Savings from Fewer Engines
ENTREPPRUNEUR
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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?
So the question is can you configure an engine to make major savings?
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
). This is very top level and general, but hopefully you get the concept.
ENTREPPRUNEUR
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Reading our minds
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
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
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.
Big scheduled air transport is another matter.
Last edited by abgd; 15th Nov 2015 at 01:14.
Optimum engines and ETOPs history
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!!
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!!
Last edited by slast; 14th Nov 2015 at 16:23. Reason: PS
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.
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.
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.)
Last edited by slast; 15th Nov 2015 at 09:20.
ENTREPPRUNEUR
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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?
