Two USAF-operated McDonnell Douglas F15E fighter aircraft declared as operating as a formation were pulling up from low level after encountering potential delays at their intended post-exercise destination due to deteriorating weather in preparation for a diversion. Despite being cleared to FL150 only, both continued to climb and both passed through the level of an Embraer 145 being operated by British Airways Regional on a scheduled passenger flight from Birmingham to Hannover, one seen at an estimated range of 100 feet. The aircraft were on different ATC control frequencies, being Operational Air Traffic (OAT) and General Air Traffic (GAT) respectively.
An Investigation into this event was carried out by the UK AAIB because it was assessed as too dangerous to be handled by the UK AIRPROX Board which routinely deals with routine losses of separation.
The Investigation found that the infringement of controlled airspace by the two USAF aircraft had followed a level bust through their cleared level of FL150 and that Mode ’C’ altitude returns from both aircraft temporarily disappeared during the 1 minute 21 seconds they were passing through the airway. Visual contact with the civil aircraft was admitted by the F15 involved on their military ATC frequency with an apology.
It was found that "despite replying to the transmission correcting their cleared level to FL150, by the time the aircraft were handed over to London Military Tahoe 52 was already passing FL160 in the climb and both aircraft continued until level at FL230, suggesting the clearance was either misunderstood or ignored."
The Investigation had “concerns about the fact that the secondary radar data disappeared as the aircraft entered controlled airspace and only re-appeared once the aircraft had cleared the airway it has been suggested that the disappearance was due to a failure of the ground radar, however because the secondary data from the F15E disappeared on more than one ground radar but other aircraft were unaffected, this does not seem to be the case.” It was noted that the absence of the secondary data….effectively disabled both the ground radar STCA and the TCAS RA protection on the Embraer 145, “representing a serious loss in conflict warning and resolution ability for all the aircraft and ATC”.
The Investigation conformed that the USAF aircraft had both climbed through their cleared flight level and then climbed though controlled airspace without transponding into conflict with the Embraer 145 which was in level flight at FL210. It found that “inadequate transmission and acknowledgement of clearances within the USAF formation plus the crews inability to fly either as a coherent formation or as two independent aircraft during their diversion were major contributory factors to the ensuing general confusion.”
It was also noted that the USAF aircraft had made “poor use… of the highly sophisticated aids available to the crews in monitoring fuel loads, monitoring ground position and using airborne radar” and that “whilst it is accepted that aircraft such as the F15E necessarily operate at times close to their minimum fuel requirements, this places an even greater emphasis on the need to make early decisions when a deteriorating weather situation makes a diversion more probable. This is especially so when the diverting aircraft are required to negotiate some of the UK’s busiest areas of civil controlled airspace.”
The full AAIB Report of the Investigation was published on 2 February 2006. No Safety Recommendations were made.