Procedural control

It is used in regions of the world, specifically sparsely populated land areas and oceans, where radar coverage is either prohibitively expensive or is simply not feasible.

In air traffic control, the risk of aircraft colliding is managed by applying separation rules.

The central rule of procedural control is that each aircraft is cleared onto a predetermined route (airway), and no aircraft traveling on the same or intersecting routes at the same level shall come within 10 minutes' flying time of another (or sometimes 15 minutes depending on the accuracy of the available radio navigation beacons).

Because aircraft cruising at different altitudes will never conflict regardless of route of flight or estimated times, they don't need to be checked against each other.

The fix posting area method is easier however when there are a lot of altitude changes because of numerous departures and arrivals within the sector.

This is a more obscure method today because most airports around the world with significant enough traffic to warrant this technique have at least a radar covering the climb out and arrival area, and only need procedural control for points between (in which case altitude grouping is likely more efficient).

As new altitudes are issued, all of the strips for that aircraft must be annotated with the new data and separation checked again at all of the intersections it passes through.

This is more laborious than the altitude method of grouping where only one strip is needed, but possible conflicts between multiple aircraft climbing and descending are easier to discern or discard this way since all intersection points and times are pre-calculated and laid out separately.

At airports equipped with radar, procedural approaches normally are used only in the event of an aircraft suffering a radio failure, or for training purposes.