Lot of comments in here about the high maint load on the Tristar.
I was there at the start of BA service. When the Tristars were introduced at LHR, the engineers refused to work it. So the turnrounds at LHR were performed by management, and the maint was done on the line, mainly CDG and AMS. The aircraft kept flying. Being used to the Trident it was amazing how the (new) Tristars just kept on going. I was an apprentice, but got involved when we were sent off to AMS to change the first Nbr 2 engine. 3 Line trainers, 6 ex apprentices, and 6 Gulf Air engineers on OJT used the fly away nbr 2 engine staging for the first time in KLM's hangar and changed the engine in two days.
two years later I was in BAH with Gulf Air's brand new L1011s. They also kept flying. It was amazing how well developed the MEL was and how it kept the aircraft out of the hangar. But there we were comparing them to the BAC111s!!
I think that now people are compring them to new modern aircraft, and yes they were not that reliable, but compared to the aircraft in service in 1976, they were good.