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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.

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Safety issues

AO-2009-004-SI-01 - AO-2009-004-SI-02 - AO-2009-004-SI-03 - AO-2009-004-SI-04 -  

Aircraft design permitted avionics malfunction via moisture

The design and relative positioning of the external air vent and avionics modules permitted the ingress of moisture and particulates that led to corrosion and contamination of electronic avionics components and consequently the generation of multiple erroneous crew alerting system (CAS) messages due to electrical shorting.

Safety issue details
Issue number:AO-2009-004-SI-01
Who it affects:Operators of ‘long nose configuration’ AW139 helicopters
Status:Adequately addressed


Lack of formalised monitoring role

The lack of formalised procedures in place requiring the Air Crew Officer (ACO) to monitor key instrument indications probably contributed to the undetected altitude loss.

Safety issue details
Issue number:AO-2009-004-SI-02
Who it affects:Helicopter operator
Status:Adequately addressed


Lack of deviation alert on ATC system

The absence of an altitude deviation alert within the Australian Defence Air Traffic System increases the risk of undetected altitude variation and contributed to the significant loss of altitude.

Safety issue details
Issue number:AO-2009-004-SI-03
Who it affects:Operations receiving Air Traffic Services via the Australian Defence Air Traffic System
Status:Adequately addressed


Electrical interuption due to water ingress

Interruption of electrical power to the multi purpose flight recorder due to water ingress removed of an important source of information used to identify safety issues.

Safety issue details
Issue number:AO-2009-004-SI-04
Who it affects:Operators of ‘long nose configuration’ AW139 helicopters
Status:Adequately addressed

General details
Date: 02 February 2009 Investigation status: Completed 
Time: 1030 EST  
Location   (show map):89 km south-east of Townsville Airport Investigation type: Occurrence Investigation 
State: Queensland Occurrence type: Warning devices 
Release date: 05 October 2010 Occurrence class: Operational 
Report status: Final Occurrence category: Serious Incident 
 Highest injury level: None 
Aircraft details
Aircraft manufacturer: Agusta, S.p.A, Construzioni Aeronautiche 
Aircraft model: AW139 
Aircraft registration: VH-ESH 
Serial number: 31083 
Type of operation: Aerial Work 
Damage to aircraft: Nil 
Departure point:Mackay Qld
Destination:Townsville Qld
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Last update 14 November 2018