ltbewr
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Can A 3 Engine Freight A/C Cruise Using 2?

Sat Sep 25, 2004 1:17 pm

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
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sat Sep 25, 2004 1:25 pm

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.
A318, A320, A332, A333, B1900, B722, B732, B733, B734, B735, B737, B738, B772, CR1, CR2, CR7, CR9, MD80, MD81, MD82, MD8
 
JETPILOT
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sat Sep 25, 2004 3:34 pm

The # 2 engine on the 727 has an engine driven hydraulic pump powering A system hydraulics. So that answers that question.

JET
 
rendezvous
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sat Sep 25, 2004 7:23 pm

The engine that is not running would cause excess drag, therefor requiring more power overall to make the aircraft cruise at the same speed. This increases fuel burn.

Basically drag = fuel.

I think it would also require the other two engines to run at pretty high power settings for the whole flight, which probably doesn't help their life.
 
SPREE34
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sun Sep 26, 2004 6:01 am

Not and stay at cruise altitude.
I don't understand everything I don't know about this.
 
411A
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sun Sep 26, 2004 7:31 am

It wouldn't save any fuel either.

Example,
Lockheed L1011, 3 engine cruise (mid weight), FL330, 485 KTAS, M.85

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.

 
COFreqFlyer
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Sun Sep 26, 2004 11:53 am

As I recall, the RAF Nimrod (Military version of the Comet) will frequently shut down two of the four engines while on the extended maritime patrols they fly. Soooo, 411A is correct.
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AGrayson514
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Mon Sep 27, 2004 5:08 am

Could they not run one engine on a lower setting or maybe even idle instead of just carrying dead weight?

~ Andrew Grayson
Give a little bit...
 
air2gxs
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Mon Sep 27, 2004 5:31 am

Basically, if its not producing thrust, its creating drag. Not only that; Operating on 2 engines vs. 3 engines means you're flying slower, taking longer to get to your destiantion and burning fuel.

Cargo airlines live by the clock. Their schedules are built around getting in on time. Sure, you can build your schedule around the slower aircraft, but that will reduce your ground time.
 
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Starlionblue
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Mon Sep 27, 2004 6:10 am

Could they not run one engine on a lower setting or maybe even idle instead of just carrying dead weight?


Sure, but it would produce drag since it's going at a speed not "coordinated" (for lack of a better word) with the speed of the aircraft.

Also, this would mean the other engines would need to go faster, increasing their fuel burn.

Generally, one can say that beyond special cases (Nimrod), aircraft are designed to cruise most efficiently with all their engines running. Any attempt to fudge this will increase cost.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
XXXX10
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Mon Sep 27, 2004 6:15 am

I would imagine that the pilots would try to fly higher if the a/c was light this would give them better consumption. If a higher altitude was not available then I'm not sure what would happen.

It might be the case that if they didn't need as much power they would use a lower thrust setting on each engine rather than idle/shut down one.
 
TimT
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Mon Sep 27, 2004 3:16 pm

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
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Tue Sep 28, 2004 7:21 pm

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).

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prebennorholm
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Wed Oct 06, 2004 5:06 am

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.
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411A
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Wed Oct 06, 2004 12:12 pm

Not two engines shut down with the Comet 4, Prebennorholm, it was ONE engine that was the DanAir request to the UK CAA...and rejected.
 
skysurfer
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RE: Can A 3 Engine Freight A/C Cruise Using 2?

Tue Oct 12, 2004 6:50 am

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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