The key performance area of capacity is measured by the so-called en-route Air Traffic Flow Management Delay (ATFM). This indicator monitors the delay of an aircraft at departure due to a decision by air traffic control. The reasons behind such measures are manifold and range from congested airspace, limited capacity at the destination airport, weather conditions or ATC capacity. Hence, the indicator provides an indication if the airspace is overloaded – but it does not provide any insight into whether air navigation service providers have sufficient capacity. In addition, EUROCONTROL states that about a quarter of all ATFM delay is necessary to stabilise the network. One practical example is that an ATFM delay might be useful if the airport at the destination is crowded. Hence, it would be useful to keep the aircraft on the ground until the airport is accessible. In many cases those flights – and the passengers – reach their destination on time as airlines plan buffers between the flight plan and the scheduled time on the ticket.