I believe there was some sort of water system for the engines which might have been needed to help reach the Balearic's definitely Teesside Ibiza. Sure someone will know.
Yes, absolutely correct - it was a water injection system which basically allowed uprated thrust settings on take-off. There were barrels and barrels of de-mineralised water all over the network as needed, and the control and labelling of it was pretty tight to make sure that the Paninternational 1971 accident couldn't happen again - the de-min water tanks were accidentally filled with jet fuel which was injected into the engines on take-off, which set off a disastrous chain of events.
I never saw a system anywhere for doing the pumping of the water from barrel to aircraft by anything other than hand-pump and it was an absolute sod of a job. Other 1-11 gotchas included the cargo door coming off its runners and CSDS (drive-shaft) failures on engine start which you could hear from a few miles away.
I can't remember if the BA 1-11-510s had the water injection system (something tells me they didn't but happy to stand corrected!) but the Dan-Air and BIA aircraft definitely did. The Tarom / Adria ROMBAC aircraft also had. The BA -510s also didn't have forward airstairs and I think has been covered elsewhere that they flew around with a lump of concrete in their place for weight & balance. Incredible really.
In latter years, BIA probably took the 1-11 performance towards the boundaries more than most. Gatwick-Athens on a Friday night in a 1-11-500 was always quite interesting, particularly when half the fleet was out doing night Athens flying. There was a fine morning when, for performance reasons, none (including the -400, which was normally OK-ish on this sector) could make it back direct and the whole lot ended up in Ostend for tech-stops in the early hours of Saturday morning where crews were going out of hours due to call-outs from standby, night duty times and all the rest. Ops had worked out that if the First Officer from one 1-11 could quickly get across to another one on a nearby stand, they could swap F/Os and at least get one full crew who had the hours to fly one of the 1-11s on to Gatwick. The plan was fine until the F/O, running across the ramp, tripped over a ground power cable and broke (or at least badly injured, can't remember which!) his ankle so couldn't continue the duty.
It wasn't one of the finest Saturday mornings in BIA Ops with half the fleet stuck in Ostend and a full Saturday charter programme to fulfil.