Whats the Plan B if this two runway airport closes due to say a fire Eg Eddl a few years ago or LHBP if you want one with an alternate a bit further away.
The plan B revolves once more around the difference between planning requirements and in-flight requirements. Once you're in flight, all bets are off and you are required to land with more than RESERVE. The fuel policy provides suitable guidance to minimise the chances of this happening on more than an occasional basis.
For example, if I were to fly to LHR, commit to the destination in accordance with the fuel policy and it subsequently shut on me because someone had an incident on 27L, a Mayday would in all likelihood open LHR again for me to land on 27R. Hence the two-runway requirement at the planning stage.
Once you're in flight, depending on the company policy, you can find yourself in a situation where you may be able to commit to a single runway airport given certain provisos.
However, another good example (back at the planning stage) would be flying to a remote island where it is impractical to have an alternate airfield. In that particular case, you will carry extra fuel known as "Island Reserve" which will give you sufficient holding time for them to bulldoze the wreckage of the previous arrival off the runway.
