What happened
On 24 July 2015, a scheduled British Airways flight from London Heathrow to Zurich, operating an Airbus A319-131 with registration G-EUPJ, encountered a strong, unpleasant odor in the cabin and cockpit while in a holding pattern near Basel. The crew and cabin crew noticed a smell described as evaporated oil or "old socks." As the flight progressed, the odor intensified, and visible fine smoke appeared in the cabin.
In response to the deteriorating air quality, the flight crew declared an urgency condition (PAN PAN). To protect themselves, both the captain and the first officer donned oxygen masks. The crew performed the necessary smoke and fumes checklists, which included cutting power to the galleys and managing the ventilation. The aircraft completed a direct approach to Zurich and landed at 20:23 UTC, where it was met by the fire service. Following the landing, the crew opened the cockpit windows to ventilate the area, and passengers were able to disembark safely.
The investigation
Investigators examined the aircraft's engines and the auxiliary power unit (APU) to identify the source of the contamination. While borescope inspections of the engines showed no anomalies, a detailed inspection of the APU bay revealed an oil leak originating from the APU generator drain plug. This leak was dripping onto the APU air inlet duct.
Technicians determined that the oil was being drawn into the APU air inlet during ground operations, likely aided by a tailwind. This contaminated oil was then distributed throughout the aircraft's ventilation system via the air conditioning packs when the APU was providing bleed air. The investigation also noted that a previous attempt to rectify the issue at London Heathrow—where technicians flushed the packs using engine bleed air—had failed to address the underlying leak, leading to the secondary contamination event during the flight.