so they couldn't make up their minds in the case of the tri star, L1011 and DC10 eh?????????????
So they had 90,000 Lb thrust engines available back in the 70's when these aircraft were designed, did they?
Given that the aircraft mentioned had 3 engines of APPROXIMATELY 45,000 Lb thrust each, 90,000 Lb (2 engines) would be required to meet the OEI requirement. To make the L1011 and DC10/MD11 two engined aircrafrt would have required engines of a little less than 90,000 Lb thrust each.
Nice post,
Mad (Flt) Scientist, a good summary in simple terms. I think that you neglected to mention that the IFSD (In-Flight Shutdown) Rate for the 4 engined aircraft is somewhat more than twice that for the 2 engined aircraft. In Normal operations (i.e. the 99.999% of the time that an engine DOESN'T fail), the 2 engined aircraft is operating with 100% thrust excess above the minimum requirement, whilst the 4 engined aircraft has a mere 33% excess. Thus, the 4 engined aircraft is operating at high thrust levels (Takeoff and Climb) for a MUCH longer period, and, as most turbine engine failures are due to cumulative stress, and the cumulative stress for the 4 engined aircraft is much higher, with a commensurately higher IFSD Rate per engine than for the twin.
"Four engined aircraft can afford to lose two engines" - True, but not on Takeoff, or early in the flight at high gross weights, unless you can dump fuel fast enough before impact.
"Four engined aircraft may well end up lighter (due to less thrust excess by design). " - Absolutely true, plus lighter wing structure needed to provide wing bending relief due to the outboard engines. The 3 engined aircraft is WORST in this respect.
"And twice the number of engines may cost more to operate due to maintenance and the like." - Absolutely true again. Increased size does not necessarily mean increased labour for overhaul. Roughly, twice as many engines cost twice as much to overhaul, irrespective of size.
Mad (Flt) Scientist, not in any way a criticism of your post, it was an excellent summary provoking further thought and comment.
Given that 99.999% of the flights that I've operated in 2, 3, and 4 engined aircraft did NOT suffer an engine failure, my clear preference for safety is for the aircraft with the greatest performance margin to get out of a sticky situation - the twin.
Pray that the IFSD Rate never reaches ZERO, because when it does, the accountants will be calling for single engined aircraft
Regards,
Old Smokey