Whether you’re traveling to the Sahara Desert or to Tahiti, it’s probably best to pack a blanket or a sweater for the flight.

If you’ve ever been on a plane, you’d quickly realize airlines routinely crank up the air conditioning to frigid levels.

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Of course, we’ve all had the experience of boarding a plane to find the air stagnant and hot. So why can’t airlines find a happy medium between the extremes?

Part of the problem lies in the fact that anywhere from dozens to hundreds of people with varying temperature preferences occupy a plane at the same time.

While you can generally control a small fan overhead, there’s no option to open a window or turn down the AC in your area only.

Making matters worse, older planes can have wildly different temperatures throughout a plane. A window seat in the back could be baking while an aisle seat by a cooling duct could be freezing.

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Newer planes are much better at regulating temperature.

When they’re in the air and have full control of the air conditioning, they can regulate the temperature down to the row.

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When a plane is parked at the gate, the situation is a little different. Patrick Smith, the author Cockpit Confidential, told Traveller, “Cold air is plumbed in from outside through ducting connected to a huge air conditioning unit mounted below the jetway. Usually this is adequate. If not, we will start the auxiliary turbine engine that provides air and electricity when the main engines aren’t running.”

Waiting on the tarmac can be a different story altogether.

Because the plane does not have access to the gate’s air conditioner, the plane can heat up quickly.

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Because of this potentially dangerous situation, airlines set maximum safe temperatures. American Airlines’ current maximum temperature is 90 degrees, though the flight attendants’ union is fighting to lower that to a more reasonable number.

Once the airplane is in the sky, air conditioning units mix fresh cold air with recirculated air from the cabin.

This is why the cabin air is much fresher than most passengers would assume; think of the smells that would otherwise accumulate after 100 passengers sat in the same space for a couple hours.

The airlines choose an unusually cold temperature to reduce the likelihood of fainting while flying. A study by the American Society for Testing and Materials International proved that people suffer from in-flight syncope (which is the technical term for fainting on a plane) when the temperate is warmer. 

So, to keep everyone conscious and healthy (even if freezing), the pilots crank up the AC.

When the temperature or cabin pressure is too high, a passenger can faint because their body tissue does not receive enough oxygen. 

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Airlines want to keep passengers from fainting for multiple reasons. For one, it can be a scary experience for the passenger, their family, and even the rest of the plane. More importantly, when someone faints, an entire plane full of passengers can be diverted if the severity of the medical issue is unknown.

Nonetheless, flight crews do their best to keep the temperature in the cabin comfortable. For a variety of reasons, that’s not always possible. Just remember to take a blanket and wear long sleeves, and you’ll have no problem (with the temperature, at least).