Inside cabin fume events: How bleed-air contamination happens and how it’s managed
October 19, 2025
Aircraft cabin fume events occur when bleed air from engines contaminates the cabin during pressurisation and air conditioning. Engine oil, hydraulic fluids, or other hazardous chemicals leaking from the engine can enter the cabin, causing odours, smoke, or even fire.
The Federal Aviation Administration (FAA) suggests that, on average, three fume or smoke events occur each day on US aircraft. Despite the frequency, minor fume events are generally detectable and controllable, ensuring minimal risk to passengers.
Are engines to blame for cabin fume events?
Most cabin fume or smoke events occur as a result of the bleed air contamination. Aircraft engines ingest large amounts of air from the atmosphere before pressurising and heating it for combustion. A small portion of the compressor air, before combustion, is routed to the aircraft through what is commonly called customer bleed air.
Cabin pressurisation, air conditioning, and other aircraft systems use high-pressure, high-temperature air from the compressor. A leak in any part of the compressor can contaminate the bleed air routed to the cabin.
For example, a worn tube within the heat exchanger can allow fumes of oil to enter the cabin through the customer bleed. Similarly, excessive wear on air-oil seals within the pressurised sump may cause unwanted fluids to enter the air stream.
Moreover, a faulty component within the Auxiliary Power Unit (APU) can cause oil to leak into the air conditioning hot-air ducts, allowing continuous smoke in the cabin and the cockpit.
Handling of fume events on board
Cabin air systems on modern commercial airliners use a mix of engine bleed air and recirculating air. While the circulating air is often filtered, most onboard systems do not filter specific fumes resulting from external contamination.
Chemical analyses of contaminated bleed air have detected trace amounts of organophosphates and other toxic compounds from synthetic engine oils, including tricresyl phosphate (TCP), a known neurotoxin in higher doses.
Not all fume events are the same, and their impact on passengers varies based on the severity and toxicity of the contaminant. Fumes in the cabin can cause headache, dizziness, throat aches, or major respiratory illnesses. Passengers often describe the smell as oily, metallic, or like wet dog or smelly socks.
With varying extents of events onboard, pilots and flight attendants must recognise a fume event and take steps to mitigate safety risks. The International Civil Aviation Organization (ICAO) provides guidelines for fume identification and reporting during such events.
.@British_Airways terrifying experience on flight to Valencia. Felt like horror film. Thankfully everyone safe. Flight filled with smoke and had to be emergency evacuated. #britishairways pic.twitter.com/NT4Gtme9kl
— Professor Lucy Brown (@lucyaabrown) August 5, 2019
Flight attendants pay close attention to passenger behaviours, recognising symptoms resulting from usual events. They actively handle all aspects of fume events while ensuring passenger safety. Flight attendants use personal protective equipment, including pressurised oxygen masks, to identify the source of the fumes and attempt to mitigate it. In severe fume events, the aircraft descends below 3,000 meters (10,000 ft), where the cabin can be safely depressurised.
Although debate continues over the long-term health effects of exposure, some severe fume events have caused short-term neurological symptoms in crew, consistent with exposure to neurotoxic compounds such as organophosphates.
In-flight fume events can compromise flight safety
Cabin fumes can also enter the cockpit, limiting pilots’ ability to aviate, navigate, and communicate. Smoke events in particular can reduce visibility to cockpit systems and compromise flight safety.
Aviation regulatory authorities such as EASA and FAA require OEMs to establish and test passenger compartments that are free from hazardous concentrations of fumes, smoke, or other toxic vapours.
Aviation authorities also recommend the use of supplemental equipment and technologies to minimise safety hazards.
Technology can ensure pilot visibility in a smoke or fume event
Pilots use various specialised equipment to retain visibility during severe fumes and smoke events in-flight. VisionSafe has developed the Emergency Vision Assurance System (EVAS) that creates a clear space of air for pilots during such events.
Highlighting the importance of such innovations, Global Air states that the self-contained EVAS system can inflate a smoke-free viewing chamber within 30 seconds, offering pilots an unobstructed view of cockpit instruments as well as forward visibility through the windshield.
According to VisionSafe, “When smoke evacuation procedures are not sufficient, EVAS® provides emergency backup allowing the pilot to see and fly the aircraft to a safe landing. Statistics from FAA Service Difficulty Reports clearly show that in-flight fires, smoke, or fumes are some of the most significant causes of unscheduled or emergency landings.“
The self-powered system deploys an Inflatable Vision Unit (IVU) to displace dense, continuous smoke and offer clear vision through to the instruments and the windshield.
How designs are evolving to lower the risk of fume events
Unlike most modern jet engines, the GE Aerospace GEnx engine powering the Boeing 787 Dreamliner features bleedless technology. The type eliminates the use of engine bleed air for cabin and other aircraft systems.
Instead, the GEnx engines power generators that drive electric compressors to provide cabin pressurisation and air conditioning.
The design philosophy onboard the 787 substitutes the use of pneumatic and hydraulic power with electricity. Instead, the GEnx engines power generators that drive electric compressors to provide cabin pressurisation and air conditioning.
The system consists of two air conditioning packs. The cabin crew controls the air temperature and humidity based on the passenger count. Furthermore, to increase passenger comfort, the pressure altitude is set to approximately 6,000 feet at maximum cruise altitude, a 25% reduction compared to most other aircraft.
Four engine-mounted generators and two APUs power the electric compressors on the 787, generating sufficient energy to compress the atmospheric air. The bleedless system eliminates the risk of cabin air contamination while enabling accurate control of pressure, temperature, and humidity inside the cabin.
In addition to the safety benefits stated, bleedless architecture improves the overall engine efficiency by conserving energy. With minimum or no “robbing” of bleed air, less fuel is required to generate the necessary thrust.
Moreover, the removal of the bleed system eliminates manifolds, valves, and tubes, reducing the overall weight of the engine. That, in return, further enhances fuel efficiency and lowers operating costs.
