In designing a helipad I notice that the Australian requirements (AC139.R-01) for obstacle clear gradients specify that the slope category (12.5% slope) generally applies to PC2 operations, whereas categories A (4.5% slope) and B (8% slope) are said to generally apply to PC1 and PC3 operations respectively:
The slope design categories in Table 2 may not be restricted to a specific performance class of operations and may be applicable to more than one performance class of operation. The slope design categories depicted in Table 2 represent minimum design slope angles and not operational slopes. Slope category "A" generally corresponds with helicopters operated in performance class 1; slope category "B" generally corresponds with helicopters operated in performance class 3; and slope category "C" generally corresponds with helicopters operated in perfromance class 2. Consultation with helicopter operators will help to determine the appropriate slope category to apply according to the heliport environment and the most critical helicopter type for which the heliport is intended.
This is in contrast with ICAO's notes in Doc 9261, which explain category A as applying to PC1 operations with one engine inoperative, and Cat B and C applying to different scenarios where All Engines Operating (AEO) is assumed.
Is there a logical explanation as to why the Australian rules might consider categories B and C to be applicable to PC3 and PC2 respectively? As a former CPL-H myself I just can't see the reasoning behind it.
