CHAPTER 1

SURFACES

1.1 GENERAL

The effective utilisation of an aerodrome may be considerably influenced by natural features and manmade constructions inside and outside its boundary. These may result in limitations on the distances available for take‑off and landing and on the range of meteorological conditions in which take‑off and landing can be undertaken. For these reasons certain areas of the local airspace must be regarded as integral parts of the aerodrome environment. The degree of freedom from obstacles in these areas is as important to the safe and efficient use of the aerodrome as are the more obvious physical requirements of the runways and their associated strips.

1.1.2 The significance of any existing or proposed object within the aerodrome boundary or in the vicinity of the aerodrome is assessed by the use of two separate sets of criteria defining airspace requirements. The first of these comprises the obstacle limitation surfaces particular to a runway and its intended use detailed in Chapter 4 of ECAR 139 ‑ Aerodromes. The broad purpose of these surfaces is to define the volume of airspace that should ideally be kept free from obstacles in order to minimise the dangers presented by obstacles to an aircraft, either during an entirely visual approach or during the visual segment of an instrument approach. The second set of criteria comprises the surfaces described in the Procedures for Air Navigation Services ‑ Aircraft Operations (PANSOPS) (Doc: 8168), Volume II ‑ Construction of Visual and Instrument Flight Procedures. The PANS‑OPS surfaces are intended for use by procedure designers for the construction of instrument flight procedures and for specifying minimum safe altitudes/heights for each segment of the procedure. The procedure and/or minimum heights may vary with aeroplane speed, the navigational aid being used, and in some cases the equipment fitted to the aeroplane.

1.1.3 The surfaces of ECAR 139 are intended to be of a permanent nature. To be effective, they should therefore be enacted in local zoning laws or ordinances or as part of a national planning consultation scheme. The surfaces established should allow not only for existing operations but also for the ultimate development envisaged for each aerodrome. There may also be a need to restrict obstacles in areas other than those covered by ECAR 139 if operational minima calculated using the PANS‑OPS criteria are not to be increased, thereby limiting aerodrome utilisation.

1.2 ECAR 139 –OBSTACLE LIMITATION SURFACES

1.2.1 Function of the surfaces

1.2.1.1 The following paragraphs describe the function of the various surfaces defined in Chapter 4, and in certain instances include additional information concerning their characteristics. For the benefit of the reader, several illustrations of obstacle limitation surfaces are included in Appendix 1.

1.2.2 Outer horizontal surface

1.2.2.1 In the experience of some States, significant operational problems can arise from the erection of tall structures in the vicinity of airports beyond the areas currently recognised in ECAR 139 as areas in which restriction of new construction may be necessary. The operational implications fall broadly under the headings of safety and efficiency.

1.2.2.2 Safety implications. It is particularly desirable to review carefully any proposal to erect high masts or other skeletal structures in areas which would otherwise be suitable for use by aircraft on wide visual circuits, on arrival routes towards the airport or circuit, or on departure or missed approach climb‑paths. Avoidance by marking or lighting cannot be relied upon in view of the relatively inconspicuous character of these structures, especially in conditions of reduced visibility, and notification of their existence will similarly not always guarantee avoidance.

1.2.2.3 Efficiency Implications. If tall structures are erected in or near areas otherwise suitable for instrument approach procedures, increased procedure heights may need to be adopted, with consequent adverse effects on regularity and on the duration of the approach procedure, such as the denial of useful altitude allocations to aircraft in associated holding patterns. Such structures furthermore limit desirable flexibility for radar vectored initial approaches and the facility to turn en route during the departure climb or missed approach.

1.2.2.4 In view of these potentially important operational considerations, authorities may consider it desirable to adopt measures to ensure that they have advance notice of any proposals to erect tall structures. This will enable them to study the aeronautical implications and take such action as may be at their disposal to protect aviation interests. In assessing the operational effect of proposed new construction, tall structures would not be of immediate significance if they are proposed to be located in:

(a) an area already substantially obstructed by terrain or existing structures of equivalent height; and

(b) an area which would be safely avoided by prescribed procedures associated with navigational guidance when appropriate.

1.2.2.5 As a broad specification for the outer horizontal surface, tall structures can be considered to be of possible significance if they are both higher than 30 m above local ground level, and higher than 150 m above aerodrome elevation within a radius of 15 000 m of the centre of the airport where the runway code number is 3 or 4. The area of concern may need to be extended to coincide with the obstacle‑accountable areas of PANS-OPS for the individual approach procedures at the airport under consideration.

1.2.3 Inner horizontal surface and conical surface

1.2.3.1 The purpose of the inner horizontal surface is to protect airspace for visual circling prior to landing, possibly after a descent through cloud aligned with a runway other than that in use for landing.

1.2.3.2 In some instances, certain sectors of the visual circling areas will not be essential to aircraft operations and, provided procedures are established to ensure that aircraft do not fly in these sectors, the protection afforded by the inner horizontal surface need not extend into those sectors. Similar discretion can be exercised by the appropriate authorities when procedures have been established and navigational guidance provided to ensure that defined approach and missed approach paths will be followed.

1.2.3.3 Whilst visual circling protection for slower aircraft using shorter runways may be achieved by a single circular inner horizontal surface, with an increase in speed it becomes essential to adopt a race‑track pattern (similar to PANS‑OPS) and use circular arcs centred on runway ends joined tangentially by straight lines. To protect two or more widely spaced runways, a more complex pattern could become necessary, involving four or more circular arcs. These situations are illustrated at Figures 1‑1 and 1‑2 respectively.

1.2.3.4 Inner horizontal surface ‑ elevation datum. To satisfy the intention of the inner horizontal surface described above, it is desirable that authorities select a datum elevation from which the top elevation of the surface is determined. Selection of the datum should take account of:

(a) the elevations of the most frequently used altimeter setting datum points;

(b) minimum circling altitudes in use or required; and c) the nature of operations at the airport.

For relatively level runways the choice of datum is not critical, but when the thresholds differ by more than 6rn, the datum selected should have particular regard to the factors above. For complex inner horizontal surfaces (Figure 1‑2) a common elevation is not essential, but where surfaces overlap the lower surface should be regarded as dominant.

1.2.4 Approach and transitional surfaces

1.2.4.1 These surfaces define the volume of airspace that should be kept free from obstacles to protect an aeroplane in the final phase of the approach‑to‑land manoeuvre. Their slopes and dimensions will vary with the aerodrome reference code and whether the runway is used for visual, non‑precision or precision approaches.

1.2.5 Take‑off climb surface

1.2.5.1 This surface provides protection for an aircraft on take‑off by indicating which obstacles should be removed if possible, and marked or lighted if removal is impossible. The dimensions and slopes also vary with the aerodrome reference code.

1.2.6 The inner approach, inner transitional an balked landing surfaces

1.2.6.1 Together, these surfaces (see Figure 1‑3) define a volume of airspace in the immediate vicinity of a

precision approach runway which is known as the obstacle‑free zone (OFZ). This zone shall be kept free from fixed objects, other than lightweight frangibly mounted aids to air navigation which must be near the runway to perform their function, and from transient objects such as aircraft and vehicles when the runway is being used for category 11 or 111 ILS approaches. When an OFZ is established for a precision approach runway category 1, it shall be clear of such objects when the runway is used for category I ILS approaches.

1.2.6.2 The OFZ provided on a precision approach runway where the code number is 3 or 4 is designed to protect an aeroplane with a wingspan of 60 m on a precision approach below a height of 30 m having been correctly aligned with the runway at that height, to climb at a gradient of 3.33 per cent and diverge from the runway centre line at a splay no greater than 10 per cent. The gradient of 3.33 per cent is the lowest permitted for an all‑engine‑operating balked landing. A horizontal distance of 1800 m from threshold to the start of the balked landing surface assumes that the latest point for a pilot to initiate a balked landing is the end of the touchdown zone lighting, and that changes to aircraft configuration to achieve a positive climb gradient will normally require a further distance of 900 m which is equivalent to a maximum time of about 15 s. A slope of 33.33 per cent for the inner transitional surfaces results from a 3.33 per cent climb gradient with a splay of 10 per cent. The splay of 10 per cent is based upon recorded dispersion data in programmes conducted by two States.

1.2.6.3 The OFZ for a precision approach runway category 1 where the code number is 1 or 2 is designed to protect an aeroplane with a wing span of 30 m to climb at a gradient of 4 per cent and diverge from the runway centre line at a splay no greater than 10 per cent. The gradient of 4 per cent is that of the normal take‑off climb surface for these aeroplanes. When allied to a 10 per cent splay, it results in a slope for the inner transitional surfaces of 40 per cent. The balked landing surface originates at 60 m beyond the far end of the runway from threshold and is coincident with the take‑off climb surface for the runway.

1.3 PANS‑OPS SURFACES

General

1.3.1.1 The PANS‑OPS surfaces are intended for use by procedure designers primarily in the construction of instrument flight procedures which are designed to safeguard an aeroplane from collision with obstacles when flying on instruments. In designing procedures, the designer will determine areas (horizontally) needed for various segments of the procedure. Then he will analyse the obstacles within the determined areas, and based on this analysis he will specify minimum safe altitudes/heights for each segment of the procedure for use by pilots.

1.3.1.2 The minimum safe altitude/height specified for the final approach phase of a flight is called "obstacle clearance altitude/height (OCA/H)”. A missed approach procedure initiated by the pilot at or above this altitude/height will ensure that, even if the pilot has no outside visual reference to the ground at any point, the aeroplane will pass safely above all potentially dangerous obstacles. The pilot may descend below the OCA/H only if he has visually confirmed that the aeroplane is correctly aligned with the runway and that there are sufficient visual cues to continue the approach. The pilot is permitted to discontinue the approach at any point below the OCA/H, e.g. if the required visual reference ceases to be available. Such a late missed approach is called balked landing. Because the initiation point of the balked landing procedure is known more accurately than the initiation point of the missed approach procedure, a smaller airspace needs to be protected.

Note. ‑ Not all of the above is applicable to category III operations carried out with no decision height.

1.3.1.3 The size and dimensions of the obstacle‑free airspace needed for the approach, for the missed approach initiated at or above the OCA/H and for the visual manoeuvring (circling) procedure are specified in PANS‑OPS. Aeroplanes continuing their descent below the specified OCA/H, and therefore having visual confirmation that they are correctly aligned, are protected from obstacles by the ECAR 139 obstacle limitation surfaces and related obstacle limitation and marking/lighting requirements. Similarly, the ECAR 139 surfaces provide protection for the balked landing. In other than low visibilities, it may be necessary for the pilot to avoid some obstacles visually.

1.3.1.4 The airspace required for an approach (including missed approach and visual circling) is bounded by surfaces which do not usually coincide with the obstacle limitation surfaces specified in ECAR 139. In the case of a non‑precision approach, missed approach and visual manoeuvring, the surfaces have a rather simple form. Typical cross‑sections of such obstacle‑free airspace are shown in Figures 1‑4 and 1‑5. The plan view of such an obstacle‑free area depends on the characteristics of the navigational facility used for the approach but not on the characteristics of the aeroplane. A typical plan view is shown in Figure 1‑6.

1.3.1.5 In the case of a precision approach, the form of the obstacle‑free airspace becomes more complicated because it depends on several variables, such as aeroplane characteristics (dimensions, equipment, performance) and ILS facility characteristics (facility

performance category, reference datum height, localizer course width and the distance between the threshold and localizer antenna). The airspace can be bounded by plane or curved surfaces which have resulted in “basic ILS surfaces”, “obstacle assessment surfaces (OAS)” and the Collision Risk Model (CRM) (see further, 1.3.2 to 1.3.4 below).

1.3.2 Basic ILS surfaces. “The basic ILS surfaces” defined in PANS‑OPS represent the simplest form of protection for ILS operations. These surfaces are extensions of certain ECAR 139 surfaces, referenced to threshold level throughout and modified after threshold to protect the instrument missed approach. The airspace bounded by the basic ILS surfaces is however usually too conservative and therefore another set of surfaces, “obstacle assessment surfaces”, is specified in PANS-OPS.

1.3.3 Obstacle assessment surfaces. The obstacle assessment surfaces (OAS) establish a volume of airspace, inside which it is assumed the flight paths of aeroplanes making ILS approaches and subsequent missed approaches will be contained with sufficiently high probability. Accordingly, aeroplanes need normally only be protected from those obstacles that penetrate this airspace; objects that do not penetrate it usually present no danger to ILS operations. However, if the density of obstacles below the OAS is very high, these obstacles will add to the total risk and may need to be evaluated (see 1.5.2 below). The above airspace (funnel) is illustrated in Figure 1‑7. It is formed by a set of plane surfaces; an approach surface (W), a ground or "footprint" surface (A) and a missed approach surface W; all bounded by side surfaces (X and Y). The dimensions of the surfaces are tabulated in PANS‑OPS, Volume 11. The lateral boundaries of the funnel represent estimates of the maximum divergence of an aeroplane from the runway centre line during the approach and missed approach so that the probability of an aeroplane touching the funnel at any one point is 1:10‑7 or less. The probable flight paths, both vertical and lateral, for aeroplanes tracking the ILS beams during an approach, have been based on a consideration of possible tolerances in both the ground and airborne navigational equipment and the extent to which the pilot may allow the aeroplane to deviate from the beam whilst attempting to follow the ILS guidance (pilot age). The probable flight paths in the missed approach are based on arbitrary assumptions of minimum climb performance and maximum splay angle of the aeroplane in a missed approach manoeuvre. Note that, as mentioned in 1.3.1.5, the precise dimensions of a funnel do vary with a number of factors. Having defined this volume of airspace, simple calculations allow an OCA/H to be calculated which would protect the aeroplane from all obstacles. The difference between the basic ILS surfaces and the OAS is that the dimensions of the latter are based upon a collection of data on aircraft ILS precision approach performance during actual instrument meteorological conditions, rather than existing ECAR 139 surfaces.

1.3.4 ILS Collision Risk Model (CRM). The approach funnel of the OAS was designed against an over‑all risk budget of one accident in 10 million approaches (i.e. a target level of safety of I X 10‑7 per approach). One consequence was that an operational judgement was required to assess the acceptable density of obstacles in the vicinity of the OAS, although they might be below the surface itself. In addition, the OAS were overprotective in certain areas, because they were relatively simple plane surfaces designed to enclose a complex shape and to allow easy manual application. As a consequence of these factors, a more sophisticated method of relating obstacle heights and locations to total risk and OCA/H was developed. This method was embodied in a computer programme called the Collision Risk Model (CRM). It enables a far more realistic assessment of the effects of obstacles, both individually and collectively. The actual construction of the approach funnel (illustrated in Figure 1‑8) involves some fairly detailed mathematics and cannot be done manually. However, its application is easy, because all calculations will be done by a computer. The Collision Risk Model is widely available. (ECAA offers the service and the programme is available for purchase to interested users. For further details see 1.5 below).

1.3.5 Visual manoeuvring (circling procedure). Visual manoeuvring (circling procedure), described in the PANS‑OPS, is a visual extension of an instrument approach procedure. The size of the area for a visual manoeuvring (circling) varies with the flight speed. It is permissible to eliminate from consideration a particular sector where a prominent obstacle exists by establishing appropriate operational procedures. In many cases, the size of the area will be considerably larger than that covered by the ECAR 139 inner horizontal surface. Therefore circling altitudes /heights calculated according to PANS‑OPS for actual operations may be higher than those based only on obstacles penetrating the inner horizontal surface area.

1.3.6 Operational minima. In conclusion, it must be stressed that a runway protected only by the obstacle limitation surfaces of ECAR 139 will not necessarily allow the achievement of the lowest possible operational minima if it does not, at the same time, satisfy the provisions of the PANS‑OPS. Consequently, consideration needs to be given to objects which penetrate the PANS‑OPS surfaces, regardless of whether or not they penetrate an ECAR 139 obstacle limitation surface, and such obstacles may result in an operational penalty.

1.4 INNER TRANSITIONAL AND BALKED LANDING SURFACES VERSUS Y SURFACES AND MISSED APPROACH SURFACE

When establishing the obstacle‑free zone for precision approach category 11 operations, the Obstacle Clearance Panel (OCP) created the inner transitional and balked landing surfaces. When developing the new approach procedures contained in PANS‑OPS, Volume II, First Edition, instead of using these surfaces for obstacle assessment, the OCP used the Y surface and a new surface referred to as the missed approach surface (see Figure 1‑7). Both sets of surfaces are required. In determining the need for the two sets of surfaces, the difference between the objectives of ECAR 139 and PANS‑OPS has to be taken into account. The surfaces in PANS‑OPS are intended for assessing the impact of objects on the determination of the obstacle clearance height, which in turn is used in determining approach minima and ensuring that the minimum acceptable safety level is achieved (i.e. probability of collision with objects is not more than 1:10‑7) . ECAR 139 surfaces are intended to define the limits around airports to which objects can extend. A further difference, and one specifically associated with these surfaces, is that PANS‑OPS provides obstacle assessment for operations down to the obstacle clearance height and, for most aeroplanes, for a missed approach with one engine inoperative executed above or at this height. The ECAR 139 surfaces are intended to protect a landing from the obstacle clearance height, or a balked landing executed with all engines operative and initiated below the obstacle clearance height. In the missed approach case, the PANS‑OPS surfaces (see 1.3.2 to 1.3.4 above), which include a missed approach surface, are the controlling surfaces. The obstacle assessment surfaces (OAS) fall below a portion of the ECAR 139 inner approach surface and below that portion of the transitional surface near the end of the touchdown area. In cases such as these, the ECAR 139 surfaces are used to determine OCH. In the landings and balked landing, the inner transitional and balked landing surfaces are the controlling surfaces.

1.4.2 The PANS‑OPS and ECAR 139 surfaces are different for several reasons. A missed approach is to be executed at or above the obstacle clearance height. At this point, the aircraft can not be assumed to be aligned with the runway as precisely as in the case of a balked landing, as the pilot may never have had visual reference to the runway. The width required for executing the missed approach is therefore wider than for a balked landing; thus the use of the transitional surfaces, which are wider apart than the inner transitional surfaces. Secondly, since the missed approach may be assumed to be executed with one engine inoperative, the climb rate will be less than for a balked landing executed with all engines operating, and consequently the slope of the missed approach surface must be less than that of the balked landing surface. As the missed approach operation by definition has to be initiated at or above the obstacle clearance height, the origin of the missed approach surface may be closer to the threshold than that of the balked landing surface.

1.5 BACKGROUND OF THE COLLISION RISK MODEL

1.5.1 The Collision Risk Model (CRM) is a computer programme that calculates the probability of collision with obstacles by an aeroplane on an ILS approach and subsequent missed approach. The CRM was developed by the Obstacle Clearance Panel as a result of an extensive data collection programme followed by detailed mathematical analysis. The CRM is an important part of the criteria for ILS operations described in Part III of the PANS‑OPS, Volume II.

1.5.2 Obstacle assessment and obstacle clearance calculations can be carried out by using obstacle assessment surfaces (see 1.3.3 above). However, this manual method, although simple in concept, involves tedious numerical calculations and is thus time-consuming, particularly if the number of obstacles is high. Furthermore, it suffers from two main drawbacks:

(a) Firstly, the requirement that the OAS be of simple form (a set of plane surfaces) to allow easy manual application of the criteria, results in the surfaces being overprotective in certain areas, particularly in the vicinity of the runway. This is precisely the area where critical obstacles (glide path antenna, holding aircraft, etc.) are most likely to be sited. Hence, under the OAS criteria, such obstacles may unnecessarily prevent aeroplanes operating to low minima.

(b) Secondly, the use of the OAS implies that these surfaces could become solid walls without any operational penalty in terms of an increase in OCA/H. Clearly such a situation would degrade safety. If left entirely to the operational judgement of the procedures specialist to decide at what point there exists an excessive density of obstacles around the runway, an insufficient operational penalty could result.

1.5.3 Therefore, although the OAS criteria are designed to achieve a specified target level of safety, they may result in a greater level of safety being imposed and consequently unnecessarily prevent operations to low minima or, alternatively, they may result in the safety of operations being degraded below the required standards. The CRM has been developed in response to these problems. It will:

(a) provide risk computations (separately for all obstacles and for individual obstacles) to a specific set of conditions and runway environment; and

(b) provide minimum acceptable OCA/H values for a specific set of conditions and runway environment.

1.5.4 The CRM may also be used to assist:

(a) in aerodrome planning (in evaluating possible locations for new runways in a given geographical and obstacle environment);

(b) in deciding whether or not an existing object should be removed; and

(c) in deciding whether or not a particular new construction would result in an operational penalty (i.e. in an increase in OCA/H).

1.5.5 Doe 9274‑AN/904, entitled Manual on the Use of the Collision Risk Model (CRM) for ILS Operations, provides a comprehensive description of the CRM and instructions for its use.