Incomplete monitoring of instruments by flight crew is “the norm” among pilots due to how the human brain works, an inquiry into a fatal helicopter ditching that claimed four lives has heard.

Steve Jarvis, an expert on human factors in the aviation industry, said the monitoring of instruments by flight crew has arisen as an issue in many helicopter crashes but normally the resilience of the system helps protect the aircraft.

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.

Mr Jarvis said pilots can suffer from “vigilance decrement”, which is when it becomes harder for them to remain vigilant over time when monitoring instruments.

The research scientist said it can also be difficult for them to maintain attention when looking at autopilot.

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.

Sheriff principal Derek Pyle, who is hearing the fatal accident inquiry (FAI) asked: “What we are talking about is incomplete monitoring, are we not?”

Mr Jarvis replied: “I suppose we are but what I’m suggesting and what I’ve seen in my research is that incomplete monitoring is actually the norm and the resilience of the system is what happens most of the time and that protects it.

“It’s not because the pilots are just getting degraded or anything, it’s because deep inside the human brain things are happening which we have not really fully understood at this point and the aviation industry has been really good at making systems resilient enough to deal with that but until we get an even better understanding that next tiny step probably won’t happen.”

With so many factors coming together in this particular case the odds of this happening were very very low

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” and more research is needed.

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.

Asked how likely it is that such an accident will occur again, he said it is a very rare event.

He told the inquiry, which is being held virtually due to coronavirus measures: “With so many factors coming together in this particular case the odds of this happening were very very low, because it would need a whole lot of things to come together in a kind of perfect storm.”

A second expert witness disagreed with Mr Jarvis, saying vigilance decrement should not be discounted from being a potential factor in the crash.

Polly Dalton, professor in psychology at Royal Holloway, University of London, helped to prepare two reports for the inquiry.

She said: “I wouldn’t be as quick as Mr Jarvis to say vigilance decrement isn’t relevant.

“I think it’s important to bear in mind their ability to do so (passive monitoring) over time will decrease.”

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

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 on Thursday.