Originally Posted by
redsnail
The flight times the airline publishes versus the actual ones are quite different. They are usually padded to allow for the inevitable delays operating at most major/popular airports.
Now to the delays. There are airport slots and there's airways slots. The differences are like the queues to get in/out of a car park (airport slot) versus the traffic levels on a motorway (airways or CTOTs - Calculated Take off Time).
Anything can affect the airways. Weather enroute, traffic, lack of ATC, weather at departure/destination.
So you think, why board if we have a CTOT? They can change very quickly. The idea is we get you on board, then send a ready message to ATC. In UK/Europe, that goes to Eurocontrol and if possibly, you'll be released early and you can go. However, if you have to board every one, you'll miss it!
So why move? Well, the airline still needs that gate so you'll remote park to wait your turn.
Schedule planning is a black art.
Build in enough slack and you'll reduce the potential for delays.
But you'll also reduce the utilisation that you get from the fleet, and the bean-counters won't be happy (they never are ...).