The Centerville International Airport has two runways, one used exclusively for takeoffs and the other exclusively for landings.

The Centerville International Airport has two runways, one used exclusively for takeoffs and the other exclusively for landings. Airplanes arrive in the Centerville air space to re- quest landing instructions according to a Poisson process at a mean rate of 12 per hour. The time required for an airplane to land after receiving clearance to do so has an expo- nential distribution with a mean of 3 minutes, and this process must be completed before giving clearance to do so to another airplane. Airplanes awaiting clearance must circle the airport. The Federal Aviation Administration has a number of criteria regarding the safe level of congestion of airplanes waiting to land. These criteria depend on a number of factors regarding the airport involved, such as the number of runways available for land- ing. For Centerville, the criteria are (1) the average number of airplanes waiting to receive clearance to land should not exceed 1, (2) 90 percent of the time, the actual number of airplanes waiting to receive clearance to land should not exceed 3.