Bsergonomics From United Kingdom, joined Jan 2002, 462 posts, RR: 0 Reply 1, posted (11 years 8 months 1 day 1 hour ago) and read 1953 times:
Four words: Weight, Balance, Commonality and Maintenance. Oh, and aerodynamics.
The three engine design essentially means one engine at the nose or tail end of the fuselage. This causes stress on the fuselage (thus reducing lifespan) in a manner that it different to mounting the engines symmetrically on the wings.
You want all engines on the a/c to be the same, in order to minimise spares, crew training etc. When fuselage internal volume is at a premium, locating an engine in there as well is undesirable.
A fuselage-mounted engine is more difficult to access than one slung under the wing. This increases maintenance costs and turnaround times. This is a critical factor.
An engine (ideally) wants a "clean" airflow to suck in and push out. Therefore, (ideally), a fuselage-mounted engine would have air intakes at the front (or high) and exhaust at the rear, with the engine somewhere in between. This has structural, weight and accessibility implications.
Finally, why use three engines when you can use two? And if you need a larger MTOW, longer range etc., why use three engines when four will do the trick, at lower power per engine (thus increasing engine life), using variations on existing wing/fuselage designs (rather than new developments).
Not all of these arguments are applicable to all a/c and situations, but a combination of them means that the 2/4 engines, symmetrically mounted on the wings, is the preferred option.
The definition of a 'Pessimist': an Optimist with experience...
Airplay From , joined Dec 1969, posts, RR: Reply 8, posted (11 years 7 months 3 weeks 5 days 11 hours ago) and read 1661 times:
Over the years the requirements for trans-oceanic flight have evolved. 3 and 4 engine aircraft were developed for certain longer routes for safety reasons. Obviously, if a pre-determined engine failure probabilty was too low, adding engines would reduce the probability that the airplane could not make it's destination.
Some Newer modern twin engine aircraft can now be eligible for ETOPS certification (Extended Range Twin Engine Operations). This enables an operator to bypass regulations requiring 3 or 4 engines only if they pass the eligibility criteria. Not only does the aircraft need to conform, but the operations and maintenance procedures of the airline are put under scrutiny.
NDSchu777 From United States of America, joined Oct 2001, 419 posts, RR: 3 Reply 9, posted (11 years 7 months 3 weeks 3 days 17 hours ago) and read 1602 times:
and i believe that tail-mounted engines, like on the DC-10 or L1011, are pretty expensive to design and build, plus they making maintenance more difficult since unlike having 2 or four similar wing-mounted engines, your trijet has two wing mounted (or in the case of the 727, rear fusalage-mounted) engines, with the tail-mounted engine different than the other two requiring some different maintanance.