A Fokker 100 (Fokker F28 Mk.100) was scheduled to operate a passenger flight between the cities of Perth and Geraldton, both in Australia. A crew of five would operate the flight, two flight crew and three cabin crew. Boarding of the 57 passengers was delayed for a short while as heavy rain fell for a short period of time, resulting in a departure delay of 5 minutes.
The insulation blanket stuck in the secondary outflow valve(source: ATSB report)
At 07.17 lt (local time) the aircraft departed Perth and climbed to the cruise flight level 260 (26.000 feet using a standard QNH setting of 1013.25 (hPa / 29.92 inHg). Eight minutes after establishing cruise flight (at 07.38 lt) a Triple Chime, Master Warning lights and an "excessive cabin altitude' warning on the MFDU (Multi-Function Display Unit) alerted the crew. Indicating a problem with the cabin pressurisation. the MFDU also presented the crew with the required emergency procedure for this warning;
• fit oxygen masks
• establish flight crew communication
Within a minute the crew completed the procedure and initiated the emergency descent.
The Cabin Pressure indication panel (source: ATSB report)
As a result of the warning also the fasten seatbelt sign had illuminated in the cabin, which initially made the purser believe they were approaching an area of turbulence (as briefed during flight preparation). When the purser recognised the sounds coming from the flight deck as the flight crew using oxygen masks the other cabin crew was instructed to prepare for a possible decompression. A short while later the cockpit crew conducted a cabin announcement (PA) ‘attention cabin crew, descent, descent, descent’. When the oxygen masks did not deploy shortly after the announcement the purser informed the fligthdeck about this. The flight crew reported looking at the cabin altitude indication and noted it was increasing but had not yet reached the altitude where the masks would automatically deploy. In order to minimise communication with the cabin, during a period of high workload, they elected to manually deploy the oxygen masks. Four minutes after the start of the emergency descent the aircraft levelled off at an altitude of 9000 feet~and an announcement was made that the oxygen masks were no longer required. After a further update to the passengers, the cabin crew checked the passengers, and there were none. Due to their proximity to, and desire to avoid a return flight at a low level through showers and possible turbulence, the flight crew elected to continue to Geraldton. An uneventful landing was made at 08.04 lt. Prior to disembarking the aircraft, the PF stood at the front of the cabin, further detailed the event and offered that passengers could approach any flight, cabin or ground crew if they had any questions or concerns.
The incident aircraft (source: ATSB report)
The pressurisation problem was investigated by the airlines' maintenance department. They found an insulation blanket wedged in the secondary outflow valve of the pressurisation system. This prevented the valve from closing, causing a slow increase in cabin altitude during the flight.
The incident was investigated by the Australian Transport Safety Bureau. In their report, the cause as found by the airlines' maintenance department was confirmed. A non-secured insulation blanket caused an obstruction for the secondary outflow valve to close resulting in a pressurisation issue. An extensive Safety analysis is presented, as well as a list of contributing factors and findings. The report, used as a reference for this blog, is available by clicking on the .pdf file below;