A pilot’s perspective on the ‘toxic fume events’ controversy

Recent reports have highlighted the occurrence of "fume events" in aircraft. In the most extreme cases, pilots became disorientated and had to don emergency oxygen masks. Cabin crew and passengers reported feeling nauseous and dizzy.

Between January 2018 and December 2019, 362 fume events were reported to NASA, with 400 crew and passengers requiring post-flight medical attention, according to reporting by the LA Times. Of these flights, pilots on 73 of them used emergency oxygen.

So how real is this threat and is it something that you, as a passenger, should be worried about?

How aircraft are pressurized

The engines on a modern jet airliner provide more than just thrust to drive the aircraft forward. One of those other functions is to provide air to pressurize the cabin.

The fuselage of an aircraft is, for all intents and purposes, a sealed metal tube and for good reason. The atmospheric conditions outside at 39,000 feet are inhospitable, to say the least. Air temperatures can be as low as minus 94 degrees Fahrenheit and the oxygen concentrations are so low that a human will become unconscious in a matter of seconds.

With the fuselage largely sealed off, air inside would run out pretty quickly so it needs to be replaced on a continuous basis. However, there are other parts of the aircraft designed to draw in several tons of air every minute -- the engines.

The engines

Aircraft engines operate in a four-stage process, colloquially known as "suck, squeeze, bang, blow."

First, the air is sucked into the front of the engine before it is then compressed in the "squeeze" phase. It then enters the combustion chamber of the aircraft where it is ignited before the final stage where it is blown out the rear of the engine, creating thrust to drive the aircraft forward.

However, this high-pressure air created in the engines serves other purposes, too. Depending on the engine type, at certain stages of the process, some air is "bled" out of the system, known as "bleed air."

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Due to the high pressure and temperature of this air, it can be used to stop ice from forming on the wings, start other engines and pressurize the cabin. This allows passengers and crew to breathe as if they are on the ground.

On an aircraft like the Boeing 777, high-pressure bleed air is directed to the air conditioning packs, which sit in the belly of the aircraft. Using a combination of heat exchange methods, utilizing cooler (think minus 76 degrees Fahrenheit) air from outside, the hot bleed air is cooled.

Once it has been cooled to an acceptable temperature, it is then directed towards a unit that removes moisture. After this, it heads towards another unit where it is mixed with some of the original hot air. It is here where the temperature required in the cabin is created.

Finally, the conditioned air flows into the cabin, providing the air to pressurize the cabin at a temperature that keeps you comfortable.

Detecting smells and odors

When we detect a smell, there are a number of factors that determine our interpretation of the odor and how we respond to it. These include the intensity of the smell, the expectation of smelling it at that particular time, our individual experience and the frequency of the smell.

For example, if you smell fuel when filling up your car, you're not surprised because you've had this experience before and, as a result, you're not alarmed because you expect it. If, however, you were sitting in an office and started to smell fuel, you might be a little concerned.

Depending on the situation and your own personal experience, this concern would elevate your stress levels, leading to a response that may or may not be commensurate to the threat posed.

Aircraft are complex machines and, as a result, there are a whole number of reasons why an odd smell could be detected in the cabin. Exhaust fumes from other aircraft on the ground, hydraulic fluid leaks, residues from engine compressor washes and even from pollution in the air outside.

This summer, California suffered the worst wildfires on record with millions of acres of land being lost. Flying into Los Angeles airport during this time, there was a distinct layer of smoke as aircraft made their approach to land. Within seconds of flying through this, the smell of burning wood was clearly present in the aircraft. Not a threat to the safety of those on board, but still, it would have taken those in the cabin by surprise.

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In a small number of events, fumes in the cabin have been caused by contaminated bleed air from the engines. In these cases, faults in oil seals have allowed burnt oil vapors to enter the cabin air supply via the bleed air.

However, one of the main sources of odors in cabin air isn't actually from the engines, it's from the auxiliary power unit (APU).

The APU

Sat in the tail of most commercial airliners is a small engine called the auxiliary power unit (APU). This is used on the ground to provide electrical power and also to provide air for starting the engines and the air conditioning system.

Like the main engines, the air is bled off the APU to and directed towards the environmental conditioning system (ECS), which regulates the pressure and temperature of the air in the cabin. Whilst on the ground, the APU inlet is a relatively easy entry point for contaminants such as de-icing fluid, vehicle exhaust gases and oil.

Investigations into odor events have found that, once airborne with the APU switched off, the contaminants already in the system were found to be responsible for a large number of odor events; the engines were only suspected as they were providing the bleed air at the time.

What is being done to prevent this?

With the knowledge that most odor events can be sourced back to the APU, airlines and manufacturers have implemented steps to reduce the chances of this air entering the environmental control system (ECS).

Airbus recommends that pilots leave the APU bleed running for a few minutes before turning the air conditioning packs on. This will allow any contaminant to be flushed out before it enters the ECS.

In addition, some airlines have been fitting guard bars to the area around the APU inlet. This acts as a physical barrier to prevent oil and other liquids from getting into the APU in the first place.

How pilots keep the aircraft safe

Should the crew detect an odd smell during a flight, their first act is to protect themselves. If they deem it necessary, they can don their oxygen mask. If things get really bad, we can carry out a procedure that evacuates the air from the cabin.

Oxygen masks

In the flight deck, the oxygen masks are a little different (and more complicated) to those in the cabin. In order to protect us in the event of smoke or fumes, the masks also contain fully enclosed goggles to protect our eyes. Once the masks have been donned, establishing communication with the other pilot is key.

Due to the goggles, our peripheral vision is drastically reduced and all audio conversation via the intercom is conducted over a background of heavy breathing. If you think of a standard Darth Vader sound effect, you won't be far off understanding what it sounds like. This is part of the reason why we will delay making an announcement to the passengers too soon.

Smoke, fire and fumes removal

When dealing with a smoke or fumes event, pilots will initially run the smoke, fire and fumes checklist for their aircraft type. Part of this gives us the option to attempt to remove the smoke or fumes from the cabin.

This procedure involves opening up the outflow valves, normally used to control the cabin pressurisation and then opening the ram air inlet. This forces fresh air from the outside directly into the cabin, taking any contaminated air with it as it exits through the outflow valves.

However, this is quite an involved procedure so it may not be pertinent to try this when close to landing; just focus on landing the aircraft safely.

The 787 Dreamliner

As I mentioned, most aircraft use bleed air to pressurise the cabin. This means that despite being filtered before entering the air conditioning systems, the air you breathe has still come via the engine. The Boeing 787 Dreamliner, however, is different.

Instead of using air from the engines or APU, Boeing designed the aircraft to use air taken directly from the outside. As the engines aren't then losing energy to power the air conditioning system, it also makes them more efficient. More efficient engines equal lower carbon emissions.

Air is taken into the aircraft by two dedicated inlets just below where the front of the wing meets the fuselage. To protect these inlets, two deflector doors deploy in front of them during normal ground operations and the landing phase of flight. This stops large contaminants such as stones and birds from being taken into the air conditioning system.

This air is then directed into four electrically operated cabin air compressors (CACs). The air is pressurised and sent to two identical air conditioning packs. Each pack has two dedicated CACs, however, a single CAC is enough to power a single pack.

Read more: Why pilots don't have to wear masks when flying

This airflow is controlled by regulating the cabin air compressors. CAC output is automatically increased during periods of high demand, for example, to compensate for a failed pack. It is also limited during times of low aircraft electrical output to ensure that there is enough power available to run other critical systems.

From here, the supply of air to the cabin is much the same as other aircraft, except when it comes to another important factor — moister air.

The air on the 787 is much moister than on other types, particularly compared to the 777. On the Dreamliner, the crew are able to set exactly how many passengers are on board. The air-conditioning system then uses this number to optimise the humidity of the air being directed into the cabin creating an environment much more like that on the ground.

Should you be worried about toxic fumes?

When you read of 362 reported fumes events in a two-year period, it seems quite a lot. However, according to the Bureau of Transport Statistics, part of the Department of Transport, during the same time there were around 16 million flights. That means that there was a fumes event in only one in every 44,198 flights.

To put that into perspective, statistically speaking, you'd have to get on a flight every single day for 121 years before you experienced a fumes event. When put like that, it doesn't seem like much to worry about.

As a pilot myself, I am not concerned. Yes, like all threats to safety, I am respectful of the danger, but it is not something that will stop me from doing the job I love.

The 787 is a game-changer when it comes to air quality in aircraft. Due to the fact that it does not use bleed air for the air conditioning system, it is not at risk from contaminated air in the same way as the majority of other aircraft types are. If you are really concerned, try to book your next flight with an airline that flies the 787 on the route.

Bottom line

A 2017 study by the European Aviation Safety Agency (EASA) found that the air quality found in aircraft is similar, if not better, than many enclosed environments on the ground such as offices, schools and homes.

"To state the obvious, there is no contaminant-free indoor environment," it continued. "The aircraft cabin is no exception. However, due to the exceptional high air-exchange rates in aircraft, the cabin air has been proven to be less polluted compared to normal indoor environments … volatile contaminations in the cabin are thus depleted quickly."

Whilst the number of reports of fumes in the cabin do seem to be increasing, the overall numbers are still incredibly small. From the statistics mentioned previously, you have a one in 219,000 chance of being involved in a fumes event serious enough for the pilots to require the use of their masks, you're 20 times more likely to be injured by a toilet.

As your pilot, our number one priority is to keep you safe and, in a selfish kind of way, if we keep ourselves safe, we keep you safe.