Safety Issue

Following a number of reports of AW139 helicopters experiencing erroneous crew alerting system messages, the helicopter and avionics system manufacturers established that the design and relative positioning of the external air vent was the source of the problem. Once identified, the helicopter manufacturer issued mandatory service bulletin (SB) 139-166 that was applicable to ‘short nose configuration’ helicopters.

The bulletin provided instructions to identify the presence of corrosion on the Modular Avionics Unit (MAU) modules and also detailed modification of the MAU2 ventilation duct to ensure that the modules were not subject to the direct flow of air from the duct outlet. In addition, following notification of instances of moisture ingress following exposure to heavy rain, the helicopter manufacturer has developed a number of modifications to improve the overall water proofing of AW139 helicopters. Service bulletin 139-105 is currently being prepared to provide the required detail to operators.

Following the occurrence, and prior to the issue of the manufacturer’s SB, the operator developed a modification, in accordance with an engineering order, to prevent contamination of the modular avionics system. That modification was fitted to the operator’s AW139 aircraft until the release of SB 139-166.

The Civil Aviation Safety Authority (CASA) identified that, under the current system of maintenance applicable to AW139 helicopters on the Australian register, compliance with a manufacturer’s SB is not mandatory in all circumstances. Compliance is only mandatory for aircraft on the Australian register if an Airworthiness Directive (AD), issued by the National Airworthiness Authority (NAA) responsible for the type design of the aircraft, calls for compliance with a manufacturer’s SB. At the time of writing, no such AD had been issued regarding compliance with SB 139-166. Following this occurrence, CASA contacted the relevant NAA and ascertained that the NAA intends issuing an AD that will mandate compliance with SB 139-166. CASA is presently monitoring the development of this directive and, in the event that the relevant NAA does not issue the AD, CASA will consider if the issue of an Australian AD is warranted.