No pilot should be expected to 'know' the dimensions used in the design of a heliport; just as no pilot should be expected to ‘know’ exactly where the extremities of the helicopter are in space.
Heliports should be designed within criteria established by the appropriate authority – based upon the ‘Design Helicopter’ (an aggregate of measurements within which all helicopters, for which the heliport is designed, can be contained). The heliport should be marked with, and have promulgated, its ‘Design D’ and ‘Maximum Mass’.
Each of the defined areas (FATO, TLOF, Stand, Taxiway) should have visual cues (aiming points, touchdown and positioning markers, lead-in-lines, centre-lines) that ensure that a pilot observing the markings will be clear of all obstacles (including other helicopters in adjacent areas) by a safe margin. Normal errors of positioning should be accounted for within the design.
This was an ‘Organisational Accident’ (Reason 1997) because none of the above criteria was met and normal standards for design and marking (Annex 14 or AC 150-5390 2C) had not been used in the design of the heliport.
The accident investigation exists to establish all the elements that led up to, and resulted in, the accident, and publish recommendations that will prevent a further occurrence – in essence a barrier for each of the causal links.
It is not established to attribute blame – unlike PPRune!
Jim