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Instrument approaches that use distance measurement, either DME or GPS, are provided with an altitude/distance table that enables a constant angle approach to be flown. Provided, on establishing visual contact, the aircraft continues a stabilized descent to the runway, a safe margin above the approach OLS will be maintained.
In recognition of the added safety afforded by straight-in approaches, some NPAs without DME have been published as straight-in approaches. (e.g. RWY 12 VOR).  As an altitude/distance scale is not available in those cases, the distance and approach angle to the runway from the point at which the pilot becomes visual will vary.  
Descent should not be continued below the MDA until the aircraft is established on a safe approach path, in order to ensure clearance from obstacles beneath the approach.  This may necessitate flying level at the MDA until a safe approach angle to the runway is intercepted.
6.3 CIRCLING APPROACH
Circling approaches normally require manoeuvring to align the aircraft with a suitable runway.  Circling is a visual procedure that can be very hazardous if not executed correctly.
Circling rules are published in AIP ENR 1.5 Section 1.7.  These rules have been developed as the result of many years’ experience, and if followed, enable the safe termination of an instrument approach.
What is the circling area?
The circling area is an area bounded by arcs drawn from the runway ends within which obstacle protection at the MDA of not less than 300ft for CAT A/B and 400FT for Cat C/D is provided. The size of the circling area is based on the maximum IAS permitted for each aircraft Category.  In order to maintain obstacle protection the aircraft must be maintained within the circling area by visual reference to the runway.  The maximum speeds are published in PANS-OPS and reproduced in the Australian AIP in AIP ENR 1.15 Table 1.1
 
Category
 KIAS
 
A
 100
 
B
 135
 
C
 180
 
D
 205
 

 
Table 1  - Maximum circling IAS
By day, and complying with the rules for circling, a pilot may elect to descend below the MDA, but in doing so must take responsibility for obstacle clearance.  As spot heights on IAL charts do not necessarily indicate the highest terrain, or all obstacles in the circling area, pilots should only exercise this option when they are familiar with the terrain in the circling area.  Although instrument approach charts show significant obstacles they do not provide detailed terrain information.  Without detailed local knowledge, it is generally a safer option to utilize the obstacle protection afforded by remaining at the MDA.
 
 
What is the purpose of a no-circling area?
The circling MDA is calculated by adding a minimum obstacle clearance of 300ft (Cat A/B) or 400ft (Cat C/D) to the highest obstacle in the circling area.  In order to permit a lower MDA, a no-circling area is sometimes published to eliminate obstacles in part of the circling area.  Provided a pilot avoids the no-circling area, the required obstacle clearance applicable above all other obstacles is maintained.
　
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本文鏈接地址：非精密進(jìn)近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(10)