What happened
In December 2010, two separate serious incidents involving cabin depressurization occurred in Russian airspace involving Finnair-operated Airbus A330-302 aircraft.
On 11 December 2010, the aircraft registered OH-LTO, operating a scheduled flight from Osaka, Japan, to Helsinki, experienced a loss of cabin pressure. The aircraft was operated with a crew of three and no passengers. Approximately five minutes after the loss of pressure, and two minutes after an excessive cabin altitude master warning, the crew initiated an emergency descent. Due to higher than expected fuel consumption, the aircraft diverted to Kuopio.
On 2 and 22 December 2010, the aircraft registered OH-LTS, operating a chartered flight from Krabi, Thailand, to Helsinki, experienced a similar failure south of Moscow. This flight carried 286 passengers and 15 crew members. The crew initiated a rapid descent from cruise level approximately two minutes after the loss of pressure. An excessive cabin altitude warning occurred during the descent, and the crew continued with an emergency descent, eventually arriving at the original destination, Helsinki.
In both instances, the flight crews utilized emergency oxygen systems. In the case of OH-LTO, the passenger cabin oxygen masks also deployed automatically.
The investigation
The investigation was conducted in collaboration with the aircraft manufacturer, the pressure transducer manufacturer, and the operator. The inquiry examined the causes of the dual engine bleed air system failures and the resulting loss of cabin pressure. Investigators also analyzed the impact of extremely cold air masses, using data from the Finnish Meteorological Institute, and evaluated crew actions and the adequacy of cockpit voice recorder (CVR) recording durations.
Findings
The investigation established that both incidents were caused by the malfunctioning of the pressure transducers within the engine bleed regulated pressure systems. Specifically, water that had accumulated in the extremely confined pressure cell rooms of the transducers froze.
This ice formation caused the transducers to provide erroneous pressure readings to the bleed monitoring computers. Consequently, the computers automatically closed the bleed air systems for both engines, leading to the loss of cabin pressurization. The freezing was facilitated by prolonged exposure to extremely cold air masses during flight, and the relatively rapid increase in ambient temperatures during the routes may have also contributed to the system failures.