Danish civil aviation administration SLV is awaiting a detailed statement from regional airline Cimber Air after suspecting that one of its ATR 42-500 turboprops exceeded flight limits by performing a 360° longitudinal roll. Investigations began when the Danish authority was notified of the manoeuvre on 10 May, the day after it was carried out at Sonderborg, where Cimber Air is based.
Sources familiar with the incident say that Cimber Air managing director Jorgen Nielsen was at the controls with one other pilot. No-one else was on board. The sources claim the crew had been preparing for Denmark's Karup air show on 8 June, although it is unclear whether the roll was intended to be part of the display.
Sequences of images of the incident appear to show the twin-engined aircraft travelling at low level - perhaps under 100ft (30m) - with its undercarriage raised, before entering a right-hand climbing turn, inverting and completing the roll recovery.
The climb is indicative of a "barrel" roll, a manoeuvre more complex than a basic aileron roll because it requires constant changes, in all three axes, to the aircraft's direction of travel.
Cimber Air would not discuss the event. SLV says that it is to conduct an assessment and is awaiting a report from the airline, expected shortly, which will include cockpit-voice and flight-data recorder information.
SLV does not have confirmation of the height of the ATR or the extent of the aerodynamic forces involved. It says: "We suspect, although only on the basis of initial information, that the aircraft had been flown outside its operational envelope."
ATR could not immediately comment on the incident.
Following the incident the ATR was taken to Eindhoven in the Netherlands to undergo a structural inspection.
"We'll need some time to make a thorough report on the incident and assess whether there were stress factors on the airframe," says the SLV, cautioning that a sequence of photographs can be misleading.
The aircraft, serial number 501, is a 12-year-old example that was previously on lease to Middle Eastern operator Oman Air, and was still carrying much of the airline's colour scheme at the time.
SLV believes the aircraft was on its way back to airline service with Cimber Air, but that it had still to be brought up to full operational specifications.
The SLV cannot say whether any regulations might have been broken until it has had a chance to study the data and the airline's report.
In 1995 an RAF Nimrod performed a similar manoeuvre crashed into Lake Ontario - here's a video: