Savings from Fewer Engines

[twistedenginestarter]

Anybody got any idea - however rough - how the puchase and maintenance costs of eg a CFM56/IAEV2500 compare with a PW4000/GE CF6?

I'm interested in how much you save by reducing the number of engines.

I'm assuming the fuel cost and drag per megawatt are similar although I suspect weight per Mw drops if you have fewer but larger engines.

[Peter47]

I asked the same roughly the same question eight years ago - why are two engine aircraft more fuel efficient than four.

Only possible answers:

- The engines used on twins have a lower specific fuel consumption.

- Twins are lighter than quads (the A330 is certainly lighter than the A340 although it has a slightly lower range). I don't know if engine weight is a factor.

- Some aerodynamic factor - which would surprise me.

Twin jets will probably have a better power/weight ratio in order to cope with an engine out t/o (the A340 is notoriously underpowered as were early 747s) but if that is a factor just put more powerful jets on the 340.

The 77W has the same payload as the 744 but burns 20 - 25% less fuel but this may be down to design advances over the years. The 146 had four engines because they needed four of the best type available. If you put four engines of half the power with the same sfc on the 777 would you burn the same amount of fuel? I presume so.

Checking my old (and probably well out of date) coursework notes I see that a 343 of a similar vintage to a 333 burns about 12.5% less fuel. MTOW is 275t v 230t, OWE 170.3t v 124.3t so you would expect it to burn less. However range is over 4,000km greater. (Newer 330s will doubtless go further).

The 77W is more efficient than the 748 on a per seat basis but that may be because it is based on a 45 year old design.

I can't comment on how having fewer engines reduces maintenance & capital cost against the need to factor in greater reliability & would welcome anyone's comments, but evidence from the real world suggest that twins win hands down.

I presume that the reason that 757s has such low seat costs on transatlantic routes is their low weight (offset by very limited range and negligible cargo capacity).

[twistedenginestarter]

I have to confess my interest is in the concept of a Single Engine Airliner.

This has been discussed on occasions in the past, and now and again you see future designs of this nature (eg Lockheed).

I fully understand you can't actually have an airliner with one engine as the normal mitigation - low stall speed for forced landing - is not consistent with required performance.

However you could fly one engine if you had an emergency back-up. So you might have your principal engine on the centre line and then a hidden engine in the tail, APU style.

The emergency engine could be massively less sophisticated because of its rare and very short term utilsation - like a cruise missile jet. It could be cheap, noisy, polluting, dreadfully inefficient but above light. The problem would be: would all of the savings from buying and running only one super-expensive high-tech motor be entirely offset by the cost of carrying the extra engine for the life of the aircraft? If not, then to me it's a no-brainer: stop putting two identical (half-power) engines on plane when a full power one is now feasible for everything except A380.

[Wizofoz]

You might have noticed that in recent history, any aircraft that needed more than two engines went straight to four.

I think the reason for this can be pretty easily seen- the convenience of pod-mounted engines.

They allow an uninterrupted Fuselage design, have no problems with intake, are very easy to maintain and remove, and actually have a structural advantage acting as a counter-weight to the wing attachment point.

What you are proposing has all the inherent problems of a tri-jet- mounting an engine that needs air coming into the FRONT at the BACK of the aircraft, with the additional hurdle of needing TWO of them (your "emergency" engine is still going to have to intake air!!)

The pure economics of single v twin would be as persuasive as twin v quad- but the engineering would, I think, make it a non-starter.

As an aside, Douglas were proposing a twin variant of the DC 10, but figured the restrictions on twins would make it non-viable. Enter the B757/767 and ETOPS.... We'd probably still have a "Big Three" if they had gone ahead with it!

[underfire]

http://i68.tinypic.com/2ns3qjo.jpg

One engine under the tube?

http://i65.tinypic.com/15d2l2v.jpg

[barit1]

Wizofoz:Douglas were proposing a twin variant of the DC 10, but figured the restrictions on twins would make it non-viable. Enter the B757/767 and ETOPS.... We'd probably still have a "Big Three" if they had gone ahead with it!

Not quite so simple. The DC-9 had two crew and two engines back in early 60s. Boeing had more of a struggle with the early 737 - designed for two crew/two engines, but UAL's union forced a third man into that cockpit.

Douglas's big twin was simply late to the game.

[barit1]

In fact. the 1011 and DC-10 faced initial opposition from a few airlines who fear passenger rejection: "What? Flying across the pond on ONLY THREE engines?" :=

[Metro man]

DC10 and L1011 came into being because of an FAA rule exempting aircraft with four piston or three turbine engines from ETOPS requirements.

Back in those days engines were less powerful and less reliable so a DC10 sized aircraft would probably have required three engines anyway. These days a single jet engine can match the output of the four JT3D engines on a B707.

[twistedenginestarter]

In those days you were nervous about crossing oceans with less than 3 engines but then we all started getting flights to Florida on 757s and even smaller aircraft. BA use an A319 for business class to JFK. If the price is right I suspect passengers would turn a blind eye to a further leap of faith.

That said, I'm not sure Single Engine Airliner would be good for routes over risky terrain. Once you get lengthy 'ETOPS' requirements then your second engine needs to be so good (reliable and efficient) you might as well promote it back into a twin configuration. It's more the London-Paris, Frankfurt-Malaga, New York- Miami etc that could produce savings.

[er340790]

The emergency engine could be massively less sophisticated because of its rare and very short term utilsation - like a cruise missile jet. It could be cheap, noisy, polluting, dreadfully inefficient but above light. The problem would be: would all of the savings from buying and running only one super-expensive high-tech motor be entirely offset by the cost of carrying the extra engine for the life of the aircraft? If not, then to me it's a no-brainer: stop putting two identical (half-power) engines on plane when a full power one is now feasible for everything except A380.

Or... the principal in-flight engine in the tail section as DC-10 / L-1011 and a couple of smaller podded jets on the wings that would only be used for take-off / climb (performance) and landing (security) phases.

Probably still easier and cheaper to go with a conventional twin!!! :8

[BluSdUp]

Like this part of pprune, lots of positive stuff here!

Re-engine of the 707AWACS was done with 4 newish CFM56 engines I think.
One reason for not using 2 big engines was ground clearance.

Has the B52 ever been re-engined , i do not know?
With 2 big fan engines replacing 8 straight"fuel to noise\ash converters" on this bird I am shure the maintainance cost would drop, offsetting the huge price of a say 110 000lbs fan.

Boys , lets make a cargo drone from a used 100 000lbs fan, with a shute emg recovery system, that is the end for u poor freight dogs.
Then we retire to a lake i Canada and do some real flying!
Anyone?
Yours Sincerely
BSU

[Turbine D]

BluSdUp,
Has the B52 ever been re-engined , i do not know?
Not yet, but being looked at, again…

Once Again The USAF Is Looking To Re-Engine Its B-52 Fleet* (http://foxtrotalpha.jalopnik.com/once-again-the-usaf-is-looking-to-re-engine-its-b-52-fl-1685747978)

[barit1]

Metro man:DC10 and L1011 came into being because of an FAA rule exempting aircraft with four piston or three turbine engines from ETOPS requirements.

Not exactly. Four donks across the pond was the gold standard from the days of flying boats and DC-4's, four DECADES before ETOPS was ever thought of.

The trijets were readily accepted by regulators because of proven reliability of turbines in the 70s. Canadian operator Wardair ran scheduled flights Vancouver-Hawaii in a 727. Twins were restricted to 90 minutes overwater IIRC. It was Airbus' A300s and the like that wanted to "push the envelope" to operate twins further, and for this, the ETOPS (i.e. "Extended") rules were formulated in early 80s. Existing aircraft had to be modified and/or specially tested or inspected to meet ETOPS approval.

By about 1981-82, with the 757/767 in the offing, the push was on for ETOPS approval "out of the box", on initial delivery of new aircraft.

Since then regulators have been looking closer at 3- and 4-engine birds, recognizing that cargo fire suppression and other airframe-related systems are potential risks as much as engines, and are pushing to incorporate ETOPS-style thinking into 747s and A340s.

[Metro man]

Yes, the new buzz word is EROPS or extended range operations which takes a bigger view of what might happen beyond a simple engine failure.

The limit for twins was 60mins, 4 piston/3 turbine engines were exempt from the requirement.
Richard Branson wanted "Four engines for long haul." hence the B747 and A340 for Virgin Atlantic. Trams Atlantic flying became routine and the B767 became the most popular type on the route.

The A350 entered service with 180 mins approval and there is a provision to go to 370 mins or about 2500nm.

As with all things in aviation its risk management, an autoland system is allowed to fail resulting in a disaster, once in every 100 million landings. Once this reliability level is reached single engined airliners could be entering service.

However, what happens if an engineer fails to install a simple oil seal correctly resulting in that precious fluid leaking away ? The Cessna Caravan has restrictions in some countries regarding passenger flights under IFR and overwater, it's PT6 engine is extremely reliable and has been in service for decades but there have still been some failures.

[tdracer]

By about 1981-82, with the 757/767 in the offing, the push was on for ETOPS approval "out of the box", on initial delivery of new aircraft.
Not quite - the 757/767 helped pave the way for EROPS/ETOPS, but they didn't have it out of the box, nor was there much consideration of ETOPS during the design phase. During the early transition to what was initially called EROPS, the FAA required a significant amount of in-service time on the engine and an established (and good) in-flight shutdown rate (IIRC 250,000 hours before you could even petition for 90 minutes). It wasn't until the engine upgrades to the 767 in the late 1980s (PW4000/CF6-80C2/RB211-524) that the real push for 180 minutes came into play. I recall one big meeting where management explained that the 767 now had enough range to fly from Seattle to New York and back without stopping, but since there was little demand for that we needed to get 180 minute ETOPS so that the operators could put that range to good use.:E
The 777 was the first aircraft designed from the ground up for ETOPS 'out of the box'. I was on the program at the time and we were quite literally making it up as we went. The processes we came up with during the initial 777 development have been largely adopted as the requirements for early ETOPS.


To answer the original post - two engines are more economical for several reasons. First, two engines require less maintenance that three or four. Engines tend to be a bit 'draggy' as they interfere with the airflow around the wing - less is better. Engines and the associated nacelle and hardware are heavy and expensive - two big engines tend to weigh and cost less than three or four smaller engines.
But the biggie is that, due to the differing TO vs. Cruise thrust requirements between two and four engines, twins tend to operate in a more favorable area of the "TSFC bucket" at cruise relative to quads. Now, you could put more powerful engines on the quad to get that same TSFC bucket relationship, but those bigger engines weigh and cost more and have more drag that what's needed for TO performance, so it still ends up costing more.

[galaxy flyer]

tdracer,

Would care to expand on your last paragraph, please. I'm interested in learning, not doubting your analysis.

GF

[abgd]

The emergency engine could be massively less sophisticated because of its rare and very short term utilsation - like a cruise missile jet. It could be cheap, noisy, polluting, dreadfully inefficient but above light

You mean a pulse jet?

My questions would be: what do you do in an EFATO? You'd need an engine that can be started reliably and almost instantly. Hobby pulse-jets don't fall into this category, but perhaps commercial aircraft could do better.

Secondly, if it's less efficient than the usual engine, you'll need to carry significantly greater fuel reserves which might well negate the savings from not carrying the initial engine.

e.g. an A320 engine weighs about 2 tons. For the sake of argument your pulse jet weighs 1 ton. The fuel weighs about 20 tons. It wouldn't have to be much less efficient than the original engine to totally negate the weight savings. It might be less costly to maintain, I grant you.

[andrasz]

I'll just throw in a bean counter's perspective here, which usually gets a rather heavy weighing when making aircraft purchase / lease decisions. It is not common knowledge, but when it comes to the cost of an engine, over the lifetime of the engne one usually pays 1.5-2x the original purchase price on shop visits. Since the cost / power is non-linear, bigger engines cost less per unit of thrust then smaller ones (logical, the material cost may be less - and for the exoic alloys used it is not negligible - however design and assembly costs are close to being the same, regardless of big or small). Multiply that by the maintenance factor, and over the lifetime of an aircraft even if fuel consumption is similar, four holes will cost much more than two.

About a decade ago I had the opportunity of comparing Avro RJ100 / Fokker 100 for an interim lease of nine months. The two aircraft were roughly the same hours/cycles the Fokker was offered at a monthly lease rate of ~$55k. We did the maths and found that all other things being equal the lessor of the Avro would have had to give us the aircraft for free to compensate for the difference in maintenance reserves (roughly 25k per engine).

[gcal]

Someone earlier on mentioned MD were looking at a 2 engined DC10
To be honest I think we got it in the 777 and in the view from the front the cockpits look similar window wise.
I've no knowledge to back this up or how far MD were down the line before being taken over by Boeing. Or, of course how far Boeing was down the same line before the take over.

[barit1]

There's some good ETOPS timeline material here - I think it's pretty accurate: https://en.wikipedia.org/wiki/ETOPS

[twistedenginestarter]

It's obviously not easy to get figures off the Internet but it looks like an engine might be about 10% of the cost of the plane. A larger engine might be 40% more. So with maintenance, you might save 12% (taking the times 2 guideline above). From that you need to subtract extra fuel to carry the stand-by engine. And then you have to buy the engine, which needs to be much cheaper.

So the question is can you configure an engine to make major savings?

[tdracer]

tdracer,
Would care to expand on your last paragraph, please. I'm interested in learning, not doubting your analysis.
GF

I'll give it a shot :E
If you look at a plot of TSFC for a typical turbofan engine (specific fuel consumption on the vertical scale, thrust on the horizontal) it looks rather like a stretched out 'U'. At very low power and very high power, the efficiency is fairly poor, but it's really good in the middle. This is 'OK' because you're still not burning much fuel at idle, and you don't spend more than a few minutes per flight at takeoff. The efficiency at idle is so poor that for some engine types, as you accelerate from minimum ground idle to flight or approach idle, the EGT actually drops. Now this 'U' curve moves around with altitude, airspeed, and total temp (it's mainly a function of inlet total pressure) but the basic shape remains.
Now this is a broad generalization, but because twins are overpowered relative to quads they tend to cruise closer to the bottom of that 'U' shaped TSFC curve while a quad tends to move more up the increasing TSFC slope due to the higher relative thrust demand.
As an extreme example, think of the case of a 747 with an engine out. Now, a 747-400 or -8 will happily cruise on 3 engines at 35k or above provided it's not really heavy. But the fuel consumption skyrockets relative to 4 engine cruise because now you're way up on the high power side of that U curve.

Does that all make sense (or you at least confused at a higher level :O). This is very top level and general, but hopefully you get the concept.

[galaxy flyer]

tdracer,

Thanks, yes I do see your point.

GF

[twistedenginestarter]

The General Aviation Manufacturers Association (GAMA) reports that EASA has issued Opinion 06/2015 which contains a regulatory framework that would allow Commercial Air Transport (CAT) operations using single-engine turbine aeroplanes at night and in Instrument Meteorological Conditions (IMC).

Europe is the last major aviation region of the world that does not permit widespread commercial operations in single-engine aeroplanes, says GAMA. “The journey to develop the safety regulatory framework for commercial single-engine turbine operations has been long, but today’s action by EASA is an important milestone in providing the full safety and economic utility of single-engine operations in Europe,” said GAMA President and CEO Pete Bunce, speaking on 12 November

[abgd]

Personally I feel that over land - i.e. most of Europe - ballistic chutes should be considered a reasonable alternative to a second engine for small charter flights. Sure, you're likely to lose the airframe, but the chances of engine failure multiplied by the chances of dying after activating the chute are pretty low. And whilst they do take up some payload they're a lot simpler and less costly than a second engine - even a cheap emergency-only one.

Big scheduled air transport is another matter.

[slast]

Re optimum number of engines:
In the 1970s British Airways had both DC10s and L1011 Tristars in its fleet as well as 747s, 707s, VC10s etc. BA was also the launch customer (along with Eastern) for the B757. From its pre-privatisation days BA retained significant research and development expertise in both Flight Ops and Engineering that was pretty widely respected by the manufacturers. For the company's long term planning there was a project to look what should replace the DC10 and Tristar, which examined the "how many engines" issue purely from the viewpoint of what would produce the optimum economics for BA as a customer.

Unfortunately I can't lay hands on it now, but that analysis came to the conclusion that a four-engined aircraft was best for long ranges, and a twin best for short ranges. The three-engine didn't do best at any range. One of the factors favouring the four over the twin for long range in that discussion was that four smaller engines spread over the same wing span (and hence fuel tank capacity) spread the wing bending moments better than two larger and heavier ones producing the same total thrust. This resulted in a significantly lighter overall airframe. So at that time an ideal aircraft for an airline like BA with a mix of long and short routes might come in two variants with as much commonality as possible, as per A330/340.

Input from many customer airlines had led to the original B757 design, optimised for about a 500nm range, and the B767 for high density city pairs up to US and Europe cross-continent range (3500nm). The A300 was also conceived as a short-to-medium range twin. There is a clue in the name: many early presentations focused on the idea of a European Air Bus, crossing land masses with frequent stops, hence today's Airbus Industrie.

ETOPS rules. Some of the comments on this thread about ETOPS are pretty inaccurate (as is Wikipedia). For example it's nonsense to say "DC10 and L1011 came into being because of an FAA rule exempting aircraft with four piston or three turbine engines from ETOPS requirements".

What had happened was that by the early 80s Boeing, Airbus and IATA realised that although the large twins (A300/B767/B757) had NOT originally been designed for long-range over-ocean operations, they did have big fuel tanks which gave them the range to do it. So why not use it?

However such operations would by definition be international, so need international agreement and would conflict with the historic ICAO "90 minute rule". This dated from the 1954 "Standing Committee on Performance", and obviously wasn't written with twins in mind. However, it was NOT actually a rule, i.e. an ICAO Standard or Recommended Practice or - a "SARP" - but simply "guidance material" in the 3rd Example to the 3rd Attachment to part 1 of Annex 6!

This guidance material was concerned with the consequences of a second engine failing on any aircraft. It suggested that any aircraft ought to have either a specified two-engine-out performance level, or if it didn't have that performance it should remain within a distance of a suitable emergency aerodrome, defined as 90 minutes at all engines cruise speed. Clearly a twin has no performance at all after a second failure, so it effectively implied a limit for twins.

In 1982/3 Boeing/Airbus/IATA lobbied that this ICAO "rule" simply be deleted, on the basis of turbine engine reliability being greater than that of the piston engines considered in 1954. However, as they were not proposing any other factors at all should be considered, this caused great concern within knowledgeable XAAs, and as a result ICAO set an "ETOPs Study Group" in 1984.

The only participants with voting rights in ICAO are Member States but at that time 2 International Observer organisations were also normally invited to participate in helping ICAO in its deliberations. (There may be others now). ICAO annexes typically address the "Operator" and the "Pilot" as well as States; IATA speaks for "The Operator" and IFALPA for the Pilot. In the ETOPs discussion IATA basically fronted for Boeing and Airbus. The IFALPA reps were Jack Howell and me.

This "Study Group" was a very unusual forum for ICAO. The idea was to set out arguments and evidence for the XAAs to consider, before a second meeting to make rules. It was obvious that the XAA experts were very unhappy with the industry proposals, but one major state's reps said their hands were tied as they had to take a neutral position and simply listen: they had been subject to extensive political lobbying and were obliged to consider the economic wellbeing of their nation's industry as well as safety issues.

The industry proposals included a definition of required engine reliability that counted if I recall correctly only the "non-restartable, core engine, failure rate", on the basis that you could always re-start an engine that had been shut down if the 2nd one failed. This discounted any precautionary shutdown prior to a physical failure and took no consideration of reliability / redundancy in any other airframe system, or any operational issues such as weather.

Although IFALPA was not opposed in principle, we thought the industry position extraordinarily unwise, and we put in many proposals were much more comprehensive. However we simply did not have access to enough statistics as "proof" to back some of them up. This was remedied by some of the XAA senior people during coffee breaks, on the basis of "if you were to show us THESE numbers or make the particular point that just happens to be on this piece of paper that I seem to have accidentally dropped, I should find it very hard to argue with.......... "

At the end of the first 3-week meeting, Dick Taylor of Boeing (the manufacturers' leading spokesman) came up to us and said "we didn't agree with you, but you did make a lot of good points". As a result, after the second round, the "90 minute (non) rule" was indeed changed, but not just thrown away, as originally advocated by the industry. Instead in 1985 it was replaced with a much more comprehensive amendment to ICAO Annex 6 which defined the same actual distance as 120 minutes "ETOPs". 90 minutes at all-engines speed being about the same as 120 mins on one. Hence the initial 120 mins approvals, subsequently extended to 150, 180, 240 etc.
Steve

PS much of this early debate is described in IFALPA Monthly News Bulletin #131, February 1983 - someone's grandfather might have a copy!!

[Metro man]

British Airways did not have the DC10 in its fleet during the 1970s, it only operated the type after taking over British Caledonian in 1988. BA was actually anti DC10 and would have preferred the A300, but was forced by the government to buy the Tristar in order to help Rolls Royce who supplied the engines.

The L1011 was phased out from the BA fleet four years before the DC10 as the DC10 had the longer range and was the only alternative to the B747 until the T7 arrived.

[slast]

Yes you're right about the timing of BA's DC10 actual operations, I should have said that BA's ops engineering study had data for the DC10 in the exercise I was referring to. (BA's first A320s also arrived as a result of the 1988 takeover of BCal who had ordered them prior to the takeover.)

[twistedenginestarter]

So here's a question: If you stuck a profan or turboprop on the nose (like a very large Cessna Caravan - one TP400 could pull a 737-100), would you be able to see through all those blades from the cockpit?