Travellers by air to-day only too often can be heard to remark how good it is to be on the ground again. Even now flying is considered by many as an adventure and only those living near an airfield realise the regularity and reliability of air transport.

It is with the hope of removing this feeling of adventure and instilling a sense of security in air travel that this Paper has been written. An attempt has been made to reduce the power plant engineering to plain science and to show the care, knowledge and painstaking development that has gone into the design.

The second world war resulted in tremendous progress in aeronautics and while, during the war, the major factor was the aircraft efficiency as an offensive unit, that was not to say that reliability and passenger comfort were ignored.

A recent paper gave an elementary analysis of the effect of control procedures on the flow of air traffic. It was shown that the average delay experienced by aircraft landing at an airport increases rapidly as the arrival rate approaches the maximum handling capacity of the airport. It is the object of the present paper to give a more rigorous treatment of the problem and to derive the distribution of delays under different traffic conditions.

The maximum handling capacity of an airport is the greatest rate at which aircraft can enter the airport with absolute safety. It is determined by the safe minimum separation between successive landings. An aircraft arriving in the central zone with less than the safe minimum separation is withheld from landing until the minimum interval has elapsed.

In the following it is generally assumed that aircraft cross the boundaries of the control zone from random directions with random separations, the mean arrival rate being constant. Aircraft which follow their predecessors too closely are assumed to be delayed for a minimum period so that none lands with an interval less than the safe minimum. As a simplification it is assumed that no aircraft takes off from the airport.