Investigators believe that planners of a major military exercise off the Dutch-German coast incorrectly assumed there were no restrictions on airspace use before a serious incident involving a KLM Cityhopper Fokker 70.
The aircraft came within 730m horizontally and 512ft vertically of a Boeing F-15C fighter, the proximity of which triggered an order to climb from the Fokker’s collision-avoidance system.
Dutch Safety Board inquiries found that the F-15 had crossed an active airway, UN873, and that the fighter controller “unintentionally aggravated” the situation by turning the aircraft such that it crossed the airway again, in front of the Fokker, at the same altitude.
During preparation of last year’s Frisian Flag exercise – which was granted access to German airspace above 24,000ft for the first time – the organisers “assumed” that the assigned airspace block was available “without restrictions”, says the inquiry.
Although the Royal Netherlands Air Force conducted a risk assessment for Frisian Flag, the increase in available airspace above Germany was “not seen sufficiently as a risk”, it adds: “The focus of collision-avoidance in the air was on aircraft participating in the exercise.”
As a result “no adequate measures” were put in place to ensure separation with non-participating traffic, says the inquiry, and the risk of conflict with UN873 was overlooked – even though civil aircraft were seen to be operating along the airway on the first two days of the exercise.
The F-15 involved, one of four US Air Force in Europe aircraft in formation, had not adhered to a 32,000ft altitude restriction before the incident, and had climbed to 35,000ft – the same altitude as the Fokker.
While the Dutch Safety Board says the USAFE “refused” to answer its questions, and that this “hampered” the investigation, an internal USAFE report stated that the F-15 pilot was unaware of the restriction because of a simultaneous radio call.
Dutch inquiries determined that the same F-15 had been involved in another, less serious, loss of separation with a Boeing 737-400 about 8min before the Fokker encounter on 19 April 2012.