The first time I experienced this phenomenon, it was the noise that got me. It was a loud whooshing sound that felt vaguely apocalyptic—if the apocalypse was happening in a car wash. I fly out of Denver, so this is a regular occurrence for me, but that first time felt otherworldly, almost claustrophobic, especially since I was watching workers outside the plane do things I’d never seen them do before. But for many passengers, it’s the smell that hits them first.
The smell is unmistakably … sweet? Maple syrup-ish? Maybe even a little boozy, like someone cracked open a bottle of butterscotch schnapps in 14C. If you’ve ever smelled this, know that your seat mate isn’t secretly housing a stack of waffles, and you haven’t finally snapped after two hours of gate delays. That mysterious scent is actually a very normal—and very important—part of flying.
“I find it’s always helpful to explain a little bit about what is happening, because sometimes it concerns or scares people,” says Loretta Hill, a flight attendant with JetBlue who has spent a decade in the skies. “That’s why this is so important, to educate people about these strange noises and the smells so that people can just be informed before they step on the plane.”
Consider this your official sniff test. Read on as Hill and Kevin Singh, an instructor pilot and the founder and president of Icarus Jet, a Dallas-based charter and trip-support company, explain everything you need to know about that strange smell before takeoff—and whether you should be concerned.
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What is that sweet smell before takeoff?
That intriguing aroma is de-icing fluid, and it’s one of the most critical safety measures in winter aviation. If you’ve ever wondered what that bright green liquid on airplane wings is all about, this is your answer. Without it, the consequences can be catastrophic.
“If a plane is not properly de-iced, the ice and snow buildup on the wings and control surfaces disrupt the laminar airflow over the top portion of the wing and prevent the plane from generating the necessary lift for a safe takeoff,” explains Singh, whose company regularly provides de-icing services in the U.S. and Canada. “This condition is extremely dangerous and can lead to a crash.”
Singh points to Air Florida Flight 90, in 1982, as a sobering example. Ice contamination on the wings and engines prevented the aircraft from producing adequate lift after departing Ronald Reagan Washington National Airport, and the plane crashed shortly after takeoff, killing 78 of the 85 people on board. “That crash fundamentally changed winter pre-flight standards and reinforced the rule that any aircraft with contaminated wings is not airworthy and must not fly,” Singh says.
How does de-icing work?
The de-icing process itself is a carefully choreographed operation. Ground crews first spray a heated de-icing fluid under high pressure to remove existing snow, ice and frost. Then they apply a different anti-icing fluid to create a protective layer, preventing new buildup before takeoff.
The whole process typically takes 10 to 20 minutes, depending on the aircraft’s size and weather conditions. And there’s a ticking clock involved: Pilots use something called “hold over time” to ensure they’re airborne before the protective fluid loses its effectiveness. If they miss that window? Back to the de-icing pad they go.
So how does that fluid end up tickling your nose inside the cabin? Aircraft ventilation systems draw in outside air, and when de-icing occurs nearby, those vapors can temporarily mix with the cabin air supply, Hill explains. Singh adds that passengers may actually be smelling fluid “that is sprayed closer to the engine or the [auxiliary] power unit.”
Why does it smell sweet?
The distinctive aroma comes from propylene glycol, the main ingredient in de-icing fluid. It’s an alcohol-based solution, and that slightly boozy sweetness you’re detecting comes from the same type of compound found in everything from food additives to medications to some e-cigarette liquids. That familiarity might explain why the smell seems almost pleasant, or at least not alarming. Some passengers describe it as maple syrup, cotton candy or sweet antifreeze (which also contains glycol compounds, though a different, and more toxic, type).
While passengers experience the smell, Singh notes that the fluid is “mainly recognized by its color” among aviation professionals. Type I de-icing fluid is orange, Type II is clear or straw-colored, Type III is green and Type IV is a brighter emerald green. Each formulation is designed for specific conditions and aircraft speeds during takeoff.
Does de-icing fluid always smell like this?
Here’s where things get interesting—and possibly pungent. While most passengers report that pleasant, syrupy scent, de-icing fluid doesn’t always smell like breakfast at the pancake house. Some formulations or conditions can produce a decidedly less appetizing aroma: onions. And not the caramelized, French-onion-soup variety. We’re talking raw, eye-watering onions.
The variation in smell depends on the specific fluid formulation, temperature, concentration and how the fluid interacts with various aircraft components. Some passengers have reported smells ranging from slightly alcoholic to distinctly chemical to downright funky. The onion smell tends to appear when certain fluid types interact with heat or when the fluid has degraded slightly.
So if you’re settling into your seat and suddenly wondering if someone smuggled onions into their carry-on, rest assured—it’s likely just a different batch of the same safety fluid doing its job.
Is it dangerous to breathe in?
“The fluid is non-toxic and not harmful to breathe in, though the smell can certainly be strong enough to make you wrinkle your nose,” Singh says.
That said, the fluid isn’t exactly something you’d want to bathe in. Singh says that de-icing fluids “are corrosive for aircraft metal and not the best for the environment, therefore each wintry airport has a drain collection to separate these fluids once sprayed on the tarmac at the airport.” He adds that the fluids “also eat up the soles of the shoes that pilots walk around in to do the preflight checks”—which paints quite a picture of how powerful this stuff actually is.
For the brief moment you’re smelling it inside the cabin, however, there’s no cause for concern. Some passengers with heightened sensitivities might experience minor irritation or sneezing, but for the vast majority of travelers, it’s simply an unusual scent that passes quickly. Modern aircraft have HEPA filtration systems that efficiently cycle cabin air, which helps clear any residual odors.
Will you always smell this before takeoff?
Not at all. This particular olfactory experience is strictly a cold-weather phenomenon. You’re only going to encounter it during winter months when temperatures are low enough for ice, frost or snow to accumulate on aircraft surfaces, typically when temperatures are at or below freezing.
If you’re flying out of Miami in July or Los Angeles in September, you won’t smell a thing (at least not from de-icing fluid). The planes simply don’t need that protective treatment when conditions are warm and dry. Even during winter, not every flight requires de-icing. Aircraft that have been stored in heated hangars, flights departing during warmer parts of the day or airports experiencing dry cold without precipitation might skip the process entirely.
The decision comes down to actual conditions: Is there visible contamination on the aircraft surfaces? Are current weather conditions likely to cause accumulation before takeoff? This is also why planes can still fly in freezing temperatures—the de-icing process specifically addresses ground conditions and the crucial takeoff phase, not flight at altitude, where different systems protect the aircraft.
How long will the smell last?
The smell is temporary and typically dissipates within minutes of takeoff, Hill says. De-icing happens on the ground, typically at a designated de-icing pad near the runway. The crew sprays the aircraft, the hold over time clock starts, and the pilots taxi to the runway and take off—all within that protective window. The smell peaks during this ground operation and the initial moments of flight.
“Most aircraft have the HEPA filters, so it is very easy to clear that smell,” Singh says. These hospital-grade filtration systems cycle cabin air efficiently, replacing it with fresh air from outside the aircraft. By the time you’ve reached cruising altitude, that maple-syrup mystery should be nothing but a memory.
If you’re particularly sensitive to smells, you might notice it a bit longer than others, but even then, we’re talking about a matter of minutes, not the duration of your flight. Consider it a brief reminder that the ground crew just made your winter flight significantly safer. And if you’re the type who gets nervous about all the sights, sounds and other sensations during flight, “knowing what’s normal can make the whole experience less stressful,” Hill says.
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Sources:
- Loretta Hill, JetBlue flight attendant; phone interview, Dec. 12, 2025
- Kevin Singh, pilot and Icarus Jet founder and president; interviewed, Dec. 12, 2025
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