An Air New Zealand plane carrying 36 people sank about 2500ft below its planned flight path on a night approach to Timaru after the pilots became fixated on spotting the runway lights and left an incorrect autopilot mode on.
The Transport Accident Investigation Commission (TAIC) today released its final report into the June 13, 2023 incident. The Q300 turboprop, flight NZ8199 from Wellington, dropped to 1156ft in an area where the minimum safe altitude was 2000ft.
The aircraft had 33 passengers and three crew on board when it landed safely at the unattended South Canterbury airport with no injuries or damage reported.
The report found the captain switched the autopilot into vertical speed mode during a routine altimeter change at around 15,000 feet. The captain then forgot to switch back to the mode which follows the programmed approach.
In vertical speed mode, the plane holds a set rate of descent and ignores altitude limits. It kept descending at 1400 feet per minute, about double the usual rate, for around 10 minutes but neither pilot noticed because both were looking out the windscreen for the airport.
The first officer told the captain they could see the airport beacon flashing red. The beacon is white. The captain assumed the first officer had mistaken it for red sprinkler lights, the report said.
The captain told investigators they were "comfortable" the first officer could see the runway, but "uncomfortable that I couldn't".
At one point the first officer could see the approach guidance lights showing four red lights, which means the aircraft is well below its path, though they did not pass that on.
About 6.4km short of the approach's starting point, the captain checked the navigation display, realised the plane was in the wrong mode, levelled off and climbed back to 2000ft.
TAIC said the cockpit ground proximity warning would have sounded if the captain had not acted when they did.
The crew continued to land rather than abort the approach. TAIC said a go-around would have been the safest course of action.
'Could've been foreseen'
TAIC identified six earlier incidents in which Air NZ Q300s descended below their intended flight paths. These included a July 2022 approach into Rotorua that set off both an air traffic control low-altitude alarm and a cockpit terrain warning.
"The incident that occurred at Timaru could have been foreseen," the report said.
Two more incidents followed after the inquiry opened. In July 2023 a Q300 hit windshear on approach to Nelson and came within 45ft of the ground, about 280m short of the runway. The crews in both of those incidents also continued to land.
The report traced part of the problem to the plane's flight management system. When Air Nelson fitted it, the system received only one of the two altitude signals it needed to handle altimeter changes smoothly. As a result, pilots used vertical speed mode as a workaround and had to remember to switch back.
TAIC also found the Q300 fleet's safety culture was still out of step with the rest of Air NZ four years after Air Nelson and Mount Cook Airline joined the national carrier in 2019. It found the CAA's relationship with the airline had been "too close in nature".
The commission has recommended Air NZ keep working with manufacturer De Havilland Canada (DHC) on a technical fix.
In a September 16 response published in the report, Air New Zealand accepted the recommendation. The airline said it had already approached DHC, Universal Avionics, Honeywell and other Dash 8 operators.
"An engineering solution that removes the need to rely on a procedural workaround through transition would provide a stronger and more robust control," Air New Zealand said.
"The available evidence continues to indicate that no such technical solution is currently available for the Q300 configuration operated by Air New Zealand."
In March, DHC told the Transportation Safety Board of Canada the incident was "primarily influenced by flight crew operational deviations".
Two days after the incident, Air NZ changed its altitude alert rules. It has since required Q300 pilots to call out autopilot mode changes aloud and ran a "cultural reset" programme for the fleet from September 2023.
Air New Zealand chief safety and risk officer Nathan McGraw said the airline began its own investigation in June 2023 and strengthened procedures, training, monitoring and safety culture " well before the TAIC inquiry was completed".
"TAIC has recognised that work, and the final report makes one recommendation to Air New Zealand, which we've accepted and progressed," he said.
"Independent investigations, regular audits, and being open about what we can do better are all essential parts of a safe aviation system. We’re very confident in the training, systems and safety culture we have in place today.”
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