Figure 2 -  Example of New Format DME or GPS Arrival Chart
AIP/DAP DME and GPS arrival charts are being progressively redrawn in a format similar to normal NPA charts, and incorporating a constant approach path.  The constant approach path is designed to provide a 3° constant angle approach where possible, and terminating at a circling MDA within the circling area (Refer Fig 2).
5.5 CAN I USE GPS FOR TRACK GUIDANCE ON A GPS ARRIVAL?
No.  GPS arrivals are designed using the navigation tolerances applicable to the ground-based aid (NDB or VOR).  These tolerances include allowances for the accuracy of the navigation aid and piloting accuracy.  
6.1 WHEN MAY I DESCEND BELOW THE MDA?
Descent below the MDA may be made when visual contact is made with the runway or runway environment (i.e.  the runway threshold or approach lighting or other markings identifiable with the runway) and kept in sight during the subsequent approach and landing.  The approach procedure may support a straight-in visual segment or a “circle-to-land” procedure may be required.
6.2 STRAIGHT-IN APPROACH
If the NPA is a runway approach and, at an altitude above the straight-in MDA, you have the runway in sight and the required visibility you may continue descent.  In doing so you must nevertheless be sure that obstacles in the approach path are avoided.  Protection from obstacles beneath the approach path is aided by aerodrome design standards, including Obstacle Limitation Surfaces (OLS).  Aerodrome standards, based on ICAO Annex 14 and modified to suit Australian operating conditions, are published in CASR Part 139 Manual of Standards.
6.  The visual segment

Aerodrome standards supporting a straight-in non-precision approach provide for an OLS beneath the final approach, which is surveyed to identify obstacles above a 3.33% gradient from the end of the runway strip.
The approach OLS 3.3% surface provides a buffer to a normal 3-degree (5.2%) approach path.  Obstacles penetrating above the OLS that may be considered a hazard to an aircraft conducting a visual approach will be marked and/or lit, unless removed.  Other precautions such as installing visual approach guidance (PAPI or T-VASIS) may also be used.

 
Figure 3 - Approach Obstacle Limitation Surface
It will be safe to continue on a visual descent below the MDA, provided the approach path of the aircraft is consistent with the protection afforded by runway standards.  If an aircraft is flown below a normal (3 degree) approach then the safety margin between the aircraft and obstacles beneath the approach is reduced.  
　
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本文鏈接地址：非精密進近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(9)