Crab,
The Touchdown Positioning Circle should always be 0.5D of the design helicopter. This ensures that the helicopter undercarriage always fits onto the minimum dimension TLOF - i.e. the surface area (which, in ICAO should be no smaller than 0.83D).
With this aircraft (and the EC360), the rear wheel does not sit inside the circle - nor is it intended to (just as some nose wheels do not). It is these considerations that have driven the minimum size of TLOFs. When the TPDM(C) is correctly drawn/positioned, the pilot can trust that the wheels are correctly contained on the surface of the heliport when sitting on the bum-line (the inner circumference of the circle).
It is exceptionally difficult designing a turning circle lead-in line that will fit all helicopters; for stands that have turning on the ground, the area required, is left to the manufacturer to define for that reason.
There have been occasions (in my time) when the rear-wheel of the S61 has been 'dropped' off the back of a helideck.
Jim
Last edited by JimL; 22nd Jan 2018 at 12:23.
Reason: Typos