A PASSENGER plane which stalled en route to Shetland probably suffered "severe icing" and a particularly hazardous form of turbulence associated with flying over mountainous terrain, an investigation has found.

The FlyBe flight, operated by Loganair, took off from Aberdeen Airport on the morning of October 3 2014 with three crew and 25 passengers on board.

It was heading for Sumburgh Airport in Shetland in blustery conditions.

The aircraft was in the second stage of its climb after take-off when acceleration reduced and ice almost an inch thick began forming around the windscreen wipers, according to an Air Accident Investigation Branch (AAIB) report into the incident.

Once level, the propeller plane failed to pick up speed as expected and the pilot initiated a descent, believing that this would reduce the drag on the aircraft caused by the so-called "mountain wave effect" - severe turbulence caused by oscillating air currents downwind of a mountain ridge.

The phenomenon is believed responsible for many aviation tragedies, including the in-flight breakup of BOAC Flight 911 near Mt. Fuji, Japan in 1966, which killed all 124 people on board.

The pilot initially descended at a rate of 1000ft per minute but, noticing that the aircraft's nose was pitched high - around five degrees upwards - he increased the rate of descent to 2,500 ft per minute.

Moments later the airframe began to vibrate "like the sensation of a car going over a cattle grid". The plane stalled and seconds later autopilot disengaged.

The pilot took manual control and began a forced descent.

The AAIB stated: "[The commander] later reported that a little more force than usual had been required to lower the nose of the aircraft but it then responded normally as the speed increased."

The vibration ceased and nine seconds later auto-pilot re-engaged.

The flight continued without incident and the plane landed safely at Sumburgh Airport.

The AAIB report concluded that the aircraft "probably encountered both severe icing conditions and mountain wave effect while climbing".

Subsequently, the crew received extra simulator training and the manufacturer, Saab-Scania, issued updated advice to crews on flying in icing conditions.