3.0 Size of Landing Area and Obstacle Protected Surfaces

  3.1 For any particular type of single main rotor helicopter, the helicopter landing area should be sufficiently large to contain a circle of diameter D equal to the largest dimension of the helicopter when the rotors are turning. This D circle should be totally unobstructed (see Table 1 for D values). Due to the actual shape of most helicopter landing areas the D circle will be 'imaginary' but the helicopter landing area shape should be capable of accommodating such a circle within its physical boundaries. For landing areas of less than 1D width this will not be the case. See D-Circle Definition.

  3.1.1 When helicopter landing areas are provided in the bow or stern of a vessel or are purpose-built above the ships structure, they shall be regarded as purpose-built shipboard heliports. For purpose-built shipboard heliports provided in the bow or stern of a vessel, the landing area shall be either:

• 3.1.1.1 In accordance with 3.1 above, or,

• 3.1.1.2 For operations with limited touchdown directions, contain an area within which can be accomodated two opposing arcs of a circle with a diameter of not less than 1D in the helicopter's longitudinal direction. The minimum width of the landing area shall be not less tham 0.83D. In such arrangements of landing areas, the vessel will need to be manouvred to ensure that the relative wind is appropriate to the direction of the helicopter touchdown heading. The touchdown heading of the helicopter is limited to the angular distance subtended by the 1D arcs headings, minus 15 degrees at each end of the arc. See diagram below.

Permitted Landing Headings for Limited Heading Operations

  3.2 From any point on the periphery of the above mentioned D circle an obstacle-free approach and take-off sector should be provided which totally encompasses the safe landing area (and D circle) and which extends over a sector of at least 210°. Within this sector, from the periphery of the landing area and out to a distance that will allow for an unobstructed departure path appropriate to the helicopter that the landing area is intended to serve, only the following items may exceed the height of the landing area, but should not do so by more than 250 milimetres:

• the guttering (associated with the requirements in paragraph 4.2);
• the lighting required by Secton 4;
• the foam monitors;
• those handrails and other items associated with the landing area which are incapable of complete retraction or lowering for helicopter operations.

  3.3 The bisector of the 210° obstacle free sector (OFS) should normally pass through the centre of the D circle. The sector may be 'swung' by up to 15° as shown in Figure 1 below. Acceptance of the 'swung' criteria will normally only be applicable to existing vessels.

  3.3.1 If, for an existing vessel, the 210° obstacle free sector is swung, then it would be normal practice to swing the 180° falling 5:1 gradient by a corresponding amount to indicate, and align with, the swung OFS.

  3.4 The diagram at Figure 1 shows the extent of the two segments of the 150° Limited Obstacle Sector (LOS) and how these are measured from the centre of the (imaginary) 'D' Circle and from the perimeter of the safe landing area (SLA). This diagram assumes, since helicopter landing areas are designed to the minimum requirement of accommodating a 1 'D' Circle, that the 'D' Circle perimeter and SLA perimeter are coincidental. No objects above 0.05D are permitted in the first (hatched area in Figure 1) segment of the LOS. The first segment extends out to 0.62D from the centre of the 'D' Circle, or 0.12D from the SLA perimeter marking.

The second segment of the LOS, in which no obstacles are permitted within a rising 1:2 slope from the upper surface of the first segment, extends out to 0.83D from the centre of the 'D' Circle, or a further 0.21D from the edge of the first segment of the LOS.

The exact point of origin of the LOS is assumed to be at the periphery of the 'D' Circle.

Figure 1 Obstacle Limitation showing position of Aiming Circle

  Drawing Not to Scale

  3.4.1 Some helicopter landing areas are able to accommodate a SLA which covers a larger area than the declared 'D' value; a simple example being a rectangular deck with the minor dimension able to contain the 'D' Circle. In such cases it is important to ensure that the origin of the LOS (and OFS) is at the SLA perimeter as marked by the perimeter line. Any SLA perimeter should guarantee the obstacle protection afforded by both segments of the LOS. The respective measurements of 0.12D from the SLA perimeter line, plus a further 0.21D are to be applied. On these larger decks there is thus some flexibility in deciding the position of the perimeter line and SLA in order to meet the LOS requirements and when considering the position and height of fixed obstacles. Separating the origin of the LOS from the perimeter of the 'D' Circle in Figure 1 and moving it to the right of the page will demonstrate how this might apply on a rectangular SLA.

  3.4.2 The extent of the LOS segments will, in all cases, be lines parallel to the SLA perimeter line and follow the boundaries of the SLA perimeter (see Figure 1 above). Only in cases where the SLA perimeter is circular will the extent be in the form of arcs to the 'D' circle. However, taking the example of an octagonal SLA as drawn at Figure 1, it would be possible to replace the angled corners of the two LOS segments with arcs of 0.12D and 0.33D centred on the two adjacent corners of the SLA; thus cutting off the angled corners of the LOS segments. If these arcs are applied they should not extend beyond the two corners of each LOS segment so that minimum clearances of 0.12D and 0.33D from the corners of the SLA are maintained. Similar geometric construction may be made to a square or rectangular SLA but care should be taken to ensure that the LOS protected surfaces minima can be satisfied from all points on the SLA perimeter.

  3.5 Whilst application of the criteria in paragraph 3.4 above will ensure that no unacceptable obstructions exist above the helicopter landing area level over the whole 210° sector, it is necessary to consider the possibility of helicopter loss of height due to power unit failure during the latter stages of the approach or early stages of take-off. Accordingly, a clear zone should be provided below landing area level on all helicopter landing areas. This falling 5:1 protected surface should be provided over at least 180° and ideally it should cover the whole of the 210° OFS, with an origin at the centre of the 'D' Circle, and extending outwards to a distacne that will allow for a safe clearance from obstacles below the landing area in the event of an engine failure for the type of helicopter that the landing area is intended to serve (see Figure 2). All objects that are underneath anticipated final approach paths should be assessed.

  3.5.1 Research completed in 1999 demonstrated that, following a single engine failure in a twin engine helicopter after take-off decision point, and assuming avoidance of the deck edge, the resulting trajectory will carry the helicopter clear of an obstruction in the range 2:1 to 3:1. It is therefore only necessary for operators of multi-engine helicopters operated in performance classes one or two (as defined in ICAO Annex 6 Volume 3) to account for performance in relation to specified 5:1 falling gradient where infringements occur to a falling 3:1 rather than a 5:1 slope.

  3.6 For practical purposes, when a safety net is fitted, the falling obstacle limitation surface can be assumed to be defined from points on the outboard edge of the helicopter landing area perimeter safety netting supports (1.5 metres from deck edge). Minor infringements of the surface by foam monitor platforms or access/escape routes may be accepted only if they are essential to the safe operation of the helicopter landing area but these infringements may also attract landing area availability restrictions.

Figure 2 Obstacle Free Areas - Below Landing Area Level