The three male crew and a 16-year-old female cadet managed to free themselves from the wreckage and reach the shore of Llyn Padarn, beside the Snowdownian town of Llanberis.
The Wessex helicopter from RAF Valley, Anglesey, was on a training flight with the four air cadets, all from the East Lancashire wing of the Air Training Corps.
Witnesses, including holidaymakers, said the helicopter began spiralling out of control after the tail rotor stopped working. Ieuan Williams, who was working at a lakeside garage, said: 'We thought the helicopter was coming down on the garage but the pilot managed to manoeuvre it over the lake. The nose went down and it crashed into the water.'
The four survivors were taken to hospital in Bangor, 15 miles away.
All had suffered back injuries but were not thought to be in a serious condition.
The RAF will try to recover the wreck of the helicopter today.
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