UK investigators looking into the near-stall of an ATR 42-300 on approach to Glasgow believe that fatigue might have played a role.
As the Air Contractors freighter was vectored for the runway 23 approach, it was operating with a level of ice protection which required higher speeds because the stall alert threshold was lower.
With the ice protection engaged the approach speed required was 114kt. The captain did not state whether this speed, or the non-icing speed of 99kt, would be used for the approach.
The inquiry says several items were omitted from the Glasgow approach briefing, and that the captain was not operating the aircraft in line with company procedures.
During the approach the ATR - flying at around 140kt, with flaps at 15° - levelled at 2,000ft and the engine torque was reduced to 3%. Neither pilot mentioned the declining airspeed as the autopilot gradually pitched the aircraft nose-up in order to maintain 2,000ft.
The aircraft was 6.5nm from the runway when a stall alarm sounded and the stick-shaker activated. It had pitched up to an angle of attack of 11.2° and its airspeed had fallen to 111kt.
To recover from the near-stall the captain pitched the aircraft down and pushed the throttles almost to full power. As the ATR descended, and then climbed, the airspeed - which had declined to 104kt - rose to 174kt, exceeding the limit for the flap setting.
During the renewed descent to Glasgow the airspeed again reduced to 111kt and the angle of attack verged on the stall alert threshold, before the aircraft subsequently landed without further incident.
The Air Accidents Investigation Branch says the captain had been performing his first night-flying duty following a period of normal night sleep, and the incident occurred nearly 24h after the end of his last proper sleep.
He had also driven 2h 45min to his base before flying duty, it adds: "Consequently, knowingly or not, he may have been tired or fatigued."
Cockpit-voice recorder data revealed the captain yawning during the flight, as well as during the previous Paris-Newcastle sector. Standard calls and responses were not always correctly performed and a sterile cockpit environment was not maintained below 10,000ft.
In its analysis of the 22 February 2012 flight, the AAIB also notes that the captain's manner during his responses to the first officer's monitoring calls was "likely to have discouraged further input" at a point when effective communication was necessary.