. a visual leg from a point where the MDA is reached to the circling area of the aerodrome.
Note: New instrument approach procedures are in the process of development, which have characteristics that are not necessarily associated with either non-precision or precision approaches. Additionally aircraft equipment and piloting techniques can be used to permit many approaches to be flown in a manner similar to an ILS and some operators are now describing approaches as either “ILS” or “non-ILS” rather than non-precision or precision.
It is expected that the terms non-precision and precision will eventually lose their significance and a review of terminology is currently being considered within ICAO.
2. General
However, as the classification of approaches is not yet resolved, in this CAAP the term NPA and the general application of the term are retained.
2.2 VERTICAL NAVIGATION
Non-precision approaches are designed as a series of decreasing minimum altitudes to a minimum descent altitude (MDA). A fix is located at each point at which critical obstacles have been passed by an adequate margin, and it is safe to continue descent to the next safe altitude.
In the past NPAs have commonly been flown as a series of descending steps conforming to the minimum published altitudes. This technique is colloquially referred to as the “dive and drive” method. Unfortunately many Controlled Flight into Terrain (CFIT) accidents have been attributed to this technique, due to human errors such as descending before a step is reached or failing to arrest descent. In addition the aircraft’s descent is more difficult to manage due to changes in airspeed, rate of descent, and configuration.
Australian NPAs are now published with a constant angle approach path, which clears all minimum altitudes, and facilitates the use of a stabilized descent technique. This method of promulgation is now the standard adopted for Australian non-precision approach procedures and the constant angle stabilized approach technique is the recommended flight technique for all aircraft.
Figure 1 shows an example of an NPA profile.
Figure 1 – Typical Constant Angle Approach Profile
2.3 WHAT’S A PROCEDURE ALTITUDE?
AIP defines procedure altitude as follows:
Procedure Altitude: A specified altitude, flown operationally at or above the minimum altitude and established to accommodate a stabilized descent at a prescribed descent gradient/angle in the intermediate/final approach segment.
The term procedure altitude is used to identify that an altitude is promulgated to facilitate flying the procedure. This is in contrast to segment minimum safe altitudes that provide minimum obstacle clearance. The procedure altitude is therefore a recommended level, and an aircraft is not required to maintain the procedure altitude, unless instructed by ATC. A procedure altitude will always be at or above the minimum altitude for obstacle clearance.
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本文链接地址：非精密进近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(3)
