The US National Transportation Safety Board (NTSB) has highlighted the absence of altitude deconfliction briefings in its preliminary report into the mid-air collision at the Wings Over Dallas Airshow on November 12.

The fatal crash – which killed six people – involved a Bell P-63F Kingcobra and Boeing B-17G.

The report says the P-63F was number three in a three-ship formation of historic fighter aircraft and the B-17G was lead of a five-ship formation of historic bombers.

According to recorded audio from the air show radio transmissions and ADS-B data the air boss – who was responsible for overseeing the display – directed both formations to manoeuvre southwest of the runway before returning to the flying display area.

“He directed the fighter formation to transition to a trail formation, fly in front of the bomber formation, and proceed near the 500ft show line,” the NTSB report reads. “The bombers were directed to fly down the 1,000ft show line.”

The 500ft and 1,000ft show line were 500ft and 1,000ft, respectively from the airshow viewing line. Under this arrangement, the lateral separation of the aircraft should have been 500ft.

“There were no altitude deconflictions briefed before the flight or while the airplanes were in the air,” the report stated.

When the fighter formation approached the display area, the P-63F was in a left bank and collided with the left side of the B-17G, just aft of the wing section.

Both aircraft broke up in flight and impacted terrain in a grassy area on airport property south of the approach end of runway 31, the NTSB said.

The preliminary report offers little insight into the cause of the crash. The final report on the incident is expected to take between 12 and 18 months to complete.