Can a four-engine aircraft with limited fuel gain extra range by shutting down one of its engines?

Question

I was reading the book Vulcan 607 by Rowland White who examines the details of a British bomber's mission during the Falklands War in 1982. At some point, the author wrote about the fuel shortage experienced by the crew of the four-engine Vulcan bomber during their return to their base. The author wrote that the pilot "considered shutting down one of the four Rolls-Royce Conway engines to get a little more range out of the remaining fuel...".

I have not heard of this type of procedure before, so I am wondering if the decision to shut down one of the engines to maximize fuel efficiency makes sense.

I would like to add something important. The bomber returned after executing a bombing attack. This means that during the return the airplane was considerably lighter. — Paul Makris, Aug 01 '15 at 09:53
They were Olympus engines. Shutting one down might have made sense but not an easy decision. If they shut one down, then the tanker they were meeting up with would have to loiter for longer and may have itself, run short of fuel and have to return to Ascension. — Simon, Aug 01 '15 at 11:42
@Simon :Let's ignore the tanker loiter issue. Why would it make sense to shut down one of the Olympus engines to increase the range of the flight? — Paul Makris, Aug 01 '15 at 12:34
I think you'd need a qualified Vulcan pilot to answer that. My intuitive guess is no. As Eric points out, you would introduce a lot of drag since the aircraft would be crabbed all the way back. — Simon, Aug 01 '15 at 12:39
Questions of the type "Can ..." are always ambiguous. Do you mean the physics, or the legal/regulatory side? I doubt you'll gain range, as the remaining engines may have to produce more power, and you'll get asymmetry, which increases drag. Also related: https://en.wikipedia.org/wiki/British_Airways_Flight_268 — Andreas Lauschke, Aug 01 '15 at 14:21
@ Andreas: I am talking about the physics. After all, I read about this procedure in a book describing a military operation and the puzzling thing is that the author actually interviewed the pilots. The way it is presented in the book, it seems that for some reason the pilot thought that it was a viable option even though he did not implement this technique in the end because according to the author the additional range he would gain was not enough. — Paul Makris, Aug 01 '15 at 14:36
As an example answer for the question title: "Can a four-engine aircraft with limited fuel gain extra range by shutting down one of its engines?" I offer the following: In the late 1990s in a 747-200 freighter I departed Santiago, Chile with enough fuel to reach Miami. During our climb, we lost one engine. For scheduling and cost considerations, we decided to continue rather than return. However, the combination of the reduced speed and the much higher fuel burn of the remaining engines of our reduced altitude capability, meant we could not reach Miami, and we landed short at Panama. — Terry, Aug 01 '15 at 19:36
possible duplicate of How far can a 777 fly with just one engine at altitude? — fooot, Aug 03 '15 at 20:07
By Jeremy Clarkson? Are you sure you don't mean this book by Rowland White? Jeremy Clarkson gave it a review, he didn't write it. — Notts90, May 24 '17 at 19:21
You could, but it would make more sense simply to keep all four engines running and reduce your throttle settings to cruise at your optimal lift/drag ratio. — Romeo_4808N, May 25 '17 at 11:06
@fooot: No, that one's about flying on 1 of 2 engines, this is about 3 of 4 engines. You can't avoid asymmetry with 1 of 2 engines. — smci, Feb 12 '19 at 19:05

score 11 · Answer 1 · edited Oct 31 '19 at 02:48

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Assuming the aircraft is always flying at its optimal Lift/Drag ratio (= optimal angle of attack = optimal speed for a given weight and altitude), then the question is:

Is it more efficient to generate a given amount of thrust T with 4 engines x T/4, or 3 engines x T/3?

I would think that in the general case, it is not the case. Specific Fuel Consumption tends to increase with thrust (at constant speed):

Note: the above diagram is for Turbofans (Source: Airplane Aerodynamics and Performance, Roskam). It would be interesting to have similar trends for other types of engines (turboprops, turbojets, piston engines), but I could not find any!

On the other hand, it is possible that, in this particular case, the engines were simply working too far off their design point, so increasing thrust was improving their SFC.

Also, there is a problem with cutting one engine: The pilot will need to counteract the thrust asymmetry with the rudder, and this will have a significant cost in drag. Of course, the pilot could just cut 2 engines instead of one! :-)

edited Oct 31 '19 at 02:48

Vikki

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answered Aug 01 '15 at 12:12

Eric Leibenguth

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I like the idea of cutting two engines. Would they have sufficient thrust available to fly back? – vasin1987 Aug 01 '15 at 14:00
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A lightly loaded Vulcan (no bombs, low fuel) can cruise on 2 engines but, at max continuous power which if memory serves, is 102%. That would burn a lot of fuel. What I don't know is which gives max range, 4, 3 or 2 engines. Incidentally, the intakes were reinforced to allow 110% emergency power for war use only. – Simon Aug 01 '15 at 17:24
I'm confused about what your graph is showing. Ignoring sfc for now: The vertical axis is thrust and the horizontal axis is mach num. So the solid lines would indicate that, for a given altitude the engine would produce more thrust at a lower airspeed? That doesn't make sense to me. Then the dotted lines seem to show that the sfc decreases with higher thrust. I'm obviously reading this wrong. – TomMcW Aug 01 '15 at 21:06
thrust is proportional to the difference of airspeed entering and exiting the engine. This difference is the greatest when the aircraft has zero airspeed. (explaining thrust vs. Ma) 2) thrust is proportional to air density, which decreases with altitude. (explaining thrust vs altitude) 3) Evolution of SFC is harder to explain, and depends very much on engine design, unlike 1) and 2)

Eric Leibenguth

Aug 01 '15 at 23:04

score 7 · Answer 2 · edited May 24 '17 at 19:32

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Yes, it's usually done by P-3 Orion. While it's usual on the P-3 to shut down engine 1, sometimes engine 4 is shut down, but engine 4 drives a generator, thus the load on the remaining generators increases. Also, the minimum altitude to maintain in flight ASL/AGL increases.

Seems like the Nimrod's crew did the same thing while on patrol. However, we're talking about ASW aircraft, designed for very long patrol/loiter periods while on station.

edited May 24 '17 at 19:32

selectstriker2

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answered Aug 03 '15 at 08:01

Marco Sanfilippo

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Since the Nimrod was a converted Comet, it was not really designed for long loiter. But when flying at lower altitude, shutting off one or two engines will let the others (and their intakes and nozzles) run closer to their design point. The P-3 was equally a passenger aircraft in its first life, so the same principle needed to be applied there. A dedicated ASW design has much more aspect ratio than both, compare with the Neptune and the Breguet Atlantique (which won the competition for a Neptune follow-on, but was eventually refused by the US - one reason for France to leave NATO back then). – Peter Kämpf Aug 03 '15 at 10:21
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For turboprops it makes sense since the propellor blades can be put to feather. A trubofan cannot and produces quite a bit of resistance when off. – Koyovis May 24 '17 at 09:45
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The P-3 shutting one down is an example of the loitering case where you want to maximize endurance by minimizing fuel burned per hour. That is very different from the situation in the question, which is about maximizing range by minimizing fuel burned per mile. – pericynthion Oct 31 '19 at 03:11
Obviously a good valid answer, but would be improved by pointing out that a turbofan or turbojet may have different characteristics as noted in comments above. – quiet flyer Oct 31 '19 at 03:42

score 0 · Answer 3 · answered Oct 31 '19 at 03:45

I would think this would not be practical for two reasons.

The drag caused by the non-running engine coupled with the increased fuel demands of the remaining three would negate any fuel savings.
I'm not sure of the the systems in the Olympus but most jet engines do not have any oil-pressure when windmilling and this can create problems. You may also have problems with accessories On some engines when the engines have been shut done the hydraulics have to be turned on to prevent the engine driven hydraulic pump from heating up. On some PW engines you need to run the fuel pump for 3 mins every hour..

But coming back to the original question, here we are talking about a wartime situation and the mission overrides everything else.. I'm sure the pilots had a lot of time to consider their options.

Wrt drag from the dead engine, the small cross section of the Olympus compared to high-bypass engines would probably have lesser impact. Another factor reducing the impact of the dead engine is that they are located quite near the centre line, requiring less rudder trim to compensate. Less rudder-trim = less drag. So the penalty it may not be as bad as say an A340 or B747.

Can a four-engine aircraft with limited fuel gain extra range by shutting down one of its engines?

3 Answers3