Norwegian investigators believe a Danish Air Transport ATR 42-300 entered an uncommanded climb, and came close to stalling, as a result of inadequate de-icing.
The inquiry into the event has highlighted the importance of de-icing all critical surfaces.
While departing Bergen, the aircraft lifted off without any control input, 10kt below rotation speed. It continued a shallow, low-speed climb despite both control columns being moved to their fully-forward position and the engine power being increased, says Norwegian investigation board SHT.
Airspeed fell away and the ATR's stick-shaker activated, along with an audio alarm, warning the pilots that the turboprop was approaching a stall.
The aircraft eventually began to respond to the pitch-down input. Its nose lowered and its airspeed increased, and the stick-shaker stopped, but the control columns initially remained heavy to operate.
Snow and ice had been present on the turboprop while at Bergen and it was de-iced with warm water, before anti-icing fluid was applied about 8min before take-off.
But airframer ATR, which submitted a comment to the inquiry report, questioned the quantity of anti-icing fluid - some 69 litres of type-2 and 17 litres of type-1 - used on the aircraft. It said the amount of type-2 fluid "seems to be low" and suggested the level ought to be closer to 120 litres.
ATR added that the Bergen event matched the behaviour of an aircraft subjected to "improper" de-icing of its horizontal stabiliser.
"The most probable scenario is that the [aircraft] was either badly de-iced on [the] ground or that the hold-over time was exceeded with subsequent contamination on the horizontal stabiliser [or] elevator," it said.
SHT says there was a "real risk" of a stall and the investigators are "uncertain" whether the pilot or the stick-pusher could have prevented it should the nose have risen further.
"It is also uncertain whether it would have been possible to recover in time if a stall had occurred at such a low altitude," adds SHT.
The control columns gradually became easier to handle and the pilots continued to their destination, landing safely. None of the 27 occupants of the 9 November 2007 flight was injured.
SHT notes that Danish Air Transport's empennage de-icing procedure for the ATR gives "special attention" to the area between the horizontal stabiliser and elevator, to prevent the elevator from freezing.
It says it "questions" whether this focus might have led de-icing personnel to "not be sufficiently attentive" to the need to keep the upper stabiliser and elevator surfaces completely free of ice.
Source: Air Transport Intelligence news