Slowing down for fuel efficiency?

[Farjer]

Probably a silly question, but is it possible to save fuel on a long-haul flight by slowing down a bit? You can do this in a car on a motorway, but I'm not sure how this works up in the air... obviously you have to avoid the falling-out-of-the-sky problem.

thanks,
Farjer.

[BOAC]

'Normal' cruise speed is determined by many cost factors including hourly maintenance, crew, leasing and fuel plus schedule timings to some extent. Flying slower will generally use less fuel, but will increase other costs and in the case of very strong headwinds can be counter-productive, eg cruising at 300kts in a 300kt headwind........................:) .

[yowie]

Jet engines are generally more efficient at the higher end of the allowable RPM range,so it basically comes down to SGR. Headwind,slots,holding,time to arrive,schedule etc all come in to it,in my (limited) experience,if you can go faster,you will save fuel,and other associated costs.

[Lancelot37]

Probably a silly question, but is it possible to save fuel on a long-haul flight by slowing down a bit? You can do this in a car on a motorway, but I'm not sure how this works up in the air... obviously you have to avoid the falling-out-of-the-sky problem.

thanks,
Farjer.

In a similar way to a car engine I would expect a jet to have an efficiency curve which shows the most effective use of the engine. Vary it's use and other factors will come into play.

[Farjer]

so it basically comes down to SGR.

sorry, what's SGR? Google, Wikipedia and PPRuNe all come up blank...

[john_tullamarine]

Specific Air Range (SAR) gives data on fuel consumption in nil wind conditions.
Specific Ground Range (SGR) corrects this for wind.

[18-Wheeler]

All that theory is great, but yes, you will save fuel in most jets if you slow down.
Old rule-of-thumb for the 747 if you get held down below your desired cruise level -> Keep the TAS to 500kts or 320KIAS, whichever is the lesser.
If you are stuck at say 28,000 instead of being at say 35,000', then you will probably have to slow down to about 0.82 - 0.83 odd, but you'll arrive (disregarding differences in wind component) with only a tonne or two less, over a seven hour or so flight.
No big deal.

[littlejet]

All that theory is great, but yes, you will save fuel in most jets if you slow down.
Old rule-of-thumb for the 747 if you get held down below your desired cruise level -> Keep the TAS to 500kts or 320KIAS, whichever is the lesser.
If you are stuck at say 28,000 instead of being at say 35,000', then you will probably have to slow down to about 0.82 - 0.83 odd, but you'll arrive (disregarding differences in wind component) with only a tonne or two less, over a seven hour or so flight.
No big deal.

Is that for 744 or the older series? Freighter or Pax?

Why dont you just type cost index 0 and ECON speed would be equal to Max Range Speed ?

[18-Wheeler]

Is that for 744 or the older series? Freighter or Pax?
Why dont you just type cost index 0 and ECON speed would be equal to Max Range Speed ?


The Classic.
The -400 would be the same though.

[411A]

...was, can you save fuel by slowing down.
Saving fuel (quantity) not cost indexes, operating costs, just fuel quantity.

The answer is, it depends.

Example, from an aeroplane I'm familiar with, the L1011, standard body aeroplane.

Generally speaking, with this aeroplane, at the proper altitude for the weight, the desired cruise speed for maximum range is, M84.

Slow down to M.82, arrive later, burn more fuel.
Speed up, say to M.86, arrive earlier, and burn slightly more fuel...but not by much.
The L1011 has a true laminar flow wing, and as such, IF you get it too slow, it will suck up fuel like a big Hoover.
Big time.

Behind an Airboos....find another level, hopefully.:rolleyes:

[littlejet]

If you are slowin' down to save fuel for the distance to be flown, then I think you should use Max Range Cruise speed (best fuel mileage)
or if you want to stay in the air as long as possible than max endurance speed (best holding speed) should be used. Latter is slower me thinks.
Any speed slower or faster will put you on the wrong end of the curve...

[javelin]

411A

Following an Airboos - some hope, we will be thousands of feet above you :E

If you are still L1011ing, are you doing the Hajj this year - I see a couple of old knackers on the midfield ramp - PM for more info !

[Old Smokey]

...........is it possible to save fuel on a long-haul flight by slowing down a bit?.........

It's already been done, aircraft all over the world are now flying at slower speeds than before in the interests of fuel cost saving.

Typical Cost Indices currently used are already approaching Maximum Range Cruise, and after that, no further optimisation of fuel mileage is possible.

For most aircraft nowadays, even reducing all the way to MRC would only mean a further speed reduction of about M0.01, about 5 Knots of speed, we're almost there now!

Regards,

Old Smokey

[misd-agin]

Probably a silly question, but is it possible to save fuel on a long-haul flight by slowing down a bit? You can do this in a car on a motorway, but I'm not sure how this works up in the air... obviously you have to avoid the falling-out-of-the-sky problem.

thanks,
Farjer.

Yes, slowing down does help save gas. Increasing cruise speed increases the drag associated with higher Mach numbers.

757 and 767-200 most efficient cruise is .78. 767-300 is .78-.79. Most airliners fly the 757 at .79-.80 and the 767's at .80 in my experience.

Slowing the .01 Mach only saves about 100-200 pounds per hour. Increasing above .81 becomes more and more inefficient.

757 wing is very speed critical for optimum cruise altitude. 767-200 is less so(above .82) and 767-300 not at all(until above .845 Mach when OPT ALT starts to change).

[skippiebg]

Slowing down used to help with guzzlers like the Convair 880 and 990. Modern Air (a US registered West Berlin airline) bought some ex-AA 990s in the late 60s and slowed them down to M0745 with amazing fuel savings and range increases. I recall figures like 15 to 20 per cent bandied about...

Beware "the coffin" with modern jobs, though...

[ETTIV]

I understand Airways now use CI of 0 in the climb and 40 in the cruise on the 767, but don't quite understand how this impacts on efficiency. Could someone explain please? ta.

[GearDoor]

I thought that for long-range cruise, as the aircraft gets lighter due to fuel burn off, you step-climb gradually, and slow down. I saw graphical proof in a text book once.

[Old Smokey]

I thought that for long-range cruise, as the aircraft gets lighter due to fuel burn off, you step-climb gradually, and slow down. I saw graphical proof in a text book once.

Actually, for "modern" aircraft, if you conduct the absolute optimum cruise climb at MRC (or lower CI), the Mach Number remains remarkedly similar, with only minor variation. MRC cruise at a constant level DOES (after an initial small increase) demonstrate decreasing Mach No. (I didn't see it in a book, I see it every time that I do a cruise climb, which is often, yeah, it's me asking for the block levels).

I understand Airways now use CI of 0 in the climb and 40 in the cruise on the 767, but don't quite understand how this impacts on efficiency. Could someone explain please? ta.

Damned good idea. The climb is the most fuel-critical stage of any flight, and a poorly managed speed schedule during climb will induce fuel penalties which cannot be recovered even on a long 10 hour or so sector. "Pushing it" a little faster on cruise has far less effect on overall fuel burn than an "off-optimum" climb schedule.

Beware "the coffin" with modern jobs, though...

Perhaps so for older types, but typical modern aircraft offer best fuel economy at a level about where 1.5G to 1.6G can be tolerated, and 1.6G is the definition of moderate turbulence. Of course, if a positive wind gradient indicates a level higher than the still air optimum, buffet boundaries could be a problem.

Just my 3 cents worth..........

Regards,

Old Smokey

[GlueBall]

Other constraints include: Maintaining cruise Mach Number, . . . as when crossing the pond on the NAT tracks, for example. So you are not at liberty to just "slow down" when you're light enough to do so, or if you can't climb.

The fact is that your airplane is not the only one in the sky and you may rarely achieve your "optimum" FMS performance parameters for the entire sector.

And even if you got lucky with extra tailwind, or less than forcast headwind, and maybe saved 1 or 2 tonnes of fuel after 13 hours, . . . when you get to the end of the rainbow early in the morning, at a place like LHR, you may be stuck in a holding pattern, you're optimum FMS descent profile for the anticipated runway is trashed, and whatever fuel savings you had coaxed out of your magic box has evaporated.

For example, in a 74, after a max gross takeoff, upon reaching FL 290, the LRC speed wouldn't be less than M.855. And if you were to "slow down" and operate below LRC, you'd be burning more fuel because of higher "deck angle" [more body drag]. Another anology might be: As when operating a boat "on the step" . . . to do that you have to be at a certain speed.

[hawk37]

Old Smokey wrote "MRC cruise at a constant level DOES (after an initial small increase) demonstrate decreasing Mach No."

OK, I'll bite. Why/when/where/how would MRC have a small increase with weight decrease?

Hawk

[Old Smokey]

Hi Hawk37,

Note that I did say, "at a constant level". A series of constant levels is typical of an aircraft conducting periodic step climbs to be at the level closest to optimum, thus, initially the weight is above optimum, decreasing to optimum weight for the level, and then further decreasing to a weight less than optimum for the level until burned down to the weight for the next step climb.

At the initial high weights (when the weight is above optimum), Angle of Attack is high (due to the high weight) resulting in greater than desired acceleration of air over the upper wing, resulting in slightly slower Mcrit. As a result, because MRC is closely related to Mcrit (always being ABOVE it!), flight is at a slower 'than might be expected' Mach Number. As weight decreases, required AoA decreases, acceleration of air over the wing is less, thus the aircraft may be flown faster (increasing Mach Number) for the same wave drag, and MRC increases slightly as the weight decreases. After this very small increase with reducing weight, MRC Mach Number then steadily decreases with reducing weight, as might be expected.

As LRC and all Cost Index speeds between MRC and LRC are inexoribly linked to MRC, the same trend is noticeable at these speeds also.

Actually, it's quite noticeable in the cruise control tables from many AFMs, the initial small increase is usually only of the order of M0.02 or so.

Regards,

Old Smokey

[hawk37]

Very informative thanks Smokey. I haven't seen many charts so I haven't noticed what you've explained, but it makes sense.

[Old Smokey]

Hi hawk37,

Arrrgh!:eek: I'm guilty of typing too carelessly, all of the text stands uncorrected, I meant to type a difference of M0.002 or so, not M0.02! That's quite a difference.

Most noticeable on high flying aircraft like the Learjet, which have a fairly 'fat' wing, and suffer high low speed and high speed drag polars in the mid thirty thousands, quite low low speed polars above 40,000 or so, but the relatively fat (slatless) wing accelerates air to Mcrit rather quickly at higher angles of attack, a good performance in this respect at more optimal angles of attack.

Mea culpa, mea culpa!!!:bored: :\

Regards,

Old Smokey