But surely the BA mate had ACAS, so no worries
He did. But it only works if the 'intruder' is giving useful transponder information. In this incident there were periods where the F15's transponders were either not selected or not transmitting Mode C data.
Radar data
Civilian air traffic control radar recordings were obtained covering the time of the incident. They show that at 1131 hrs the formation began squawking the emergency Mode A code 7700 with a concurrent Mode C altitude of 16,800 ft. Twenty five seconds later both the Mode A and C squawks disappeared, the last Mode C altitude recorded being 18,500 ft. The Mode A emergency squawk 7700 then reappeared 1 minute and 21 seconds later concurrent with a Mode C altitude of 22,400 ft.
The recordings showed a minimum lateral separation between Tahoe 52 and the Embraer 145 of 0.53 nm and a minimum lateral separation between Tahoe 51 and the Embraer 145 of 1.18 nm. No Mode C altitude information is displayed for either of the military aircraft during this period and it has not been possible to verify the minimum vertical separation.
The full report can be found here:
http://tinyurl.com/ay2us