There's a new United Airlines plane in the skies, with a dangerous-looking feature that will actually make it more efficient.

The Boeing 737-800, which took its maiden test flight in Washington on Tuesday, is the first aircraft fitted with the new Split Scimitar Winglet, a curved attachment to the tip of the wing that is appropriately named for a sword.

The winglets reduce wind drag, and can cut fuel use on the 737 by 2%. That doesn't sound like a ton, but United says that once the winglets are installed on its 737, 757, and 767 fleets, it will save more than $200 million per year. That's believable, considering that buying fuel accounted for 33% of global airline operating costs in 2012.

United will begin retrofitting its 737-800 and 737-900ER jets with the winglets early next year, once it finishes testing and gets FAA certification.

Here's the winglet in close-up:

And the whole plane: