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An empty plane used to fly from London Heathrow to Wales every week - six times in fact.

The next day it’d make a return trip without anybody on board.

Why you ask?

Well, Half As Interesting has looked at why ghost flights like this sometimes operate from the UK’s biggest airports.

Heathrow may be one of the most crowded airports in the world with just two runways, but that didn’t stop it sending these flights out.

As Heathrow is so crowded it proves difficult to get a “slot pair” - the rights for airlines to land and take off at certain times.

(Image: Getty)

Only 650 slot pairs exist per day, so airlines are willing to pay to get prime slot pairs. They can then trade or sell them.

Half As Interesting explains Kenya Airlines sold a peak time slot pair - the early morning, when overnight flights come in - for $75m to Oman Air.

If an airline fails to use a slot then Heathrow reassigns it to the next company on the list, this happens at least 80 per cent of the time.

But this causes a weird situation where airlines are then incentivised to fly empty planes out of Heathrow - the ghost planes.

Do you know weird airport or plane facts? Share your's in the comments below.