Airbus has launched a formal program to study onboard fume incidents and is developing an upgrade for the Airbus A320 environmental control system. A UK Air Accidents Investigation Branch report told of an in-service incident that left a British Airways Airbus A320ceo first officer incapacitated following a nerve-jangling approach and landing at London Heathrow Airport.

The September 2019 incident flight originated on a rainy morning in Zurich. The crew noticed a slight odor shortly after departure, but it soon went away, and the crew opted to continue to London, an Air Accidents Investigation Branch (AAIB) final report said. As the crew passed through 4,000 ft. on its approach into London, both pilots reported a sudden, strong fume odor, the report said. Both pilots donned their oxygen masks, and the captain made a “Pan-Pan” international urgency call to Heathrow tower controllers, who told two aircraft ahead of the British Airways A320 and one trailing it to go around to give the stricken aircraft priority and a margin for error.

The A320 landed and moved to a nearby taxiway, where it was met by emergency services. The co-pilot removed his mask to see if the fumes were still present. He stated that they were, and immediately began to feel nauseated. He then went to the lavatory while the captain requested air stairs to expedite getting medical personnel onboard.

Investigators learned that the odor was limited to the cockpit—one of three separate environmental control system (ECS) climate-control zones on the A320. Nothing unusual was detected in the forward or aft cabin areas, the other two zones.

Investigators could not determine what caused the fumes. Prompted in part by British Airways’ report of hundreds of fume or odor events in their A320 fleet over the previous year, the AAIB took a deeper look at the issue, and at five incidents in particular that happened in late 2019, including one nonrevenue flight. All involved Airbus A320-family aircraft.

The AAIB did not find definitive causes for any of the incidents. It did, however, observe some common factors. Among them: The incidents usually involved aircraft that stayed overnight in damp or rainy weather and started with a faint smell shortly after departure. In each case, conditions worsened rapidly during descent, and odors were described using similar terms, including “sweaty socks, manure and farmyard smells, which were unpleasant and distinctive,” the AAIB said.

In most cases, the cabin crew did not report any issues. On the few flights where cabin occupants detected fumes, they were most prevalent in the galleys.

Analysis of flight data recorder data found no correlation between engine or systems settings and the presence of fumes. The auxiliary power unit is not considered a likely culprit, as it was often not in use during the events. Off-aircraft factors, such as application of cleaning or anti-ice fluids before departure, were ruled out. The most effective mitigation strategies included using “the smoke and fumes abnormal and emergency procedures,” the AAIB said.

While few definitive conclusions can be drawn, the AAIB has some theories. “The evidence indicates that it is likely that these fumes are derivatives of contaminants entering the ECS,” the agency said. “It may not be a single compound but a combination of compounds which react and then become airborne in the bleed air supplies passing through the ECS. The fumes may have similar traits to hydrocarbon compounds combined with water vapor in low concentration which are liberated as water vapor condenses when it enters cooler conditions, for example as it passes into the flight deck or cabin via ducts.”

Beyond analyzing data and comparing witness observations, operators, investigators and Airbus have largely been frustrated in their efforts to understand the problem. No sample of the fumes has been captured, and nobody has been able to recreate the scenario on the ground.

“The unpredictable nature of the events has also meant that it has not been possible to construct an experimental flight-test schedule to capture more data,” the AAIB said. “This has left the manufacturer reliant on reported data, making the issue difficult to resolve in practical terms.”

Airbus plans to use Project Fresh to centralize analysis of related incidents. It also plans to roll out “an enhanced ECS filtration system” that will include a retrofit option, and it has published an “information paper” that provides “all the known aspects of fumes and smoke events,” the AAIB said.

British Airways has put a post-fumes maintenance procedure in place and will consider installing the new Airbus ECS system when it is available.