On 22 March 2007 when limbing out of Stockholm Sweden, the crew of a Malmö Aviation Avro RJ100 failed to notice that the aircraft was not pressurised until cabin crew advised them of automatic cabin oxygen mask deployment.
This is the Summary from the report published by the Swedish Accident Investigation Board:
"The events consist of two independent incidents, where the second incident was a consequence of the first. The events have therefore been described here as the first incident and the second incident respectively.
The first incident
The aircraft taxied out at Stockholm/Bromma airport for a scheduled flight to Gothenburg/Landvetter Airport. Due to the changing weather conditions, the switches for the aircraft’s de-icing system and air conditioning system (which among other things pressurises the cabin) were switched on and off at various points during the take-off and climb out. The climb checklist did not contain any specific item for checking the air conditioning unit (“pack”) switches, which control the pressurisation of the cabin, only a summary item in respect of the air conditioning system in general. At about 10000 ft the “Avionics fan off” warning light lit in the cockpit. The pilots began to read the emergency checklist for this warning, which may among other things be initiated by low air pressure, but there were no instructions for checks or measures to be taken related to this warning. At about 18000 ft one of the cabin crew called and said that the oxygen masks above the passenger seats in the cabin had dropped down. The pilots discovered that the aircraft cabin was not pressurised and immediately began to descend to a safe altitude. The aircraft had reached an altitude of 19000 ft before the descent began. During the descent, the warning light for high cabin altitude came on, that according to the specifications should have warned the pilots when the cabin altitude exceeded 10 000 feet. On investigation it was found that the relevant pressure sensor was damaged. The reason why the aircraft climbed to about 19 000 feet without the cabin being pressurised was that the checklist was not defined clearly enough. A contributory factor was that the inspection interval for the cabin low pressure sensor was probably too long.
The second incident
When the oxygen masks dropped from above the passenger seats, the cabin crew could see that a large number of masks on the left side had not dropped. After checking the status of the pilots the chief cabin attendant went along the cabin without oxygen and started to move passengers from the left side to the right side of the aircraft. Shortly afterwards the chief cabin attendant was given a portable oxygen bottle by a colleague and together they tried to open more hatches with oxygen masks for the passengers. A small tool that is meant for manually opening the hatches could not be found during the incident. The oxygen pressure at an altitude of 19 000 feet is only about half the pressure at sea level, and results in an equivalent reduction of the oxygen level in the blood. Human reaction to this depends on the individual, but even at low altitudes the effects of oxygen deprivation can become apparent in the form of a lowering of both physical and mental capacity. The reason why 20 of the oxygen mask hatches did not open was that the company’s quality control was deficient in connection with the repacking of the hatches.
The Swedish Civil Aviation Authority is recommended to:
- Ensure that, within applicable areas of civil commercial air transport, checks on the status of the pilots and the institution of cabin-cockpit communication are introduced as obligatory items in the cabin staff emergency checklists in the case of an unannounced decrease of cabin pressure (RL 2008:01e R1).
It is recommended that EASA:
- Takes steps to ensure that the inspection interval for cabin pressure sensors in this particular type of aircraft is reduced (RL 2008:01e R2).
- Takes steps to ensure that the emergency checklist in this particular type of aircraft is complemented with a note in the respect of checking cabin pressure when the “Avionics Fan Off” warning is activated while airborne (RL 2008:01e R3)..."
For further information see the full Report published by the Swedish Accident Investigation Board (SHK).