The helicopter-sucking hole in the ground

Russia has recently revealed that it’s been sitting on top of “trillions of carats” of diamonds, a secret it has kept until now in order to avoid competing with its other giant diamond deposit, Mirny Mine. The idea of Russia blithely announcing that it has enough diamonds to supply the world market for 3,000 years is rather startling, but even so, that’s not what really grabbed me in the Wired article on the topic. What grabbed me was this:

The [Mirny] mine, now closed due to falling yields, is currently the second-largest excavated hole in the world, and helicopters are forbidden from flying over it in case downward air flow sucks them in.

Seriously??

A search of articles on the topic backs up this assertion. Atlas Obscura has an entry on the mine, accompanied by a photograph that must have been taken through the window of a (very carefully flown) helicopter.

Mirny diamond mine

It states that the mine is 1,722 feet (525 meters) deep, and 3,900 feet (1.25 kilometers) across. Then at the bottom of the entry, it says:

Airspace above the mine is off-limits to helicopters, after “a few accidents when they were ‘sucked in’ by downward air flow…”

Ye gods. But what are the physics involved?

After a bit more searching, I found the answer. If a hole is deep enough — and a half-kilometer deep hole qualifies — the earth will warm the air inside it. The deeper the hole, the warmer the air. Warm air rises, and cool air sinks, so with a big temperature difference between in-hole air and aboveground air, you get quite a bit of air movement.

Thus, two things are happening. First, the warm air rising from the hole is less dense and gives less lift to helicopter rotors than the cooler air it had been flying through. Since the temperature change is extremely abrupt as the helicopter flies over the hole, the pilot may lose a bunch of altitude before managing to adjust the speed enough (read: increase the spin rate of the rotors) to compensate for the loss of lift.

At the same time, the cool air pouring into that hole from all sides is going to create quite a wind shear. If a helicopter loses enough lift to hit the stream of cold air, it could easily be slammed into the side of the borehole before it ever developed enough lift or power to recover.

So being “sucked in” is a bit of a misnomer. It’s more like it falls in. Either way, that’s freaking scary.

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About oregon expat

Socialist heathen and Mac-using writer who enjoys pondering science, politics, well-honed satire (though sarcastic humor can work, too) and all things geeky.
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2 Responses to The helicopter-sucking hole in the ground

  1. Alma says:

    Good heavens! That’s fascinating.

  2. Ana_ñ says:

    How cool!!!
    And cold? In view of the physics involved, which you brilliantly explained, I guess that the phenomena (and the difficulties for the pilot) are enhanced in the particular location of this deep hole:
    Mirny, Eastern Siberia: Winter temperatures in Mirny, a town just below the Arctic Circle, average –40 Celsius.

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