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Airplane meals are always a gamble — mystery meats, strange blobs, unrecognizable colors — but at least we know the beverage menu is safe territory. Right?

Wrong.

There’s one thing flight attendants know better than to drink on a plane, and it’s thanks to their insider knowledge of the goings-on in the galley.

And it might not be what you might expect.

Of all the beverages available on flights, from soft drinks and juices to beer and hard liquor, it’s actually tea and coffee that cabin crew would never personally order.

TRUE CONFESSIONS OF A FLIGHT ATTENDANT

Why? Because it requires water from water tanks, and it turns out those water tanks are pretty gross.

The Independent reports that while water tanks on aircraft do get cleaned, flight attendants never know when or how often, so they’re not willing to risk that it’s been a long time between scrubs.

“Flight attendants will not drink hot water on the plane,” a flight attendant recently told Business Insider.

“They will not drink plain coffee, and they will not drink plain tea.”

It follows that you should probably not order even plain water that might have come from a water tank either. Try and get something that’s served in a sealed bottle.

The Association of Flight Attendants-CWA (AFA) explained that in the United States, water onboard aircraft is regulated under the Environmental Protection Agency to ensure it’s safe to drink.

But how often the water tank is cleaned is another thing entirely.

“The regulation gives broad discretion to airlines on how often they must test the water and flush the tanks,” the AFA said.

“AFA does not believe this regulation goes far enough or is sufficiently enforced.“

A recent study by the Environmental Protection Agency found about one in every 10 commercial planes in the US had tested positive at least once for coliform, a bacteria found in human feces.

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Last month, top chef Anthony Bourdain also expressed his skepticism for what’s served on flights, saying he’d rather go hungry than eat in-flight meals.

“No one has ever felt better after eating plane food,” Bourdain told Bon Appetit.

“I think people only eat it because they’re bored. I don’t eat on planes. I like to arrive hungry.”

And if he was so famished on a flight he had to resort to something, the “No Reservations” host said he would order a cheese platter and some port.

“I’d eat some cheese and drink myself stupid,” he said.

This story originally appeared on news.com.au.