Japanese investigators have disclosed that a De Havilland Dash 8-300’s departure from Tokyo Haneda was supposed to be threaded between two arriving aircraft, before it entered the runway and was fatally struck by the first of them, a Japan Airlines Airbus A350-900.

The collision left only one survivor, the captain, from the six occupants of the Dash 8, but all 379 on board the A350 escaped with just five sustaining injuries.

Japanese investigation authority JTSB has yet to reach final conclusions on the 2 January 2024 accident, stating in an update that completing the inquiry before the anniversary of the collision will be “difficult”.

But it states that the Coast Guard Dash 8’s flight – to take earthquake-relief supplies to Niigata and Komatsu – was delayed for various reasons, including a technical issue with the auxiliary power unit generator.

Its crew had discussed whether they would be able to source suitable ground power-supply equipment at the destination.

As the Dash 8 travelled along taxiway C, which runs parallel to runway 34R, it was in a queue with several other aircraft heading for the C1 intersection at the end of the runway.

The inquiry says that, in order to facilitate operations on 34R, given the Dash 8’s slow speed, controllers in the east tower planned to slot the Dash 8’s departure between the arrival of the A350 and that of another aircraft behind it.

Tower control instructed the Dash 8 to exit the queue by taking the turn-off to the C5 intersection, clearing it to the holding point and stating it was “number one” for take-off.

But the Dash 8 crew did not stop at the holding point, instead continuing onto the active runway before the A350 had landed.

The inquiry has yet to explain the pilots’ failure to stop. It says, however, that a number of factors could have led the crew to perceive that they had clearance to enter the runway.

Stop-bar lights for the C5 intersection were not in operation, the inquiry says, because they were undergoing renovation work. It adds that the crew was in a hurry owing to the delayed departure and, given its humanitarian purpose, thought the Dash 8 had been allocated a priority take-off slot.

The inquiry also notes that the pilots switched to the east tower frequency only after the A350 had been given landing clearance, and were therefore “unaware” of its presence.

Just 13s after the Dash 8 passed the holding point, the aircraft’s radio operator was contacted regarding the earlier query about sourcing ground equipment at Komatsu, and sought the captain’s judgement on the matter.

While the pilots stated that they would respond later, the exchange took place at a critical point, as the aircraft lined up on the runway about 560m from the approach end of 34R.

Some 30s after the conversation, the A350 touched down on 34R and almost immediately struck the Dash 8 from behind.

The A350’s nose had not derotated – the jet was still pitched 3.5° nose-up – and it hit the upper part of the Dash 8’s vertical fin, destroying the turboprop which was instantly set ablaze. The A350’s nose-gear assembly fractured, but although the wheels broke away the strut remained intact and prevented the aircraft’s nose from contacting the runway.

Impact with the Dash 8 occurred just as the A350 crew had selected reverse thrust. The turboprop’s wings cut into the A350’s Rolls-Royce Trent XWB engines, causing them to ingest fuel. Data from the right-hand engine was immediately lost while the left engine’s fan speed dropped sharply, and the reverse-thrust command had no effect on braking.

Flight-control damage meant the A350’s rudder did not respond, and the loss of the nose-wheels also left the pilots without steering control. The twinjet veered to the right of the runway and struck precision-approach lighting before coming to rest about 1,420m from the collision point, where it was consumed by fire.

JTSB is still assessing several aspects of the accident, including the tower controllers’ lack of awareness that the Dash 8 had entered the runway.

It states that the east tower had five departing and arriving aircraft directly under its control while others, including aircraft taxiing for runway 05, were being monitored. The Dash 8 crew also correctly read back the instruction to taxi to the C5 holding point.

Seven seconds after the Dash 8 taxied past the holding point, a runway occupancy monitor activated an alert, which continued until just after the collision.

But the east tower controllers “did not recognise” the visual warning signs, says the inquiry, adding that there appears to be a lack of information and training regarding use of the system.