A communication issue led two Boeing-737 airplanes to come within 807m of each other as one landed and one took off at Dublin airport earlier this year, the Air Accident Investigation Unit (AAIU) has said.
In its report into what it described as a “serious incident”, the AAIU said that due to a high volume of inbound traffic at the airport on March 8th last, the spacing between arrivals had been decreased.
“…increased coordination was necessary between the Air Movements Controller (AMC) and the Approach Controller (APP),” the report says.
At about 9.56pm, the departing aircraft (EI-EFF) was cleared for take-off while the second place (EI-DHA) was on its final approach to the same runway at the airport.
The departing plane was carrying 179 passengers and a crew of six and the incoming plane had 135 passengers as well as six crew on board.
The report says the air movements controller attempted to stop the departing aircraft at the holding point, however, as a result of a blocked transmission it continued past the stop line which was in accordance with its previously issued clearance
“Clearing the departing aircraft for an immediate take-off, the departing aircraft became airborne prior to the arriving aircraft touching down,” it states.
Plane EI-EFF had been asked by air traffic control if it would be ready for “an immediate rolling departure” but the report notes the reply was “indistinct with the word ‘negative’ but was understood by the AMC as an affirmative reply”.
The plane was consequently instructed to line up and take off on runway 28 and after receiving a signal that the cabin was secure replied “Cleared for immediate take-off…..eh affirm (callsign 342) cleared for line up take-off we’ll expedite”.
The report says the planes speed slowed to 5kts (9 km/h) after making a turn on the runway and such a significant drop in speed was not expected by those in the control room. There was a request for the plane to “hold short” on the runway.
“This transmission crossed with another transmission and was not acknowledged by EI-EFF…,” the report adds. “EI-EFF then requested confirmation of its take-off clearance which was confirmed by the AMC using the word ‘immediate’.”
EI-DHA, which was coming into Dublin, was advised of the departing traffic and told to expect late landing clearance.
A runway occupation alert was activated and data seen in the investigation showed “the departing aircraft became airborne prior to the landing aircraft touching down”.
“At their closest point the aircraft were 807 metres apart; however the rate of closure was low,” the report said, adding that the departing aircraft was travelling at 138kts (255 km/h)and the landing aircraft at 146kts (270 km/h).
The AAIU said good communications “are essential to the safe and orderly flow of air traffic” and that with traffic levels “building up at that particular time of the evening, increased coordination was necessary” in order to identify gaps for departing traffic.
It said that following the incident the Irish Aviation Authority installed a ‘Time to Touchdown’ facility (TTT) for the tower controller “which displays 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 TTT should greatly reduce the risk of the event occurring in the future,” it said.