Ltbewr From United States of America, joined Jan 2004, 13692 posts, RR: 17 Posted (11 years 2 months 3 days ago) and read 3521 times:
Can a 3 engine aircraft like a 727, DC-10/MD-11 in freighter use be operated on just 2 engines at cruise and intentionally not using the 3rd engine to save fuel? For example, if an aircraft is running a deadhead with no or very little frieght or maybe at less than 1/2 of freight capacity, why not operate only 2 3 engines, perhaps shutting down the center engine after takeoff? Does the center engine have to be operated due to systems, support equipment, too hard to restart?
Theflcowboy From United States of America, joined Jan 2004, 405 posts, RR: 0
Reply 1, posted (11 years 2 months 3 days ago) and read 3477 times:
I think there was a post on this a couple months ago.
The answer ended up being no because the engine that has been shut down doesnt recieve the proper lubrication when its windmilling. The cost to replace an engine is MUCH greater than the cost of fuel to run it.
Same aircraft, 2 engine cruise, 420 KTAS, burning 500kg/hour MORE fuel, due to lower altitude achievable...ie: FL240.
The ONLY jet transport that could actually save fuel shutting down one engine in cruise was the deHavilland Comet 4.
This was requested as a normal operating practise by DanAir, but the UK CAA said (respectfully) NO.
TimT From United States of America, joined Jun 2001, 168 posts, RR: 0
Reply 11, posted (11 years 2 months 22 hours ago) and read 3047 times:
In the early days of the Rolls Royce RB-211 and the L-1011, they used to use all 3 for takeoff and then pull one engine's power back because they knew they were going to lose an engine before they got to the destination. Thanks to TWA, Eastern and Delta, it didn't take too many years before they got the reliability. TWA had one do some 20,000 hours on wing.
MxCtrlr From United States of America, joined Nov 2001, 2485 posts, RR: 32
Reply 12, posted (11 years 1 month 4 weeks 1 day 18 hours ago) and read 2925 times:
Some of the guys I work with have throttled an engine back to flight idle as a fuel conservation method but shutting the engine down in flight, for a reason other than abnormal operations, is a violation of FAR's (not to mention, really stupid. What happens if you can't get that engine restarted in flight? You then have to declare an emergency and explain to your company and the FAA why you shut down a good engine in flight).
I also know of several pilots flying "Valsan-mod" 727's that routinely retard #2 to flight idle after initial climb (usually, the #2 engine is a JT8D-17 or -17A, while #1 & #3 are JT8D-217's).
Freight Dogs Anonymous - O.O.T.S.K.
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Prebennorholm From Denmark, joined Mar 2000, 6953 posts, RR: 54
Reply 13, posted (11 years 1 month 3 weeks 1 day 8 hours ago) and read 2746 times:
The Nimrod is a very special case. In its maritime patrol role it may cruise at a few hundred feet altitude for hours.
In the thinner air at airliner cruising altitudes the max power output is reduced as much as 70 - 75 % and only a lightly loaded four holer can maintain speed and altitude with an engine shut down.
Shutting down an engine will always mean increased fuel consumption because of lower altitude, thicker air and more drag.
The Nimrod also uses much more fuel on two engines at sea level than it uses at 30,000 feet on four engines. Anyway it may spend less fuel with two engines at half power than with four engines at quarter power.
I don't believe in the Danair procedure shutting down two engines on the Comet 4. No way could a Comet 4 maintain a sensible altitude on two engines, and no way could they save fuel on going lower and slower on two engines instead of four engines at a sensible altitude. Even if their straight jet RR Avons had a slightly smaller thrust loss at altitude compared to modern high bypass ratio turbofans.
It might have been possible for Danair to save a substantial amount of fuel shutting down two engines during descend and keeping the rest at flight idle. In any case they would then have to restart the engines well before the approach anticipating a possible go around in dirty landing configuration. A low compression ratio straight jet like the Avon uses a significant amount of fuel also at idle.
The Comet cousin - the Nimrod - does it at an extra fuel cost only when it is demanded by the mission.
Always keep your number of landings equal to your number of take-offs
Skysurfer From United Kingdom, joined Sep 2004, 1145 posts, RR: 11
Reply 15, posted (11 years 1 month 2 weeks 2 days 6 hours ago) and read 2528 times:
I was reading an incident report from the UK's AAIB just yesterday regarding an Air Luxor L-1011. It had a tailstrike performing an autoland at Stansted airport in the UK, and the company chose to ferry the aircraft to Amman in Jordan for repairs. Because of the low flight level needed and reduced take off weight (due to the possibly damaged #2 engine) the aircraft was cleared to ferry to Amman in 3 stages. Unfortunately, the aircraft used up so much fuel due to the #2 engine being run at idle that they had to shut it down altogether inflight.
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