On 2 February 2009, an Agusta Westland AW139 helicopter, registered VH-ESH, departed under the instrument flight rules from Mackay Aerodrome for Townsville Aerodrome, Queensland. The purpose of the flight was to return the helicopter to Townsville following the passage of a tropical cyclone.

Shortly after departure from Mackay, the crew were presented with an increasing number of alert messages indicating the failure of various helicopter systems. While the crew were focussed on assessing these messages, the autopilot altitude hold function disengaged, leading to a descent that was not detected by either the flight crew or air traffic services (ATS).

The helicopter descended undetected for over 6 minutes while flying towards an area of rising terrain, losing about 3,300 ft of altitude, before ATS observed the descent and alerted the crew.

The investigation determined that the helicopter sustained two independent technical problems; one associated with water and particulate ingress and a second related to the probable susceptibility of the air data system to in-flight turbulence. These failures resulted in multiple erroneous alert messages and uncommanded disengagement of the altitude hold function. The investigation also identified a number of safety factors relating to workload and task management by the crew and monitoring of the aircraft by ATS. That included the lack of an altitude deviation alert within the Australian Defence Air Traffic System (ADATS).

Following the occurrence, the helicopter manufacturer and operator, and the Civil Aviation Safety Authority (CASA) implemented a number of safety actions relating to the identified technical and operational safety issues. In addition, the manufacturer and operator of the helicopter, and CASA undertook proactive safety action in a number of cases for which no safety issues were identified.

A replacement common ATS system for ADATS and the civilian Australian Advanced Air Traffic System (TAAATS) has been announced by the Government that could be expected to include common alert functions, such as the existing cleared level adherence monitoring alert in TAAATS.