Contaminated Air Onboard
Due to various causes, various harmful substances can enter the cabin through the air conditioning system. This occurs almost continuously, even in new and well-maintained aircraft. However, the concentration of this contamination is usually too low to be directly noticeable (no smell, not visible). Most people do not immediately experience any symptoms. Complaints after the flight (such as throat discomfort, coughing, a flu-like feeling) are often incorrectly not attributed to contaminated air onboard. Nevertheless, long-term or repeated exposure, known as “Long Term Low Level Exposure,” can indeed lead to more serious health issues. This depends on each individual’s immune system, but it applies to both crews and frequent flyers.
Under certain circumstances, such as a technical malfunction like a worn-out or damaged oil seal in the engine, the concentration of harmful substances in the cabin can suddenly increase significantly. This is referred to as a Fume Event. Many airlines prefer to label this as an “Odor Incident,” as it sounds less severe. However, it is indeed a serious incident because the high concentration of often (very) toxic substances that occur during a Fume Event can lead to acute poisoning symptoms. A Fume Event may be accompanied by visible smoke or mist and/or an unpleasant odor (“dirty sock smell,” “wet dog”). However, this is not always the case, as some of the toxic substances that can be released are odorless and invisible (e.g., carbon monoxide).
Can a Fume Event happen at any time?
Fume events can indeed occur at any time, for example, if an oil seal in an engine suddenly fails while in flight. Fortunately, this doesn’t happen often. However, the likelihood of a Fume Event is significantly higher when engine power is increased or reduced, known as “power transients.” This includes scenarios like engine start-up, take-off, climbing, descent, and landing. This is because, during these moments, the airflow through the engine is not stable.
The oil seals used are of the labyrinth type and always allow a small amount of oil to pass through. The engines are designed so that the air pressure on the outside of the seal is generally higher than the oil pressure on the inside. In theory, oil that gets between the seal should never leak outward but rather return inward. This mechanism works well under stable conditions with a constant airflow but is less reliable during such “power transients.”
Incidents are still underreported
Crew Reporting of Fume Events has been discouraged by many airlines for years. The (sometimes severe) complaints about smoke and odor in the cabin are often recorded in the Technical Logbook as “Odor Incidents.” All too often, they are not even reported at all, in order to avoid significant delays, extended maintenance, and high costs.
Drastically Extended Maintenance Schedules
Until the 1990s, engine manufacturers prescribed major maintenance for engines every 5,000 flight hours. This required the engine to be removed from the aircraft and completely disassembled in the workshop. During this process, standard maintenance included the replacement of all mentioned oil seals. Naturally, this was an expensive operation!
However, newer generations of engines became increasingly reliable and less prone to malfunctions. Partly under pressure from airlines to cut costs, the time between maintenance intervals became longer. Nowadays, the condition of the engine is monitored from the home base via datalink. Major maintenance is scheduled solely based on the engine’s condition (wear and tear). In June 2012, engine manufacturer CFM (SNECMA/General Electric) proudly reported that one of its engines had been hanging under a wing for up to 50,000 hours without a single “shop visit!”
Those engines may have improved, but…
While modern jet engines have indeed become much more reliable, the oil seals used in them are still virtually the same and wear out just as quickly as before. As they remain in use for longer periods, they tend to leak more oil, especially during “power transients.” As long as the oil consumption remains within the manufacturer’s standards (up to a liter per hour!), it doesn’t immediately warrant major maintenance. However, the contamination of Bleed Air does increase, along with the likelihood of Fume Events.
…even these maintenance guidelines are often not followed!
Take the Airbus A320, for instance, one of the aircraft types where relatively many Fume Events occur. Airbus specifies in its maintenance guidelines that after every Fume Event, the entire air conditioning system should be thoroughly cleaned internally. This is an expensive procedure that takes at least 2-3 days in the hangar. Often, a Fume Event is labeled as an “Odor Incident.” Consequently, the aircraft is back in the air after just a few hours. However, this approach leads to increasingly serious contamination of the aircraft. It is increasingly involved in “Odor Incidents,” with all the consequences for crew and passengers. Lists of notorious “stinkers,” aircraft repeatedly involved in Fume Events without the required measures to prevent recurrence, are now circulating on the internet.
In recent years, we have seen a significant increase in the number of reports of Fume Events. The bad news is that these events are indeed becoming more common. The good news is that crews are becoming more aware of the dangers. There are more reports in the media, and they hear more often from colleagues who have suffered (often permanent) health damage. In the Netherlands alone, it is estimated that several hundred cockpit and cabin crew members have become occupationally disabled as a result, with some facing permanent disabilities. Worldwide, this number reaches into the thousands.
Taking More Appropriate Measures
Crews who are aware of this issue and find themselves in such a situation tend to handle it differently. There is a greater likelihood that such a crew will take immediate appropriate measures to ensure safety. This can range from using oxygen masks in the cockpit to making a precautionary or emergency landing. Until just a few years ago, this problem was often downplayed, especially by cockpit crews, with all the resulting consequences…