What happened

On 16 July 2012 at about 0830 New Zealand Standard Time, an Airbus A320-232 aircraft, registered VH-VQA and operated by Jetstar Airways, was conducting an Area Navigation (Required Navigation Performance) approach to runway 05 at Queenstown, New Zealand. During the approach the aircraft descended below two segment minimum safe altitudes. Upon recognising the descent profile error, the crew climbed the aircraft to intercept the correct profile and continued the approach to land.

What the ATSB found

The ATSB found that, contrary to their intentions, the crew continued descent with the auto-flight system in open descent mode, which did not provide protection against infringing the instrument approach procedure’s segment minimum safe altitudes. The ATSB also found that the crew were not strictly adhering to the operator’s sterile flight deck procedures, which probably allowed the crew to become distracted.

The ATSB found that the operator’s procedures did not specifically draw the crew’s attention to unchanged auto-flight system modes during descent or prompt crew reconsideration of the most suitable descent mode at any point during descent. Additionally, the operator’s procedures allowed the crew to select the altitude to which they were cleared by air traffic control on the Flight Control Unit altitude selector, irrespective of intervening altitude constraints. This combination of procedures provided limited protection against descent through segment minimum safe altitudes.

What's been done as a result

Following this occurrence, the operator included additional guidance material in its Flight Crew Training Manual regarding mode awareness. It also included a warning on its Queenstown approach charts to state that managed descent was required beyond the initial approach fix.

Safety message

The ATSB reminds operators and flight crew of the importance of continuous attention to active and armed auto-flight system modes. Equally, the ATSB 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 appropriately managed by the auto-flight system. For flight crew, this occurrence illustrates once again the fallibility of prospective memory and the potentially serious effects of pilot distraction. For operators, it highlights the importance to safe operations of robust management procedures for auto-flight systems.