A fatal North Sea helicopter crash happened because pilots were not monitoring their instruments properly, an investigation has found.

The Super Puma aircraft had 18 people aboard when it plunged into the North Sea near Shetland in August 2013, killing four.

Both pilots and 12 passengers escaped but Sarah Darnley, 45, Duncan Munro, 46, George Allison, 57, and Gary McCrossan, 59, lost their lives.

A report published by the Air Accidents Investigation Branch (AAIB) on Tuesday said the helicopter's instruments were not effectively monitored during the final seconds of its approach to Sumburgh Airport on August 23.

Experts reported the helicopter’s airspeed dropped as it approached Sumburgh but the crew did not notice.

They were unable to pull up and the helicopter plunged into the sea around two miles west of Sumburgh. It rapidly filled with water and rolled over but was kept afloat by floatation bags.

No mechanical faults which could have caused the crash were found.

The AAIB reported: "The helicopter's flight instruments were not monitored effectively during the latter stages of the approach. This allowed the helicopter to enter a critically low energy state, from which recovery was not possible.

"Visual references had not been acquired and no effective action was taken to level the helicopter."

Crash investigators said it would have been "crucial" for the pilots to pay close attention to their instruments as they approached Sumburgh. The AAIB has made 28 safety recommendations as a result of the incident.

The AAIB reported that the Super Puma hit the water at such speed many of its passengers were unable to brace themselves or take a breath before it went under.

Investigators said: "Some of the passengers could see that they were about the hit the sea, but the impact and speed with which the helicopter rolled over and filled with water resulted in only a few being able to take a breath before becoming submerged.

"Most survivors had difficulty with the poor visibility under the water and although a few initially attempted to use their emergency breathing system, they were unable to locate the cover for the mouthpiece and concentrated on escaping from the cabin instead."

The AAIB said some passengers who survived the crash reported having problems removing windows designed to pop out in case of emergency.

The report continued: "Of the three fatalities caused by drowning, one did not release their seat harness and was still in their seat when the wreckage was recovered. A second person released their harness but did not escape from the cabin.

"There was some evidence to suggest the third person escaped the cabin but subsequently drowned."

A fourth passenger managed to escape the helicopter but then suffered a heart attack in the liferaft, investigators added.

In February 2014, the Civil Aviation Authority outlined a series of measures aimed at improving offshore helicopter safety following the crash.

Their recommendations included helicopters not being allowed to fly offshore in severe sea conditions and a size limit for passengers.

Scientists at the Robert Gordon University recently said they believed the North Sea is safer as a result of the new rules.

A spokesman for helicopter operator CHC said: "We deeply regret the tragic Sumburgh accident and we continue to extend our sympathies to the survivors and loved ones of those who died.

"Our priority is always the safety of our passengers and crews and we would like to thanks the AAIB for its thorough investigation.

"Although the AAIB makes no recommendations specific to CHC, immediately after the accident and in the time since we have put in place additional measures to improve safety.

"Among those actions we have enhanced crew monitoring procedures and supported the industry-wide deployment of improved emergency breathing systems and training for offshore passengers and crew."

A spokesman for the British Airline Pilots Association added: "Many safety improvements have already been made to helicopter operations since this tragic accident and safety experts will be examining the report to identify what more can be done to avoid a repeat."