An Air New Zealand De Havilland Dash 8-300 descended 2,500ft below its programmed vertical navigation path on a night approach to Timaru, reaching a minimum altitude of 1,156ft — about 1,022ft above ground — before the captain recovered the aircraft and landed. None of the 33 passengers and three crew members was injured.

New Zealand's Transport Accident Investigation Commission (TAIC) says the crew, operating a service from Wellington on 13 June 2023, never re-engaged VNAV mode after switching to vertical-speed mode during altimeter transition at 15,000ft. The aircraft, ZK-NEM, had been part of the Air Nelson regional operation before its integration into Air New Zealand in 2019.

What went wrong on the approach?

The crew intercepted the required VNAV descent path from cruise at 18,000ft for the RNAV approach to runway 20, via the waypoints ELDAK, BUDPA and SUNPA. During transition checks at 15,000ft, the captain changed the altimeter setting and switched to vertical-speed mode — a practised technique to avoid abrupt pitch movements during large pressure-setting adjustments.

TAIC found "no specific requirement" within the standard operating procedures to ensure the aircraft was returned to the correct descent mode. The crew did not revert to VNAV.

Analysis shows the deviation from the planned profile began at 8,000ft, about 27nm from the airport. At roughly 7,000ft the captain reduced thrust to slow to 160kt; he felt the aircraft was not slowing, queried whether there was a tailwind, and struggled to locate visual references to the airport while the first officer tried to point out the runway 20 precision approach path indicator lamps.

"The flight crew had become fixated on visual acquisition of the Timaru aerodrome lights," the inquiry states. "The preoccupation with sighting the lights led to degraded instrument scanning and a lack of appropriate flight path monitoring by both flight crew members. Because of this, they were unaware that the aeroplane was descending at a higher rate than usual."

The captain, familiar with night flying over South Island, had shown the first officer the Timaru city lights during the descent.

How close did it come?

The captain extended the landing gear at 1,500ft while crossing BUDPA, still 14nm from the runway and 6nm from SUNPA. Aircraft within 25nm of SUNPA were required to remain above 2,000ft.

Within seconds of gear extension, the captain commented that the aircraft appeared low for its distance from Timaru and requested a reference height check. Before the first officer could respond, the captain checked the position, saw the aircraft was still 3.5nm from SUNPA, realised it was below the VNAV profile, and observed it descending in vertical-speed mode at 1,400ft/min.

At 2,500ft below the programmed flightpath and 11nm from the runway, the aircraft continued down to 1,156ft before the captain levelled and climbed gradually back to 2,000ft to regain the approach profile. The flight landed without further event.

Had the risk been flagged before?

Yes — by the operator itself. The use of vertical-speed mode during transition had previously been identified by the operator as "problematic," and a safety bulletin on the importance of mode awareness was issued to Dash 8-300 pilots three months before the incident.

TAIC points out that such reminders to follow procedures are "rarely an effective method" for changing human behaviour — a finding that puts the onus back on procedure design rather than crew admonition.

"The captain could not offer any reason why they did not re-engage VNAV," the inquiry says.

The case illustrates the residual exposure in turboprop operations where SOPs permit mode changes without a mandated verification step: the omission produced no cue in the cockpit until the captain's own positional check nearly three minutes of descent later. Whether Air New Zealand will hard-wire a VNAV re-engagement check into its Dash 8-300 procedures in response to the finding remains the operational question the commission's report now leaves with the carrier.