View Full Version : Why don't aircraft have a choice of engine size?

2nd May 2008, 22:46
I can't believe there is always one correct thrust rating for a jet transport. Surely the lower the power/lower the cruise speed, the lower the fuel used. The higher the power/cruise speed, the higher the number of sectors per day per hull/per crew hour. Even if there is a sweet spot at any given moment, that must change as prices change over time? So why do we never see engine power choice, or engines varying over the life of a model?

Max Angle
2nd May 2008, 23:05
Quite a lot of jet transports have different thrust ratings for the same airframe, normally to increase take off performance rather than to optimise cruise.

3rd May 2008, 02:29
Absolutely. If your route structure includes a transatlantic nonstop leg with takeoff from Mexico City, you're going to need a few extra KN of thrust - and you'll pay for it.

The Japanese domestic airlines often buy derated engines for their short routes (0:45 to 2:00) - trading lower thrust rating for longer parts cyclic life. Search for 747SR to get the picture here.

3rd May 2008, 02:54
Most jet transports have multiple engine choices. You can put dash 7s, dash 9s, dash 15s or dash 17s on 727s for example. What really affects the cost of the flight is the speed it is flown at. Long Range Cruise will normally give you the best return on the fuel burned. You can set a cost factor into your flight planning and balance higher fuel costs v. lower crew costs. Run the engine hotter and you'll pay more when it gets to the shop. Are you really going to be able to fly an extra section if you save 7 minutes on a two hour leg. Even if you cut an hour off a 14 hour leg, it's hard to make much use of it in a day. Over months and years it will pay off if you build a schedule around it. But if you do 6 two hour back and forth flights every day with a crew change in the middle, what do you gain? Hiring one more crew for a flight in the middle of the night?

Old Fella
3rd May 2008, 06:24
Some airlines use "Minimum Cost Cruise" procedures. This sets down a given Mach number to be flown for various weight/altitude combinations and takes many factors into consideration such as fuel burn, flight time, crew costs, maintenance costs etc and, based on data gained over time, the way in which the aircraft is flown is determined to be the most cost effective way. Having various engine choices only adds to overall operating costs against single type in areas of engineering training, spares holdings etc. Most airlines will be acutely aware of how to save a dollar, those who are not don't survive longterm.

3rd May 2008, 07:23
... And many modern jets have selctable fixed de-rates, giving the best of both worlds. Ecconomy but with the power there when needed.

3rd May 2008, 19:37
When you lose an engine on take-off the remaining engine(s) must meet performance standards, its not simply a case of less power = more economical.

3rd May 2008, 20:49
Frankly in no other mechanical/transport endeavor do owners have more choice. Requirements of Aviation vary wildly depending on mission profile, acquisition cost, maintenance, training and useful life. Keep in mind the ultimate owners and manufacturers are locked in design from blank page. This is not a trip to the showroom and a hurried decision between DOHC-6 and QohcV-8. By the time metal is poured and designs are set, the powerplant is an inextricably dependent part of the airframe. Still, some options re: manufacturer can be exercised. The RRTrent @ 95k thrust, is every bit the equal of the GE90. Packaged decisions by this time have little if anything to do with power, economy, spares etc. The complete airframe with powerplant is a design from scratch endeavour. When power leads aerodynamics (leRhone, Liberty) frames are designed around powerplant. When aero leads power, (F-16) the opposite can be done. They are no less completely inter-dependent.

Hot Rod
3rd May 2008, 20:54
At todays very very very very high fuel prices no-one flies faster than necessary. Itīs too expensive.

High thrust engines are used for companies that uses higher takeoff weight or needs extra takeoff performance, not higher cruise speed.

4th May 2008, 06:13
It comes down to money sonny. Manufacturers offer various engine types & thrust ratings to suit the purchaser (Airlines). Money talks BIG TIME.

Flight Detent
4th May 2008, 13:17
That's what the -

* Cost Index
* Derated takeoff power
* Assumed temperature power

settings are decided upon during the (FMC) preflight phase, are for!


4th May 2008, 14:40
Using the analogy of car engine size, if I have a V8 and am light on the gas, OR I have a V6 and am a bit heavier (but not heavy) on the gas (to get the same mild exceleration) then I will use more gas in the V8 than the V6 for the same route profile.

Basically, a V8 needs more energy just to turn over than a V6, but gives you more power when needed.

Strictly from a fuel point of view in aviation, is a bigger engine at lower thrust on an airplane as efficent as a less powerful engine at higher thrust? That seems to be what a few posts above suggest, but that seems hard to believe. If so, how does this depend on take off vs cruize?

4th May 2008, 15:49
Furthermore - just because the nameplate declares more or less thrust, the engine hardware may be exactly the same. Look at all the different ratings available on the CF6-80C2 for example; the only difference is software in the FADEC.

Thus fuel burned in cruise, given the same GW, Mach, ALT etc. is independent of the thrust rating chosen.

glad rag
4th May 2008, 16:32
Strictly from a fuel point of view in aviation, is a bigger engine at lower thrust on an airplane as efficent as a less powerful engine at higher thrust? That seems to be what a few posts above suggest, but that seems hard to believe. If so, how does this depend on take off vs cruize?

from a stricly fuel burn point yes but it also is not working as hard and your service light will stay off for longer

Pugilistic Animus
6th May 2008, 19:02
Aside from the derating mentioned above--- the use of reduced thrust procedures allows for a constant thrust weight ratio minimizing engine wear--but worse in some cases for noise abatement---and perhaps a very negligible effect on fuel savings---although too complex to say which way:\

--- less thrust less fuel--but more time to clean up--more fuel, hence negligible effect--more for maintenance---but many takeoff will be too close to maximum TO weight for use of that procedure anyways

in cruise as also mentioned before complex cost indexing schedules are required

Capt Claret
7th May 2008, 01:41
The Douglas Boeing 717 has a choice. You can have a BR715A or a BR715C.

The BR715 offers 18,500 lb thrust (A) or 21,000 lb thrust (C). All with the tweak of a screwdriver! :8

7th May 2008, 21:23
If I look at Boeing's site, a 737-600 comes with a CFMI CFM56-7 22,700 lb. End of story. No choices.

No doubt you can de-rate but that's not the point. Surely some parts of the World have real cheap crew so it would pay to have smaller engines flying at lower speeds to save fuel. Or someone else would like high density ops off short runways, and would like a pair of monster fans to pack the punters in?

These are not niche aircraft; they're made in their thousands.

8th May 2008, 00:10
I respectfully disagree. These are "niche" aircraft, to the extent that they are tailored to a particular mission. A carrier who carries pax from Cat III to Cat III has a specific and well-defined need. In 3rd world ops, a runout classic 74 may pack in 500 souls but slurp Jet at hideous volumes. By definition, your question targets new aircraft, and new aircraft are prohibitively costly to other than first rate carriers. Also, you mention the lack of availability of "thrust" choices. Thrust choice is the result of cockpit selection, unless you meant supplying A/C with low peak thrust, which would be dangerous in the hands of marginal mission demands. Max thrust is an availability tailored by engineered requirements suggested by the operator, and Manufacturing safety and performance demands at minimum levels. You see, building an A/C with "smaller" engines to cruise economically at lower speeds would limit useful load and runway balanced length, etc. The market in used aircraft is wide open, and troubling in some respects concerning age, airworthiness. Customers buying dozens and hundreds of new A/C will always drive the new market, not marginal cash strapped operators with frightening line procedures.

From my earlier post, engines are irretrievably mated to airframes as a package, if you want slower and cheaper you'll need to choose another A/C, because putting small engines on an airframe that has load limits, is an invitation to disaster. "You'd like the weak donks? no problem, just don't fill the aft cabin with people." Oh-Oh.

8th May 2008, 01:03
twistedenginestarter seems to be missing a key point: The size (thrust rating) of the engine is driven primarily by takeoff conditions - RW length & altitude, ambient temp range, TOGW (ie payload plus fuel)... or perhaps by TOC (top of climb).

After that, the engines are run at whatever setting is needed to maintain altitude and Mach; If it's a short flight, the fuel load is probably light, so thrust required isn't so high. Slowing down the aircraft beyond some optimum point will not save much fuel, regardless of the engines' rating.

(The neighbor lady always cautioned me to fly low & slow... := )

8th May 2008, 01:09
Just judging from the Boeing website about what they really offer is misleading. You can actually buy the engine with different thrustsettings, however the core engine remains the same (CFM56-7, used for -600 to -900).

For example we bought the 22k version for our -700s (and use a mix of 20k and 22k for our -300s), but we could have bought thrustsettings up to the 26k3 version or down to 18k5. The base engine remains the same, but specific fuel usage and maintenance costs are proportional to the thrustsetting, not to mention the pricetag on the engine. So every airlines has to analyze the usual route structure and performance demands before they decide which version of the engine they buy, exactly what you expected.

8th May 2008, 02:13
I seem to remember an instructor when I did my ATPL way back discussing range / endurance when he was a nav on Bristol Brittanias.
Seemed a suggestion was made to BOAC that three engine holding or long range cruise gave dramatic decreases in fuel consumption with corresponding increase in range / endurance. Don't Nimrods and Orions do similar (shut down two) in their various roles?
Seeing one engine stopped might spook a few passengers, but that's never bothered anybody before!!
Seem to remember something about a concept business jet with two engines of differing power output designe dto have both running for T/o or landing but only one operating in cruise.
Just some ramblings from a dinosaur.

8th May 2008, 03:13
yarrayarra points to some valid issues. Turbine engines tend to be less efficient at lower power settings (ie thrust falls off faster than fuel burn).

Thus shutting down an engine or two, and running the others at higher power, can definitely save fuel - especially if you are not in a hurry. It's been done for decades on the P-3 Orion when loitering or low-speed patrolling.

Capt Pit Bull
8th May 2008, 14:43
Of course the Nimrod and Orion are unusual, compared to airliners, in the need to be able to conduct prolonged low level cruising.