A pilot lost control of a mountain rescue helicopter and had to abort the mission after losing sight of where he was in darkness.
Three climbers got into difficulty at night at Beinn Narnain near Loch Lomond and required help.
The aircraft crew made several attempts from different directions to reach them but were unable to do so because of low cloud.
Conditions “deteriorated rapidly” on the final approach and the pilot was forced to abandon the rescue, according to an Air Accidents Investigation Branch (AAIB) report.
There was also loss of visual references through the night vision at a “critical time” and the commander became “distracted” by trying to regain sight.
This meant he was not concentrating on the angle of the helicopter and it dropped in speed when he tried to elevate it to exit the mountain bowl.
He tried to use autopilot but this made the matter worse by reducing the elevation rate even more and the coastguard rescue helicopter eventually lost airspeed.
The AgustaWestland aircraft then started to spin and the commander lost control.
This forced the pilot to take over the controls manually and despite difficult conditions he was able to stabilise the flight and raise it above cloud level, where he was able to use visual references.
Mountain rescuers were contacted who were able to complete the recovery mission.
The AAIB report said new six-monthly training sessions have been introduced since the incident, which happened February 17 last year.