Do They Ever Need To "rest"?  
User currently offlineAguilo From United States of America, joined Apr 2004, 243 posts, RR: 0
Posted (12 years 2 weeks 6 days 17 hours ago) and read 25505 times:

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?

16 replies: All unread, jump to last
User currently offlineAirxLiban From Lebanon, joined Oct 2003, 4536 posts, RR: 52
Reply 1, posted (12 years 2 weeks 6 days 16 hours ago) and read 25423 times:

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.

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17625 posts, RR: 65
Reply 2, posted (12 years 2 weeks 6 days 16 hours ago) and read 25401 times:

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."
User currently offlineAguilo From United States of America, joined Apr 2004, 243 posts, RR: 0
Reply 3, posted (12 years 2 weeks 6 days 16 hours ago) and read 25379 times:

I didn't know that jet engines ever needed to be lubricated?

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17625 posts, RR: 65
Reply 4, posted (12 years 2 weeks 6 days 16 hours ago) and read 25342 times:

'fraid so, but that's a question for tech ops.

"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePW100 From Netherlands, joined Jan 2002, 3098 posts, RR: 18
Reply 5, posted (12 years 2 weeks 6 days 12 hours ago) and read 25165 times:

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.


PS. Indeed this is more for tech/ops.

Immigration officer: "What's the purpose of your visit to the USA?" Spotter: "Shooting airliners with my Canon!"
User currently offlineBroke From United States of America, joined Apr 2002, 1322 posts, RR: 3
Reply 6, posted (12 years 2 weeks 6 days 12 hours ago) and read 25055 times:

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.

User currently offlineAir2gxs From , joined Dec 1969, posts, RR:
Reply 7, posted (12 years 2 weeks 6 days 3 hours ago) and read 24683 times:

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).

User currently offlineImisspiedmont From United States of America, joined May 2001, 6544 posts, RR: 29
Reply 8, posted (12 years 2 weeks 5 days 20 hours ago) and read 24534 times:

There are turbine engines that have been running pretty much nonstop for 50 years in the agricultural and power generation fields.

The day you stop learning is the day you should die.
User currently offlineDC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 1
Reply 9, posted (12 years 2 weeks 5 days 19 hours ago) and read 24532 times:

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.

User currently offlineGalaxy5 From United States of America, joined Mar 2000, 2034 posts, RR: 23
Reply 10, posted (12 years 2 weeks 4 days 4 hours ago) and read 24240 times:

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.

"damn, I didnt know prince could Ball like that" - Charlie Murphy
User currently offlineCanoecarrier From United States of America, joined Feb 2004, 2918 posts, RR: 13
Reply 11, posted (12 years 2 weeks 3 days 14 hours ago) and read 24111 times:

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.

The beatings will continue until morale improves
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 12, posted (12 years 2 weeks 3 days 12 hours ago) and read 24093 times:

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....

User currently offlineA/c train From United Kingdom, joined Jun 2001, 501 posts, RR: 3
Reply 13, posted (12 years 2 weeks 3 days 9 hours ago) and read 24039 times:

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.

User currently offlineMusang From United Kingdom, joined Apr 2001, 920 posts, RR: 6
Reply 14, posted (12 years 2 weeks 3 days 5 hours ago) and read 23987 times:

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

User currently offlineDeltaGuy From , joined Dec 1969, posts, RR:
Reply 15, posted (12 years 2 weeks 3 days 1 hour ago) and read 23952 times:

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.


User currently offlineImisspiedmont From United States of America, joined May 2001, 6544 posts, RR: 29
Reply 16, posted (12 years 2 weeks 2 days 22 hours ago) and read 23914 times:

"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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