A five-minute phone call from air traffic controllers to police delayed the emergency response to a fatal North Sea helicopter crash, investigators say.

Eighteen people were aboard the Super Puma helicopter which plunged into the sea near Sumburgh Airport in Shetland in August 2013. Both pilots and twelve passengers escaped, but four others lost their lives.

An inquiry by the Air Accidents Investigation Branch (AAIB) ruled the crash happened because the pilots failed to monitor their instruments properly in bad weather.

But investigators also raised concerns about the length of time it took to organise the emergency response to the incident.

They said a "brief initial notification should have been sufficient" from air traffic controllers but highlighted that the delay caused by the call had "no bearing on the outcome of the rescue".

5.23pm

ATSA: Hello, this is Sumburgh Airport.

Police: Oh, hello there, have you got an emergency?

ATSA: Yes, aircraft accident.

Police: Oh, right, OK, just bear with me a moment please. This aircraft accident, is this actually at the terminal?

ATSA: No, it’s at sea.

Police: At sea, OK, just bear with me a second, sorry to keep you.

ATSA: OK.

Police: I’ll just get the basic details on and then, you know, people can start doing things while I'm on the phone to you, OK? I'm sorry to keep you.

ATSA: That’s OK.

5.24pm

Police: Sorry, I was just telling my sergeant so he can put it on there it's an aircraft accident at sea. What aircraft is it?

ATSA: It's a helicopter.

Police: OK, em.

ATSA: I don't have any persons on board at this time.

Police: There's nobody in.

ATSA: We don't know how many persons on board at this time but I can find that out and let you know.

Police: Yes, if you will, please.

ATSA: OK.

5.25pm

Police: OK, sorry to keep you. Whereabouts is it?

ATSA: He was about four miles out on approach to Sumburgh. So he was just about four miles for runway nine, he was coming in for runway nine.

Police: So he was heading south?

ATSA: He was approaching runway nine at Sumburgh Airport.

Police: Yeah.

ATSA: He was four miles out the last time we knew of him.

Police: OK, all right.

ATSA: OK.

Police: Yes. Any other details at all?

ATSA: No, that's all the details I have.

Police: That's all you've got, OK. Em, who is the helicopter owned by, which company?

ATSA: It's Scotia CHC.

Police: CHC Scotia?

ATSA: Yes. So you call out the coastguard now then?

Police: We'll do all of that, yeah. Have you advised anybody, the Aeronatical Rescue Coordination Centre, anybody at all? Coastguard, hospital, anything?

ATSA: No, you do all that as far as I know. I just call you, you're my first port of contact and then I go down my list from here.

5.26pm

Police: No, it's all right, just so we know. OK then. All right. As far as you know it was approximately four miles out but you've got no definite location.

ATSA: It was on approach to runway nine.

Police: OK, so would you have any coordinates for it at all.

ATSA: No, not at this time. I have no coordinates for it at this time.

Police: OK, sorry, I'm just double-checking if there's anything else I need to say. OK, was the mayday message sent?

ATSA: No, he was just ... they lost contact with him.

Police: OK, did you lose radar contact as well?

ATSA: We're just Sumburgh Tower, we don't have a radar here in the tower.

Police: OK, so how do you actually know that [the aircraft] has gone down.

ATSA: Because he's not contacting us. We've been calling him and I don't think they can see him on radar.

5.27pm

Police: All right. Have you got a flight identification number for this helicopter?

ATSA: What do you mean, flight identification number? Is that a squawk?

Police: I don't know what you mean by that, sorry. Is there any reference for the aircraft at all?

ATSA: No, I'm sorry that's all the details I have at the moment. So, eh, you will alert the coastguard?

Police: Oh, yeah, yeah, that's all being done now. That's being done now, I'm just double-checking to make sure everything the sergeant wants to know. But we'll be doing that. OK, em, right, so you'll let us know how many people are on board and any other details if you get them at all?

ATSA: Yes, I'll do that, I'll phone you back.

5.28pm

Police: OK, that's lovely. Sorry, what's your name please?

ATSA: It's the assistant, the tower assistant.

Police: Yeah, what's your name?

ATSA: It's [redacted].

Police: Sorry?

ATSA: [Redacted].

Police: OK and what will be the best number to contact you on.

ATSA: [Redacted].

Police: OK, all right, no problem, and can we contact you on that number?

ATSA: OK.

Police: Yep, OK, thank you.

ATSA: Thank you.

Police: Bye.

ATSA: Bye, bye.