A system warning of high ground was not turned on before a helicopter crashed, killing three men - one a friend of the Prince of Wales - an inquest has heard.
The helicopter hit the ground in the Mourne Mountains, Co Down, in October 2010.
The Prince's friend Charles Stisted, 47, chief executive of the Guards Polo Club at Windsor, was a passenger on the flight which was on its way back to England after the men attended a shooting party at an estate in Co Tyrone.
Construction company businessman and fellow polo player Ian Wooldridge, 52, and pilot Anthony Smith, 63, formerly of the RAF and army with service in Northern Ireland, also died.
Lawyer for the inquiry Ronan Daly told the Belfast inquest the helicopter had a ground proximity warning system.
"It was not actually in use, it was not the requirement for a helicopter to operate that it should be using the system and the alerts that the system would (produce) could be regarded as a nuisance to those on board," he said.
