Departing and arriving aircraft came within 800 metres of colliding at Dublin Airport, an air accident investigation report has said.
The airport was busy on March 8 this year and a scheduled flight to London Luton was cleared for take-off as an incoming plane approached.
An air traffic controller misheard a radio signal from the pilot of the departing aircraft requesting more time to prepare and asked it to take off speedily. As the plane slowed on the way to the runway the controller changed his mind and tried but failed to stop it.
Air accident investigators said: "At their closest point the aircraft were 807 metres apart; however the rate of closure was low."
A few weeks later two Ryanair planes clipped wings as they taxied in Dublin.
A report by Ireland's Air Accident Investigation Unit (AAIU) said t he departing aircraft was cleared for take-off with a landing aircraft on final approach to the same runway.
"Re-appraising the situation, the controller attempted to stop the departing aircraft at the holding point; however, as a result of a blocked transmission the departing aircraft continued past the stop line which was in accordance with its previously issued clearance," the review added.
"Clearing the departing aircraft for an immediate take-off, the departing aircraft became airborne prior to the arriving aircraft touching down."
Investigators said the air traffic controller, assessing the position and speed of both aircraft after the departing flight passed a stop line on the runway, deemed it safer at that point to let the arrival continue and land.
The controller had sought confirmation if the departing aircraft was ready for an "immediate rolling departure".
He thought the reply was affirmative and "didn't hear anything else", so he cleared the departing aircraft to line up and take off. In fact the reply from the flight deck was negative because the cabin had not been secured.
The controller remarked that if he had clearly heard a "negative" in response he would have instructed a "stop" at the stop-bar without hesitation, the report from the AAIU said.
He said when turning on to a taxiway, the aircraft "seemed to slow down".
"This caught the air traffic control officer by surprise, as the flight crew were asked to keep a good taxi speed to facilitate the departure sequence.
"The air traffic control officer considered the separation with the landing aircraft and reviewing his plan, instructed EI-EFF to hold short of the runway."
The instruction was not received and the departing aircraft passed the stop line.
The report said: "Air traffic control should be aware that it may be difficult for flight crews to know early in the taxi 'if they will be ready on reaching', as often the flight crew will not have received confirmation (intercom or chime) of the cabin being secured."
Subsequent to this incident, the Irish Aviation Authority installed a "time to touchdown" facility for the tower controller displaying the time to touchdown of the next aircraft on final approach.
"Associated air traffic control procedures have been developed, safety assessed and implemented and the use of time to touch down should greatly reduce the risk of the event occurring in the future."
A Ryanair spokesman said: "We note this report which contains no recommendations for either Dublin air traffic control or Ryanair."