A distorted public address message on a Skippers Aviation Bombardier Dash 8-100 meant that cabin crew were not able to respond quickly to a depressurisation incident that occurred off the coast of Western Australia on 7 August.
The Australian Transport Safety Bureau says that the incident occurred to the twin turboprop (VH-XFP) shortly after taking off from Perth on a scheduled flight to Shark Bay. There were 30 passengers and three crew on board.
After departing normally and ascending to 17,600ft, the flight crew engaged the autopilot with instructions to climb to 18,000ft. As it approached that altitude, the master warning activated and the cabin pressure warning light illuminated, and the crew observed that the cabin pressure was rising at a fast rate.
The two pilots donned oxygen masks, ran through their checklists, and commenced an emergency descent. The captain took control of the aircraft and made a PA announcement.
“However, the announcement was muffled and distorted and the cabin crewmember did not understand what was being said. The cabin crewmember tried contacting the flight crew but did not get a response,” the ATSB says in its report.
Shortly after, the crewmember realised that the aircraft was in descent, took their seat and instructed the passengers to do the same. They did not use supplemental oxygen, as they were unaware of the nature of the emergency.
The aircraft was not required to be fitted with drop-down oxygen masks, and the cabin crewmember did not give portable oxygen bottles to any of the passengers.
Levelling off at 10,000 feet, the pressure normalised and the aircraft completed some orbits off the coast to burn fuel before returning to Perth.
The depressurisation was traced to a faulty forward outward flow valve which was replaced. Skippers Aviation also tested the pilots’ oxygen mask audio system and found that it was serviceable.
“The most likely reason that the announcement was not heard clearly was that the oxygen masks might have distorted the communication,” the Bureau says.
In response, Skippers advised its pilots about the potential communications issues when fitted with oxygen masks, while its continuing airworthiness system will monitor the aircraft for any associated defects.