The co-pilot of the helicopter which crashed off Sumburgh in 2013 armed the flotation bags which kept the aircraft afloat after it struck the water.
A fatal accident inquiry has heard Alan Bell primed the bags for release after realising the Super Puma L2 was going to crash.
The inquiry is being held following the deaths of passengers Duncan Munro, 46, Sarah Darnley, 45, Gary McCrossan, 59, and George Allison, who was 57.
Giving evidence on Thursday was Adrian Cope, a senior inspector of engineering with the Air Accident Investigation Branch (AAIB).
He attended the scene with colleague Mark Ford, who has also been giving evidence to the inquiry.
“The co-pilot realised what was going to happen and he armed the flotation bags,” Mr Cope told Sheriff Principal Derek Pyle.
“They are automatically triggered, but they needed to be armed. That kept the helicopter on the surface.
“We know from previous experience that, should a helicopter sink, that’s a dire situation for all on board.”
• The inquiry continues.