Section 8.3: DME/GPS Arrivals
8.3.1 Introduction to DME/GPS Arrival Procedures
8.3.1.1 A DME/GPS arrival procedure is designed to permit an aircraft to descend from an en-route altitude at or above the lowest safe altitude to a minimum altitude at an aerodrome using DME or GPS distance measurement and ground based azimuth guidance facilities. The procedure is prescribed for particular tracks or sectors and takes the form of a series of descending steps at appropriate distances.
8.3.1.2 The procedure normally uses a DME that is located close to the azimuth facility but in some cases use is made of a remote beacon. Procedures must be contained within the demonstrated coverage of the navigation aids used. Where more than one tracking aid exists at an aerodrome, tracking is normally permitted to either aid. However, only one GPS reference waypoint is nominated as the GPS distance source. The GPS reference waypoint is either the position of the azimuth aid nearest the DME for DME/GPS procedures, or the position of the primary azimuth aid for GPS only procedures. Azimuth and distance tolerances must be accounted for when determining obstacle or airspace clearance.
8.3.1.3 In designing a DME/GPS arrival, allowances are to be made for terrain clearance, navigational tolerances and errors in DME/GPS distance measurement. In addition, steps are to be designed to maintain an aircraft in controlled airspace where this is applicable to the particular route. Details of the allowances are set out in the following paragraphs.
8.3.2 DME/GPS Arrival Procedure Design
8.3.2.1 DME/GPS arrivals are instrument approach procedures. DME and DME/GPS arrival procedures are not included in ICAO DOC 8168 – OPS/611 Volume II, but are designed using the criteria contained in that document applicable to non-precision approaches and therefore have initial, intermediate and final approach segments. To account for control area steps, the step information is frequently extended beyond the initial approach fix (IAF) into the en-route segment. The method of blending the en-route segment to the procedure is shown in Figure 8‑18. Should the procedure be for a sector, the centreline of Figure 8‑18 is expanded by the size of the sector (Figure 8‑19).
Track DME/GPS arrival
Figure 8‑18: Track DME/GPS arrival
Sector DME/GPS arrival
Figure 8‑19: Sector DME/GPS arrival
8.3.2.2 DME/GPS arrivals shall:
(a) not terminate below the circling minima.
(b) normally have the following segment lengths (variations must be noted on charts by reference to DME distance):
(i) initial segment—5 NM;
(ii) intermediate segment—5 NM;
(iii) final segment—as required;
Example:
If a procedure is based on a DME which is 1 NM beyond the azimuth aid and the procedure has standard length segments, the DAP would have to show ‘IAF 16 DME, FAF 6 DME’.
(c) be designed with a MAPT;
(d) assume that manoeuvring across sectors is minimised after the intermediate fix;
(e) not utilise the 15% horizontal gradient provisions of PANS-OPS Vol II paragraph 2.8.4 for sector procedures except in the final segment.
8.3.2.3 The navigation tolerances applicable to the facility which may be used to assist azimuth guidance are contained in ICAO PANS-OPS Chap 2, as are those applicable to DME. The GPS fix tolerance is 2NM for segments at or beyond 25NM from the nominated reference aid (based on GPS receiver operating in the en-route mode) and 1 NM for segments within 25NM of the nominated reference aid (based on the receiver operating in the terminal mode).
8.3.3 Obstacle Clearance
8.3.3.1 Minimum obstacle clearance is as follows:
(a) initial approach and en-route segments – 1,000 ft;
(b) intermediate approach segment – 500 ft;
(c) final approach segment – 300 ft.
Note: 1. No allowance may be made in the final segment MOC for the use of a FAF.
Note: 2. When determining the height of terrain from contours, the next highest contour to that included in this splay will be used for clearance purposes.
8.3.4 DME/GPS Steps
8.3.4.1 These steps are determined by the radius of an arc drawn from the DME beacon or GPS reference waypoint through the primary and secondary areas. It will determine the passage of obstruction and/or control area steps or boundaries significant to the last prescribed altitude. To this radius is applied the appropriate DME or GPS fix tolerance as specified in paragraph 8.3.2.3.
8.3.4.2 Except in cases of control area boundaries, DME/GPS steps will provide descent to altitudes lower than the lowest safe altitude for the route segment.
8.3.4.3 The design of the steps should be such as to ensure that an aircraft will be able to conduct a normal type of descent without detriment to the efficiency of the procedure. For instance, the steps should be so arranged that a steady descent will result, rather than a series of steps, which if followed by an aircraft, would require large variations in the rate of descent. Figure 8‑20 shows an example of good design.
DME/GPS arrival steps
Figure 8‑20: DME/GPS arrival steps
8.3.5 Minimum Altitudes
8.3.5.1 The minimum altitudes applicable to DME/GPS arrivals will normally be the circling minimum altitudes associated with the azimuth facility. In many cases, however, it is not possible for an aircraft to descend to these altitudes, due to limitations imposed by preceding DME/GPS steps. In such cases a higher MDA may be necessary. Depending on the direction of approach it may be decided to integrate the DME/GPS arrivals with a procedure already prescribed in AIP DAP.
8.3.6 Missed Approach
8.3.6.1 The MAPT is either the azimuth facility or a point prior to this facility, and is identified for each procedure together with the ‘Missed Approach’ instruction on the procedure plate.
8.3.6.2 The missed approach areas and the obstacle clearance criteria are the same as those for Instrument Approach and Landing Procedures and are outlined in ICAO PANS-OPS, Vol II Part 3 Chap 7.
8.3.7 Method of Production
8.3.7.1 The following steps must be followed in designing DME/GPS Arrivals.
(a) Obtain appropriate WAC, 1:250 000 Military Map, 1:100,000 MAP, ERC (Area Chart VTC if available) and instrument approach chart. Ensure that the latest issue of each chart is obtained.
(b) Plot the required track or sector on the WAC and largest scale map available.
(c) Plot the primary and secondary axes around the required track or section in accordance with Figure 8‑18 and Figure 8‑19.
(d) From the ERC or Area Chart determine En-route, LSALT and airspace steps (CTA).
(e) List the elevation and DME/GPS distances of terrain and obstructions in descending order within the tolerance areas, commencing at the point where descent below the LSALT first becomes possible. In the calculation of obstacle heights, additional clearance should be made for chart error, mountainous terrain, vegetation and built-up areas.
(f) Add to the identified obstacles the applicable MOC required by paragraph 8.3.3.
(g) Plot the obstacles on a sheet of graph paper, after the determination of a suitable DME distance v Altitude scale. Then, at the top of each obstacle construct a vector parallel to the DME/ALT line and toward the navigation aid, equal to the applicable DME or GPS tolerances (see paragraph 8.3.2.3).
(h) Construct MOC DME/GPS steps by drawing a broken line from en-route LSALT to each of the corrected obstacle positions to minimum altitude (see Figure 8‑21).
DME/GPS steps
Figure 8‑21: DME/GPS steps
(i) If the route or sector is in controlled airspace, it will be necessary to arrange the descent steps so that an aircraft is kept 1,000 ft or 500 ft (as applicable) above the lower limit of the controlled airspace at all times, as well as maintaining the minimum terrain clearance.
(j) It is desirable to provide a steady rate of descent through the steps, (that is, one that can be comfortably achieved by unpressurised aircraft and easily achieved by pressurised aircraft). For this purpose 160 ft per NM is a satisfactory figure. This rate of decent, represented by the full line in Figure 8‑21, extends to the minimum altitude without infringing any of the terrain clearance steps that are represented by the dotted line.