By Nigel Pankhurst
Six people are dead and one is missing after a helicopter carrying gas platform workers crashed in the sea off Morecambe Bay in Lancashire.
Investigators now face the task of establishing what caused the aircraft to come down.
The Eurocopter Dauphin had a good safety record
The Eurocopter AS365N was on a routine flight over the gas fields of the Irish Sea when disaster struck 24 miles offshore.
The job of determining what happened will be passed to inspectors from the Department for Transport's Air Accident Investigations Branch (AAIB).
According to David Learmount, safety expert with Flight International magazine, there are many reasons to be surprised that the tragedy occurred.
"The company that operates the helicopter, which is CHC Europe, is the European branch of the biggest helicopter operator in the world. It is a global operator based in Canada," he said.
"Its specialisation is in the area of offshore operations.
"They're a very professional outfit. They're not inclined to cut corners, and even if they were they wouldn't be operating in the UK because the Civil Aviation Authority (CAA) keeps a very tight watch.
"The reason why the CAA do that is because the North Sea and the Irish Sea are very demanding environments. In this case the weather was relatively benign.
"This helicopter is always crewed by two pilots although it can be flown by one. They have got two engines but can fly on one of them. It's a very well designed, modern helicopter."
Mr Learmount said that, at 20 years old, the aircraft that crashed was "fairly old but a very, very well-tried machine".
It had "a lot of updated control systems", he said.
And while its age would be a factor considered by crash investigators, "just about every part will have been replaced - in some cases several times - since it was built", he added.
Mr Learmount said this type of event in the harsh offshore environment was not rare in the 1970s, 80s and early 90s, but things had changed dramatically since then.
"Events like this are very rare now, not just because helicopters are more modern but regulations are very strict now."
Helicopters such as the AS365N - known as the "Dauphin" - feature warning systems which alert pilots in advance if anything is wrong.
A British invention - the Health and Usage Monitoring System (HUMS) - was developed by commercial companies in association with the CAA about 15 years ago.
"Helicopters are mechanically complex. The system measures for vibrations which are abnormal. That means something's getting out of balance somewhere, so the pilot should get the helicopter on the deck," said Mr Learmount.
He says there are other features which would have added to the safety of the helicopter.
"In addition, this helicopter can ditch and float because they have flotation bags which can be deployed if you know you're going to set down in the sea," he said.
"If the sea state is relatively calm the helicopter can float and even be towed to shore.
"Even if the helicopter turned upside down, all the people that travel in these things get trained in a 'dunker', which is basically a cage in a swimming pool where they practise escape. They would all have wetsuits, they would all have life-jackets."
Helicopter pilot Paul Beaver, a former editor of Helicopter World magazine, said the investigation would be "very thorough".
"At the moment I think it's very much an open book on this one. The helicopter is well known, it's been in service for over 20 years, it's got an exceptionally good record, it's used by royal flights, by police, by the military around the world," he said.
"So there's no question mark over the helicopter itself. It could be something to do with human error, it could be a seagull being ingested into the aircraft, it could be a number of things."
He said recovering the helicopter's on-board data recorder would be important for the investigation.
"It's got a beacon on it. It'll be in the safest place on the aircraft - probably behind the after-bulkhead - and it will have a beacon that will be able to be picked up by ships that have got sonar.
"The data recorder will go to Farnborough to the AAIB's workshops there. The data is recorded on very thin wire.
"It gives all sorts of information, not just what the pilots were saying to themselves but also there will be the interplay with air traffic control, vital information about the aircraft itself should be stored on these and they tend to be 99% reliable when these recorders are picked up."
He said the investigation would be "a very thorough process".
"At the end of the day we will know why this aircraft crashed."
Mr Learmount says whatever did happen to the aircraft - which is not clear at the moment - it would have happened very quickly.
"This helicopter has got a good safety record. There's no such thing as a dangerous helicopter. If they were dangerous they wouldn't be flying," he said.
"There was no emergency call so whatever happened very fast. If something goes wrong with a helicopter it tends to happen very fast. You haven't got long before you hit the deck.
"There is a very, very long list of possible causes. This was sudden, this was disastrous."