London Heathrow Airport is one of the most heavily slot restricted airports in the world. There’s a limited amount of capacity, and there’s a lot of demand. As a result, airlines have to purchase “slots,” which give them the right to operate a flight at a specific time.
These slots can be sold between airlines, and in some cases can go for a lot of money. For example, just under a year ago Oman Air paid a record amount for a Heathrow slot. Oman Air paid $75 million to buy a Heathrow slot from Air France/KLM.
The value of a slot can vary significantly based on when the flight is due to operate. Part of the reason the slot that Oman Air bought was so expensive was because it was for a peak early morning arrival and departure, which is generally the most desirable.