The three engine formula pioneered by the De Havilland division of Hawker Siddeley Aviation has been accepted as giving the best economics for short -to-medium-haul operations.Three engines are less conplex than four.Engine out asymmetry is less than with two or four wing mounted engines.The power loss percentage with one engine "out" is nearer the four engine case than the two-engine case-a fact which is particularly important at take off.Ferrying on two engines is possible.
The practcability of three engines is one of several advantages of the rear engine position-a possibility that was considered when the Comet 1 formula was being determined.
Other advantages are the clean wing unencumbered by engine pods,the quiet passenger cabin,engine protection against stone damage,the virtual elimation of the noise damage problem,superior handling qualities in asymetric-thrust and reverse thrust conditions.
philbky,The Trident 3B used the RB162-86 throughout it's flying life and were never removed,only when unservicable and then replaced with a servicable one.