Another topic of intensive study has been the estimation, through the use of queueing models, of the delays caused by the lack of sufficient runway capacity. This is a problem that poses a serious challenge to operations researchers: The closed-form results developed in the voluminous literature of classical steadystate queueing theory are largely nonapplicable—at least when it comes to the really interesting cases. The reason is that airport queues are, in general, strongly nonstationary. The demand rates and, in changing weather conditions, the service rates at most major airports vary strongly over the course of a typical day. Moreover, the demand rates may exceed capacity