Four years ago, there were 50 helicopters registered in Ireland. Today, that number has more than quadrupled to 215, including 70 US and UK registered helicopters which are based in Ireland. But this fourfold increase has started to show up in the number of accidents.

Eight out of the 28 accidents and incidents, or almost one-third investigated by the Air Accident Investigation Unit, from January 2006 to date, involved helicopters, even though helicopters constitute less than one-fifth of the 1,100 aircraft in the country.

These eight accidents include one death last July when a helicopter carrying out a routine gas pipeline and flying at low altitude crashed in farmland near the village of Ballynacally, Co Clare.

And earlier this month, two men were seriously injured when their helicopter crashed near Ballycumber in Co Offaly.

"There has been a significant rise in the number of helicopter accidents recently and while the AAIU is concerned at any accident, this is related to the increase in activity, " the chief inspector of the AAIU, Jurgen Whyte, told the Sunday Tribune.

Whyte said he was happy with the required training for helicopter pilots and with the flight regulations covering their use in Ireland.

An applicant for a helicopter licence has to have completed 45 flying hours as a pilot, of which five may have been in a flight simulator.

With regard to training by a registered training school, the applicant must have completed 25 hours dual instruction including at least 10 hours of supervised solo flight time.

Whyte added that there was no discernible trend in the recent spate of helicopter accidents and that each accident was caused by specific incidents.

"In the last year, we have had helicopter accidents attributed to weather, technical issues and fuel, " he said.

"Pilots must comply with the rules of the air and fly within the limitations of the aircraft.

To fly in Irish weather can be challenging and a helicopter pilot must be alert to this, " he said.

A spokeswoman for the Irish Aviation Industry said helicopters can take off and land anywhere as long as they have the landowners' permission. Helicopters also do not have to inform air traffic control (ATC) of flights as long as the flight is below 8,000ft and outside a 20-mile radius of all airports.

Many helicopters do, however, use the ATC service as this will log their position if something goes wrong. But the IAA spokeswoman said the helicopters do not have to pay for this service even though all other aircraft . . . even those crossing Irish airspace . . . must pay.

A spokeswoman for the Department of Transport said charges for helicopters were a matter for the Commission for Aviation Regulation.