As simple as adequate visual cueing. Whilst this may not be an issue with ground level helipads, it surely is for elevated.
Take for example a helicopter which has a dynamic vertical procedure with plenty of power in reserve; if the engine failure is recognised at just before TDP of say 30ft, the aircraft will continue to accelerate upwards (some to higher than 60ft).
If the elevated helipad were
only 1D, sight of the landing surface would in all likelihood be lost and the reject would be problematical (not for those test pilots who do this every day but for the average pilot).
For this reason, AC 29-2C has a provision that:
Quote:
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Conduct of the Test. Vertical takeoff profiles should be flown from a pad simulating operational conditions because the sight picture may be critical to successful OEI operations, particularly for elevated heliports. At all points on the vertical takeoff flight path up to the TDP, the pilot, with reasonable head movement, shall be able to keep sufficient portions of two heliport boundaries (front and one side) or equivalent markings in view to achieve a safe landing in case of engine failure...
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This is exactly the problem for those pilots who fly from rooftop helipads or from wellhead platforms where there are no obstructions topside.
One of the issues is that test flying for helipad procedures is sometimes conducted from a square painted on the runway; this provides no problem for hitting the target on rejects as there is sufficient peripheral visual cueing.
A further issue is the designation of elevated helipad; in JARs (not sure about other regulations) this is when the operating surface is 3m above the surrounding surface (triggering the higher requirement). This has been recognised in JARs and there is a facility to apply for a derogation (to the lower limit) based on an assessment of the pad and the visual cues.
The issue is more serious with larger helicopters with less than optimum Fields of View (FOV); with smaller helicopters the pilot tends to sit close to the door - where the visual cues can be maintained for longer. The best vertical procedure I have seen is the one which is provided for the Bell 427/429 where the pad can still be seen at a height of 120ft with the disc at the back and side of the pad (a very steep vertical/oblique). This profile could obtain a 1D CAT A approval.
Jim