The terminal arrival altitude (TAA) is the lowest altitude that will provide a minimum clearance of 300 m (1 000 ft) above all objects located in an arc of a circle defined by a 46 km (25 NM) radius centred on the initial approach fix (IAF), or where there is no IAF on the intermediate approach fix (IF), delimited by straight lines joining the extremity of the arc to the IF. The combined TAAs associated with an approach procedure shall account for an area of 360 degrees around the IF. (ICAO Doc 8168 PANS-OPS)
The purpose of the terminal arrival altitude (TAA) is to provide a transition from the en-route structure to an Area Navigation Systems approach procedure.
For further information, see ICAO Doc 8168 PANS-OPS Part II, Section 1, Chapter 2