A Skippers Aviation de Havilland Dash 8-100, registration VH-XFP performing flight JW-1921 from Perth,WA to Monkey Mia,WA (Australia) with 30 passengers and 3 crew, was climbing through 10,000 feet out of Perth when the crew checked the cabin pressure readings, found them normal and continued the climb to cruise FL180. While levelling off at FL180 the crew received a master warning and a cabin pressure warning, observed the cabin altitude had increased to 12,600 feet with a rate of climb in excess of 4000 fpm. The crew worked the related memory checklist items, donned their oxygen masks and initiated an emergency descent to FL100. During the descent the captain, pilot monitoring so far, took control of the aircraft and made an announcemnet to the cabin, which was muffled however and could not be understood by the flight attendant, who therefore did not don her oxygen mask remaining unaware of the loss of cabin pressure. She noticed however that the aircraft was rapidly descending. The aircraft did not feature passenger oxygen masks and was not legally required to carry such oxygen masks. After levelling off at FL100 the crew worked the cabin pressure checklists but was unable to regain control of the cabin pressure and decided to return to Perth. The aircraft entered a hold to burn off fuel to reduce weight to maximum landing weight and landed safely back in Perth. The ATSB released their final report releasing following findings: - The aircraft’s cabin pressurisation system failed resulting in the aircraft depressurising. - The PA to the cabin was muffled as the crew were using oxygen masks and potentially critical information was not communicated to the flight attendant and the passengers. - The flight crew fitted their oxygen masks immediately, when they noticed, at about 12,600 ft cabin altitude that the cabin pressure warning system had activated. The ATSB analysed: "The forward outflow valve was not operating as required which led to a rapid depressurisation of the aircraft as it approached its cruising altitude. The PA announcement from the captain was muffled, resulting in the cabin crewmember not initially realising that the flight crew were conducting an emergency descent and that they should use supplemental oxygen." The ATBS released following safety message in particular also dealing with hypoxia: The reaction time for pilots to fit oxygen masks is of critical importance when there is a cabin pressurization failure. For the crew in this occurrence, it was the first action taken when they detected that the cabin pressure warning light was on. A misconception is that it is easy to recognize the symptoms of hypoxia and take corrective action before becoming seriously impaired. The signs and symptoms vary depending on the individual, the altitude and the extent of the exposure. While other significant effects of hypoxia usually do not occur in a healthy person in an unpressurized aircraft below 12,000 ft above mean sea level (AMSL), there is no assurance that this will always be the case. Furthermore, the altitude range of impairment due to hypoxia is best described as a continuum; there is no definitive altitude at which the effects of hypoxia begin or end. To mitigate the risk associated with these variations, if hypoxia is suspected, a descent to altitudes below 10,000 ft AMSL is suggested.