Autopilot set too low for Queenstown descent
An incorrect autopilot mode led to a Jetstar plane flying too low during its approach over mountainous terrain to Queenstown.
The Australian Transport Safety Bureau found the pilots of the Airbus A320 twice descended below minimum safe altitudes by 1000 feet (304 metres) and airline procedures did not draw their attention to automatic system modes.
Settings for the autopilot were incorrect for part of the descent into Queenstown.
Flying below safe altitude limits did not endanger the aircraft but all operators of highly automated aircraft should take note, the bureau's 22-page report says.
Jetstar has added guidance material to its flight training manual and included a warning about flying into Queenstown.
"The bureau stresses the importance of continually monitoring descent profiles and an aircraft's proximity to segment-minimum safe altitudes, irrespective of any expectation that descent is being managed by the auto-flight system.
"For flight crew this occurrence illustrates . . . the potentially serious effects of pilot distraction."
At 8.15am the crew started descent procedures into Queenstown from 36,000 feet using the auto-flight system in "open descent" mode, which disregards programmed altitude limits.
At 8.21am the first officer was alerted by "going too fast". The captain checked the altimeter and started a climb to a minimum safe altitude.
The captain later told his employer they had intended to switch from open descent to "managed decent" mode, which follows programmed altitude constraints, but this did not happen.
Descent continued for seven minutes between the point when the captain had intended to switch modes and the point when the first officer alerted the captain, who then corrected the manoeuvre.
In open descent mode an aircraft automatically descends to a selected altitude without regard to procedures for a particular approach, such as terrain or minimum altitudes for descent.
The aircraft was below limits for two minutes but crashing into the mountains was highly unlikely, given clear conditions and other risk controls, the report says.
Investigators said the incident highlighted important information about automatic flight mode selection.
Jetstar had discussed mandating the use of the managed descent mode during its regulatory approval for Queenstown operations but elected instead to use "strongly recommended" in its flight procedures.
Bureau investigators said there was no specific Jetstar procedure to guard against descending in an unintended mode, although using managed descent was emphasised in the flight manuals.
This highlighted the need for "deliberate and continuous" monitoring of autopilot modes.
Jetstar's proactive response to the safety issue included embedding "Airbus golden rules" in its flight operations, expanding its training, adding guidance on autopilot mode awareness for flight crews and a specific warning about Queenstown.
Both pilots had flown the Queenstown route previously and continue to fly with Jetstar. Pilots need a special rating for Queenstown because of its terrain.
- The Southland Times
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