Here's Why Planes Smell Like Burning Fuel Before Takeoff, According to Aviation Experts

And are there health risks associated with it?

As a frequent traveler, I do everything I can to take care of myself en route, including drinking plenty of water and being pat down at security instead of walking through the metal detectors. Once I board, I'm ready to relax and settle in for a long flight, though lately, I've noticed a slight burning smell as the plane idles at the gate. While it's not present before every flight, it's happened often enough that I wanted to understand why it occurs and whether or not there are any health risks associated with it.

According to two aviation experts, the likely source is the APU (auxiliary power unit), a small turbine engine in the aircraft's tail cone. "We use this engine to power up the aircraft electrical system and to provide air for air conditioning when on the ground," says Mohamed Anas Maaz, a senior first officer for a major airline in the Middle East. Because the main engines aren't on yet, the APU functions to keep passengers comfortable during boarding.

Scott Welch, a remote site aircraft mechanic at Pentastar Aviation, says the air discharged from the APU's compressor could be the culprit. "The initial start-up of the APU could/will often have a lingering burnt petroleum smell," he says. Maaz adds that pressurized air from the APU helps start the main engines. "As rotors rotate, air is sucked in, which is then burned to light up the main engine," he notes.

Related: 11 Unique Airplane Facts You Probably Didn’t Know

But what exactly is burning to cause the odor? Welch says it's typically superheated air passing through the aircraft's air system. While both experts say the smell isn't jet fuel, depending on the air surrounding the plane, a slight amount of spent fuel may intersperse with incoming air.

As the primary engine ignites, it takes in air from the outside environment, while the APU continues to supply air conditioning inside the aircraft. "Some of the exhaust smell — burning jet fuel — makes it into the cabin," adds Maaz. After turning on the main engines, the APU shuts down, causing the smell to cease and quickly dissipate.

And is there cause for concern? A spokesperson from the Environmental Defense Fund directed me to an article showing that consistent long-term exposure can pose health risks. However, Maaz says there's no reason to worry because a typical passenger's exposure time is limited.

Dr. Jagdish Khubchandani, a professor of public health at New Mexico State University, agrees that the fumes aren’t a significant concern as flights have strict protocols and regulations to protect passengers. “Once in flight or away from the airport, these smells should vanish as they are left behind,” he adds.

Welch advises that you should only be alarmed if you see smoke. "If the smell is accompanied by smoke and continues or becomes progressively worse, passengers should alert the crew and make plans to exit the aircraft," he says. So, despite the occasional presence of an unfamiliar scent during pushback, I can sit back and relax, after all — and you can, too.

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