What are Heathrow slots?

Since International Consolidated Airlines Group (IAG) first approached Aer Lingus last month with a take over offer, there has been a lot of debate about the likely impact of a deal on the Irish carrier’s “Heathrow slots”.
The phrase refers to the rights that airlines have to land and take off from the airport.

Aer Lingus has 24 slot at Heathrow, all of them paired with matching rights at Irish airports, and the general consensus is that they are worth a total of around €400 million.
When the airline floated in 2006, the Government kept a 25.1 per cent stake in the company partly to give itself a veto should the airline ever want to sell any of its rights at Heathrow.
These are seen in some quarters as being critical to the Republic’s air links with key markets for exports, tourism and inward investment.
The high price-tag attached to the Heathrow particular rights is simply down to the fact that supply is limited.
Heathrow is operating at close to full capacity and is unlikely to be able to expand for at least the next decade.
Also, airports grant these rights on a “use it or lose it” basis, if an airline does not use the slots to fly passengers to an from an aiport, it loses them.
As a result, landing/take-off rights at Heathrow only become available occasionally.
The last time a substantial number became vacant at the hub was in 2012 when British Midland was wound up.
IAG bought the business and acquired its slots, some of which are now used for British Airways flights from Dublin to Heathrow.
There is no formal market for trading slots in any airport, so it is not possible to put a firm value on such rights, but they are traded or swapped from time to time. If an airline were to sell all its slots, it would have no landing rights at heathrow, and if it were to sell a few of them, then the amaunt of times it could land at Heathrow would be reduced accordingly.
The basic rules is that the busier the airport, the more valuable its slots.