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!!
Last edited by slast; 14th Nov 2015 at 16:23.
Reason: PS