Two British jets, from Monarch and Ambassador, came within 300ft of each other near Malaga on 3 September and last Monday two aircraft, from Britannia and Air 2000, were reported to be only 400ft apart near Palma, Majorca.
In both cases, one aircraft had been cleared by air traffic control to pass through the other's flight level and after warnings from the equipment, pilots took evasive action.
The traffic collision avoidance system (TCAS) was only in use by chance because the jets concerned are used on transatlantic routes and the equipment is being made mandatory at the end of the year in the United States. The US Federal Aviation Authority made the move after a series of mid-air collisions, and US pilots have welcomed the measure. In Britain there is substantial support from the industry for the TCAS to be mandatory but in both countries air traffic controllers have opposed its introduction.
Here, the Civil Aviation Authority has refused to make British aircraft install the equipment. It has commissioned a survey into its use and Eurocontrol, which links European air traffic controllers, is also preparing a report, but this is not due until March 1995.
The CAA does not want to be rushed into using American technology, which is all that is available, it does not want to impose the cost on British airlines - estimated at pounds 150,000 per aircraft - and its researchers are not convinced that the TCAS is sufficiently developed.
David Harrison, the CAA's head of collision avoidance, said: 'There have been several versions of TCAS already and the software is being changed all the time. The early versions gave a lot of unnecessary warnings.' He said false warnings could lead aircraft into potentially dangerous situations.
The system works by creating a radar bubble around the aircraft and when an intruder enters it, a warning is sounded. It tells the pilot to descend or climb to avoid a collision but issues no instructions on the heading of the aircraft. Nuisance warnings, where there is no risk of collision but the computer thinks there is, are recognised even by the manufacturers as a problem.
British Airways, which has a third of its fleet fitted with the TCAS - those aircraft which fly to the US - said: 'This is one of the biggest contributors to air safety since radar. We are convinced its use is ultimately inevitable.' However, the airline is refusing to install the equipment in the rest of its fleet until the CAA acts because 'we don't feel that European air traffic control systems can cope with it'. Air traffic controllers are generally against its introduction, fearing it will take their control of the airspace away.