A retired Lieutenant Colonel with the Air Corps has described a helicopter door falling into the grounds of a secondary school in Dublin yesterday as "a most fortunate and unfortunate incident".

The helicopter, part of the emergency air service, was on its way back to Casement Aerodrome in Baldonnell when the rear door came off and and fell into Moyle Park College in Clondalkin.

Speaking on Morning Ireland, retired Lt Col Kevin Byrne said it was lucky the door fell on to the ground without making contact with a person or building, meaning there were no injuries.

He said the door fluttered to the ground like "a sycamore leaf" because it is quite light.

Mr Byrne is not certain what height the helicopter was flying at and said it landed without the door, which would have been an emergency in its own right.

He said there would be a very important incident investigation being carried out into it and the door, which has been recovered, would be "an important part" of that investigation.

Fianna Fáil Spokesperson on Defence Jack Chambers said he hopes that the investigation will be completed and published without delay.

"Thankfully no one was hurt in this very serious incident," said Deputy Chambers.

"Had the door hit someone on the ground it could have fatally injured them. A member of crew in the aircraft or indeed any patient being transported to hospital could have been injured too.

"I understand the Defence Forces have launched an internal investigation to find out how this incident occurred. I think the findings of this investigations should be published as soon as it is completed."