EADS unit Eurocopter must improve the monitoring and warning systems on one of its best-selling helicopters after a gearbox fault that caused a fatal crash in the North Sea went undetected, an air accident report said.

All 14 passengers and two crew lost their lives in April 2009 when a Bond operated Super Puma helicopter crashed off Peterhead on the east cost of Scotland while returning from a visit to BP's Millier oil platform.

A report into the incident published on Thursday by Britain's Air Accidents Investigation Branch (AAIB) said an indication of gear degradation had not been adequately followed up, leading to a catastrophic failure of the main rotor gearbox, which suffered a fatigue fracture.

The AAIB's report made 17 safety recommendations, including one stating that Eurocopter must improve the gearbox monitoring and warning systems on the AS332 L2 Super Puma to identify degradation and provide adequate alerts.

The AAIB said a magnetic particle had been found on the chip detector in the gearbox of the helicopter -- manufactured by Eurocopter -- a week before the disaster.

This led the operator, Bond, to initiate a plan to remove the main rotor gearbox and replace it with one from another helicopter, the AAIB said. However, this did not happen because of miscommunication between Bond and Eurocopter.

The use of verbal and email communication between the operator and manufacturer on 25 March led to a misunderstanding or miscommunication of the issue, the AAIB said in its report published on Thursday. It added that the gearbox was declared serviceable by the operator and its planned replacement cancelled.

Some 700 Super Puma/EC225 helicopters are currently in service with 100 operators in 50 countries. They are used in a civil capacity, especially the oil and gas industry, as well as for military missions.

Eurocopter said it continued to work closely with the regulatory authorities, investigators and its operators to prevent the risk of accidents and that it had already made some revisions to its monitoring systems after the AAIB's initial report into the crash in April 2009.

As soon as initial findings regarding the accident became known, Eurocopter took steps to enhance the monitoring of the Super Puma's main rotor gearbox with the aim of increasing the number of alerts, thereby improving tolerance to any possible misleading interpretations of the maintenance manual, a Eurocopter spokesman told Reuters.

The Super Puma was, is and will remain a safe aircraft, and is the preferred choice of numerous operators globally.

Shares in EADS, which have fallen 5 percent in the last three months, were 3 percent up at 20.82 euros by 2:15 p.m., valuing the company at around 16.5 billion euros (14.2 billion pounds).

(Additional reporting by Tim Hepher in Paris; Editing by Adveith Nair and Mike Nesbit)