Procedural control

Procedural control is a method of providing Air Traffic Control services without the use of radar. It is used in regions of the world, specifically sparsely-populated countries and oceans, where provision of contiguous radar coverage is either prohibitively expensive compared to traffic levels, or is simply not feasible. It also may be used at very low-traffic airports, or at other airports at night when when the traffic levels may not justify staffing the radar control positions, or as a back-up system in the case of radar failure.

The Ten Minute Rule

The central rule of procedural control is that no aircraft travelling on the same route at the same level shall come within 10 minutes' flying time of another aircraft. There are, however, many exceptions to this rule, for example in the case of a slower aircraft following a faster aircraft, where there is obviously no risk of the aircraft colliding.

A particular aircraft will have a clearance to fly a pre-determined route. Controllers then determine separation between aircraft based upon pilots' estimates for and reports over Reporting Points, which are defined by radio navigation beacons or specific map co-ordinates. Controllers may use speed control to maintain separation, or may climb or descend aircraft to different levels to allow a faster aircraft to overtake a slower aircraft.

En-route jet aircraft tend to fly at roughly 8nm per minute: 10 minutes' flying could therefore equate to 80nm. Standard separation on radar whilst en-route is 5nm. It is therefore easy to see what a large increase in airspace capacity the provision of radar gives.

Procedural Approaches

Every runway used by IFR aircraft normally has a procedural approach defined and published. This generally involves the aircraft flying over a radio navigation beacon on or close to the airport (where the aircraft can hold if necessary), flying away from the airport whilst descending on a set track, then at some point approximately ten miles from the airport turning back towards the airport aligned with the runway to make an approach.

At airports equipped with radar, procedural approaches are only normally used in the event of an aircraft suffering a radio failure, or for training purposes. At non-radar airports or when radar is not available, these become the only means of IFR flights making approaches to the airport. Separation is maintained by only allowing one aircraft to be making an approach at any one time - the proceding aircraft must either be visual with the airport or have landed before the next aircraft is allowed to leave the hold at the beacon and commence an approach.