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Absence of runway lights preceded Q400's taxiway landing

US investigators probing the inadvertent landing on a taxiway by a Horizon Air Bombardier Q400 have disclosed that no NOTAM information had been issued warning of inoperative runway lights at the time.

The aircraft landed in darkness on a taxiway parallel to runway 06 at Pullman regional airport, Washington, on 29 December 2017.

Testing of the runway lighting system carried out at 16:00 local time, nearly 3h before the landing, had indicated that all lights were operational, says the National Transportation Safety Board in a factual update.

But it states that heavy rain and snow on the previous day had flooded electrical wiring vaults linked to the lighting system – a situation which was not discovered by airport operations personnel until after the incident.

When the Q400 departed Seattle at 17:48 local time there was no NOTAM showing the runway lights being out of service.

Cockpit-voice recordings were retrieved but found to be virtually unintelligible owing to broadband noise on one channel which masked almost all other sounds.

But the captain testified that the aircraft had been flying in cloud when he attempted to use remote control to intensify the airport lighting. This technique involves repeatedly keying the microphone on the common traffic advisory frequency.

When the aircraft exited cloud, the crew fixated on the illuminated taxiway which lay 200ft to the left of runway 06.

Further attempts were made by the captain to brighten the lights remotely.

"The captain stated he clearly felt he had the runway in sight and the runway lights were on," says the inquiry. "It was just a question of the lights not going up to bright – not to an 'on' setting."

It adds that the captain, after checking the landing-gear had deployed, looked out at the airport and saw "nothing but black" except "identifiable pavement that was lit up", and lined up the Q400 with this pavement.

Only after the aircraft had entered the flare did the first officer notice blue taxiway lighting. "Right before touchdown he saw that they were on the taxiway," says a summary of the first officer's testimony. "He and the captain were both surprised."

The aircraft landed at 18:40.

NOTAMs were subsequently published, 43min later, stating that the lighting was "out of service" after the flooding in the electrical wiring vaults was discovered.

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