What happened
On 12 November 1999, a BAe 146-200 with registration SE-DRE was operating a scheduled flight between Stockholm and Malmö. The incident was the culmination of a series of unusual events occurring across three separate flight legs that day. During the first two flights, cabin crew members reported various symptoms, including feelings of fainting, nasal itching, ear pain, and a sensation of "moon walking."
On the third flight, while cruising at FL 280, the symptoms became more severe. The cabin crew experienced intensified physical discomfort, and the commander began to feel dizzy. During the descent through FL 150, the co-pilot suddenly became nauseated and donned an oxygen mask. Shortly thereafter, the commander also became highly nauseated and was forced to use an oxygen mask. The commander experienced significant impairment, including difficulty with motor responses, focus, and coordination, necessitating that he hand over control of the aircraft to the co-pilot. The aircraft eventually completed a normal landing at Malmö/Sturup airport.
The investigation
The Swedish Accident Investigation Authority (SHK) conducted an extensive technical examination of the aircraft and its engines. Investigators performed engine tests in a test cell and conducted flight tests, taking air samples from the air-conditioning and bleed air systems. While a minor external oil leak was identified on engine #2, the chemical analysis of the air samples failed to identify any specific substances responsible for the crew's symptoms. No definitive technical fault in the air-conditioning or pressurization systems was discovered.
Findings
- The pilots were fully qualified and the aircraft held a valid certificate of airworthiness.
- The incident was caused by the pilots becoming temporarily affected by probably polluted cabin air.
- The design of the engine type is a contributing factor, as the location of the customer bleed port for the air-conditioning system is not optimal.
- There is a significant lack of scientific knowledge regarding how modern lubrication oils behave under extreme pressure and temperature, and how their by-products affect human health.
- There was a lack of clear instructions for flight crews on how to respond when cabin air contamination is suspected.
Safety action
The SHK recommended that civil aviation authorities work to:
- Update emergency checklists and training to mandate the immediate use of oxygen masks at 100% when air pollution is suspected.
- Develop standardized post-landing procedures for aircraft and crews following air contamination incidents.
- Establish an international database to track and analyze flights involving suspected cabin air pollution.
- Initiate research into the health impacts of modern lubricating oils under high-temperature and high-pressure conditions.