*Ghost planes still subject to all applicable laws of physics
Screenshot: Half as Interesting (Youtube)

Six times per week, an empty plane used to fly from London’s Heathrow Airport to Cardiff, Wales. The next day, the plane would make the return trip without a single passenger. Half As Interesting, the second channel from Planelopnik-approved Wendover Productions, details why ghost flights like this sometimes operate from Britain’s biggest airport in his new video.

Despite being one of the most crowded airports in the world, Heathrow operates with only two runways. As a result, it’s extremely difficult to get a “slot pair”—rights for airlines to land and take off at a certain time. Only 650 slot pairs exist per day, so airlines are prepared to drop massive cash in order to get prime slot pairs. And they can trade and sell them, too.

In the most extreme instance, Half As Interesting reports that Kenya Airlines sold a peak-time slot pair—early morning, when overnight flights get in—for a whopping $75 million to Oman Air. Given that multimillion-dollar sales are common, it’s unsurprising that airlines are incredibly protective of their assigned slots.

Should an airline fail to use their slot at least 80 percent of the time, Heathrow will reassign it to the next company on the waiting list. As the Half As Interesting video details, this leads to a curious situation wherein airlines are sometimes incentivized to fly empty planes out of Heathrow.

As they say, use it or lose it.