An interesting thread and a wide spread of ideas: all credit to ‘roundwego’ for the quote from CAP 437; and to ‘Silberfuchs’ for a pointer to the Canadian Regulations and CAP 437 (not yet the JAA Spec.). Interestingly in the Canadian regulations in Fig. 4 Helicopter Decks, the ‘aiming circle’ appeared to be about 0.25D which - if in accordance with the requirement for it to be 6m - would have made the size of the deck about 24m. This appears to be in conflict with the un-amended ICAO Standard which calls for the inner dimensions of the circle to be “half the D value of the helideck or 6m whichever is the greater” (the amended version will specify only an ‘Aiming/Positioning Marking” with the inner dimensions half the D value of the helideck).
Leaving aside the discussion on the net - which was not the subject of the thread but which has been well covered; and an interesting side discussion about the ovality of the deck lights in assessing the glide slope at night (not really sure what the relevance of that was to pinnacle landings); on the original question there appear to be three different schools of thought:
1. Those who thought that the aiming circle was an S61 anachronism, the pilots landing in the centre of the deck; this could work and is an interesting method but does rely upon the (exceptional) judgement of the pilot knowing (a) where the centre of the deck is and (b) more importantly, where the centre of the helicopter is - and matching one to the other.
2. One view that “if you put your main wheels anywhere in the circle…the main rotor blades won’t go ‘boink’ against anything”; once again it would work but it does rather rely upon the main wheels being in the centre of the helicopter (see the discussion later) and in any case, unless it is a small circle in the centre of the deck, there is too much leeway (the size of the deck would then have to reflect that). The circle could be tailored to the particular helicopter and be made smaller to reduce the leeway but why have a circle in which to ‘put the wheels’ when what is required is a ‘visual cue’ for the pilot that can be used without having to see where the wheels are. If, as implied, the tail always has to point aft then no more than a shoulder line would be necessary. (Note that skidded helicopters were not considered.)
3. Those who considered that the circle is a ‘bum line’ on which the pilot positions himself/herself. ‘Check’ - in a well argued post - almost agreed with this but stated that it was the ‘nose’ of the helicopter that had to be on the aiming circle (which with an S76, EC155 or AW139 on a 1D deck, would put the tail two metres into the obstacle environment).
As ‘check’ says, the aiming circle should have an inner circle 0.5 times the declared size of the deck (declared in the markings at the edge of the FATO - not in the aiming circle) with a 1m yellow boundary; it is situated in the middle of the deck unless using the permitted offset of 0.1D (ICAO and CAP 437).
Positioning on the helideck by using the wheels/skids is not accurate enough; the S76, EC155, AW139 have all of their wheels ahead of the centre line; the AS332, EC225 and S92 have their main wheels aft of the centre line and their nose wheels forward; the S61 and S70 have the main wheels forward of the centre line and the tail wheel aft (in the S70 by a considerable distance). The length of undercarriages ranges extensively - suffice to say that if you want to have an area in which the wheels/skids are always contained it will require a circle of 0.83D (centred on FATO).
In spite of variations in size of helicopters and configuration of wheels and skids; for all single rotor helicopters recently examined it was observed that, for a 1D deck, when the pilots seat is positioned on a circle of 0.5D, all of the parts of most helicopters will be within the boundaries of the deck and where there are exceptions, the extent of the overlap of the boundary is measured in centimetres. (Remember that on a 1D deck the main rotor is already at the front edge and the tail rotor (fenestron) at the back edge).
Regardless of the type of helicopter; when landing on a deck (which is of a size of 1D or greater), positioning with the pilot’s seat on the 0.5D (of the size of the helideck) aiming circle will always provide the pilot with the safest option. When using any other method, or when using a circle of any other size, there is a possibility of an unsafe outcome.
Jim