Dash 8 crew stayed in vertical-speed mode on night approach to Timaru, inquiry finds

An Air New Zealand De Havilland Dash 8-300 descended well below the glideslope on a night approach to Timaru. The crew had left vertical navigation mode earlier in the descent and never re-selected it, New Zealand's Transport Accident Investigation Commission found.

The turboprop was arriving from Wellington on 13 June 2023, flying a required navigation procedure to runway 20. It had joined the vertical navigation descent path from 18,000ft. Passing 15,000ft, the captain ran transition checks, reset the altimeter and moved the aircraft into vertical-speed mode, a technique the operator used to prevent sharp pitch changes when pressure settings are altered.

The crew did not switch back to vertical navigation afterwards. The commission said the standard operating procedures contained no specific requirement to confirm the correct descent mode had been restored. The captain could not give a reason for not re-engaging the mode.

Data showed the aircraft began departing from the planned descent profile at 8,000ft, about 27nm from the airport. Investigators believe the crew became focused on outside visual references. The captain, who knew the South Island night environment, pointed out the Timaru city lights to the first officer. At about 7,000ft the captain pulled back thrust to slow to 160kt.

Using vertical-speed mode during the transition had already been flagged as problematic by the operator, the commission noted, and a safety bulletin stressing mode awareness had gone to Dash 8-300 pilots three months before the event. Reminders to follow procedures rarely change behaviour, it added.