The monitoring of instruments by flight crew has arisen as an issue in many helicopter crashes, including one off Shetland in which four people died, a fatal accident inquiry has heard.

Steve Jarvis, an expert on human factors in the aviation industry, said pilots can suffer from “vigilance decrement”.

This is when it becomes harder for them to remain vigilant over time when monitoring instruments.

Mr Jarvis said it can also be difficult for them to maintain attention when looking at autopilot.

Two crew and 12 passengers on the Super Puma L2 survived when it ditched on its approach to Sumburgh Airport, Shetland, at 6.17pm on August 23 2013.

But Sarah Darnley, 45, from Elgin, Moray; Gary McCrossan, 59, from Inverness; Duncan Munro, 46, from Bishop Auckland, County Durham, and George Allison, 57, from Winchester, Hampshire, all died in the incident.

Shetland helicopter crash
(left to right) Crash victims Duncan Munro, George Allison, Gary McCrossan and Sarah Darnley (Police Scotland/PA)

An AAIB report published in 2016 found the pilots failed to properly monitor the flight instruments and failed to notice their speed was decreasing until it was too late to avoid the Super Puma plunging into the sea.

Martin Richardson QC, who is leading the inquiry for the Crown, asked Mr Jarvis: “Should the issue of monitoring by the flight crew at Sumburgh be seen as an isolated incident?”

Mr Jarvis replied: “No. One reason is that for many accidents and incidents monitoring has come up as an issue.”

He said it is important to understand why pilots do not notice things but research into this area is “embryonic”.

Mr Jarvis said he does not believe the Sumburgh accident had anything to do with vigilance decrement but was a result of a collection of circumstances, with the issue of monitoring combined with other things.

Survivor Samuel Bull took his own life in London in 2017, which sheriff principal Derek Pyle said was “directly caused” by the crash.

The inquiry is taking place virtually due to coronavirus measures.

A statement of agreed evidence read at the start of the inquiry confirmed no mechanical fault was discovered with the helicopter, which was returning from the Borgsten Dolphin support vessel to Sumburgh Airport when it ditched.

The inquiry continues.