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Therefore the approach alignment, descent gradient, runway dimensions and runway approach surfaces all need to meet appropriate standards. Where these standards can be met, a runway approach will be designed, but in those cases where it is not possible to comply with the standards only a circling approach is published.
3.4 WHAT ARE THE STANDARDS REQUIRED FOR THE PUBLICATION OF A RUNWAY APPROACH?
Alignment. To avoid the necessity to conduct turns close to the ground, the alignment of the final approach course needs to be closely aligned to the runway centreline. PANS-OPS design rules permit the final approach course to intersect the extended runway centerline at an angle up to 30° for Category A/B procedures and at up to 15° for Category C/D. The final approach course also needs to intersect the extended runway centerline at a sufficient distance from the threshold to allow a turn onto the runway heading to be completed safely.
Note: Instrument approach procedures are designed to accommodate varying aircraft performance by the use of an Aircraft Performance Category based upon approach speed. Approach procedures in Australia are designed for Category A, B, C & D. Category A applies to aircraft with low approach speeds, and each successive category applies to aircraft with higher approach speed. A separate Category H applies to approaches designed for use by helicopters only. Refer AIP ENR 1.5
Descent gradient. For an approach to be safe the descent gradient should be neither too steep, nor too shallow. A steep approach requires high rates of descent which can result in inadvertent descent below critical altitudes. An approach that is too shallow can also increase risk. Straight-in approach procedures are normally designed with a 3° (5.2%/320 ft per NM) gradient, but where necessary this may be increased to a maximum of 3.72° for Cat A/B procedures or 3.5° for Cat C/D. A descent gradient of less than 3° is not normally published.
Runway Standards. Runways serving straight-in approaches need to be of adequate dimensions to enable an aircraft to land after becoming visual at the MDA and must provide adequate clearance from obstacles on the visual segment of the approach path. Runways that conform to these standards are termed non-precision approach runways (NPA runways). Runways that do not meet these standards may conform to a lesser standard suitable for VFR or circling IFR approaches and are referred to as non-instrument runways.
In Australia runway standards are contained in the CASR Part 139 MOS. These standards are based on ICAO Annex 14 requirements modified to meet Australian circumstances.
Obstacle Limitation Surfaces (OLS). Runway standards incorporate a set of surfaces surrounding an aerodrome referred to as the OLS. Critical to the safe conduct of a straight-in approach is the surface immediately below the approach path. For straight-in non-precision approaches, the Australian standard specifies an approach surface gradient of 3.33% along the final approach flight path and obstacles in this area should not be permitted above the approach surface. In cases where this is not possible and obstacles penetrate the approach surface an assessment of risk is required. Where obstacles are assessed to constitute an unacceptable risk, they may be required to be removed unless the risk can be mitigated by other means such as lighting and marking. Where penetrating obstacles cannot be removed or the risk reduced to an acceptable level, a straight-in approach is not published.
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非精密进近 Non-Precision Approaches Civil Aviation Advisory Publication October 2004 CAAP 178-1(1)(6)
