If all aircraft flew exactly from point to point in straight lines the life of the procedures specialist would be very easy indeed. Unfortunately they do not and so the area required to complete a turn must also be checked for obstacles that may cause safety buffers to be infringed. As we all know the speed that a turn is carried out determines the radius of the turn, the same is true in the air.

Also effecting the turning radius is the bank angle of the aircraft itself. This can increase or decrease the turning radius required. Peculiar to the air is the difference between Indicated Air Speed (IAS), what the pilot sees in the cockpit and True Air Speed (TAS) the actual speed of the plane. This difference is directly related to the altitude of the aircraft and can be calculated with the following simple formula:

TAS=IAS*171233*[ (288± VAR)-0.006496H] 0.5 /(288-0.006496H) 2.628

Aircraft prefer to increase their speed and altitude as soon as possible but do not like to use high bank angles in the turn. Unfortunately this combination increases the turn area required, which may then include critical obstacles. Procedure design is the balancing of these optimal flying requirements and the absolute need to avoid obstacles.

Because aircraft like ships are effected by the wind, this factor must also be taken into account. As it is impossible to predict the wind direction at any time in the future, an omni-directional wind is used. By using a wind from any direction at any time during a turn the worst possible case is allowed for. This increasing wind effect can be calculated using the formula.

Wind effect = (q / R) * (w / 3600)

Once all these factors have been taken into account the obstacle protection area for a turn can be constructed and checked for critical obstacles or terrain.