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Procedure altitudes are shown on the profile diagram at the commencement of the procedure and at each fix or significant point on the approach.
2.4 WHAT ARE THE SHADED AREAS ON THE PROFILE DIAGRAM OF AN NPA?
At each stage of a non-precision approach a segment minimum safe altitude, depicted as a “not below altitude” (e.g. 1700) identifies the lowest altitude that provides the required obstacle clearance. ICAO has identified that many CFIT accidents have occurred because pilots did not possess good situational awareness in regard to terrain beneath the approach flight path. Australian charts are shaded beneath the segment minimum safe altitude to graphically illustrate the presence of obstacles or terrain to aid vertical situational awareness.
2.5 CAN I DESCEND TO THE MINIMUM SEGMENT ALTITUDE INSTEAD OF FOLLOWING THE CONSTANT ANGLE APPROACH PATH?
Yes. Obstacle clearance is provided at or above the segment minimum safe altitude (shaded areas). The constant angle vertical approach path is the recommended flight path designed to enable a stabilized descent.
2.6 WHAT IS THE MINIMUM DESCENT ALTITUDE (MDA)?
The MDA is the lowest altitude that can be used in IMC. Flight below the MDA reduces the clearance above obstacles and is not permitted in IMC.
The published MDA may include an allowance for barometric error, depending upon the location and the availability of an actual aerodrome barometric pressure. Instrument approach charts identify when an allowance is added to the MDA, by use of a shaded minima box or by publishing an additional MDA.
Where a 24-hour air traffic service is available the published MDA normally is based on the use of the local aerodrome QNH as reported by ATC and no allowance is added to the MDA.
At other aerodromes, an allowance of 100ft is included in the published MDA to provide for the accuracy of a forecast aerodrome QNH. If an actual aerodrome QNH is available (e.g. AWIS) the 100ft tolerance for forecasting accuracy is not required and the MDA may be adjusted accordingly.
Where a forecast QNH (TAF) is not available and Area QNH is used the allowance of 100ft may be insufficient to take into account the forecasting tolerance and the pilot must add 50ft to the published MDA.
2.7 WHAT ALLOWANCE SHOULD BE MADE FOR WIND?
Timing specified for holding and approach procedures includes an allowance for adverse winds. Pilots should not rely solely upon the design allowances and adjustment should be made for known or estimated winds to ensure that the aircraft remains within the design obstacle protection area, and to facilitate the successful completion of the approach within normal operating limits.
2.8 CAN I USE A VNAV SYSTEM TO FLY THE APPROACH?
A variety of certified or approved vertical navigation systems are available, or are expected to become available. However, at present NPAs in Australia are not designed to provide obstacle clearance solely by use of a vertical navigation system (VNAV).
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非精密进近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(4)
