Manual of Standards Part 173—Standards Applicable to Instrument Flight Procedure Design
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8.2.1.1 ICAO Annex 11 Chap 2 outlines the requirement for the publication of Lowest Safe Altitudes (LSALT) and the criteria to be used for their determination. In particular, it states that the LSALT determined shall be at least 1,000 ft above the highest obstacle located within the area concerned.
8.2.1.2 The purpose of this standard is to outline the rules and parameters to be used for the calculation of LSALT for IFR en-route operations. The particular issues addressed are:
(a) effective areas;
(b) charts;
(c) altitude tolerances;
(d) failure of radio navigation aids;
(e) applicability.
8.2.2 Effective Areas
8.2.2.1 Route defined by Radio Navigation Aids or by Dead Reckoning. The area to be considered for the LSALT calculation for a particular route or route segment must be a 5 NM area surrounding and including the route navigation tolerance area. The navigation tolerance must be based on an expanding track tolerance of ± 10.3° from the navigation aids which provide track guidance, and ±15° when no track guidance is available and DR navigation is required to be used. The maximum width of the route navigation tolerance area is 50 NM. For route or route segments served by VORs, the navigation aid coverage to be applied for the determination of the route navigation tolerance area must be the lesser of 60 NM or the maximum coverage published in ERSA. The effective areas for various circumstances are shown at paragraph 8.2.5.
8.2.2.2 Operations with Area Navigation Systems (including GPS). The area to be considered in specifying the LSALT applicable to any route or route segment must be as defined in paragraph 8.2.2.1 with an expanding track tolerance of ±10.3° to a maximum width of 30NM either side of track for conventional area navigation (RNAV) systems and 7 NM for GPS.
8.2.2.3 RNP operations. For routes flown by aircraft operating under a RNP approval, LSALT may be calculated in accordance with the standards contained in ICAO Doc 8168.
8.2.2.4 Navigation Charts. In specifying the LSALT for any latitude/longitude grid, only the area within the latitude/longitude grid need be considered.
8.2.3 Charts
8.2.3.1 The chart recommended for plotting navigation tolerance areas and the extraction of terrain/obstacle information is the ICAO 1:1,000,000 WAC Series chart. However, whenever doubt exists concerning the accuracy of the information on the WAC, larger scale maps must be used. The RAAF Joint Operations Graphic (Air) Chart, at a scale of 1:250,000, is ideal for this purpose.
8.2.3.2 The terrain to be considered in the determination of the LSALT for any route segment must be:
(a) when heights are shown on charts by spot heights and hachuring only—the highest spot height within the navigation tolerance area.
(b) where heights are shown on charts as spot heights, contours and hypsometric tints—the highest spot height within the navigation tolerance area or, where there is no spot height, the highest terrain within the intruding contours.
8.2.3.3 As spot height information shown on charts may not necessarily indicate the highest terrain in that area, it is necessary to examine the contour information to ensure that no higher terrain exists that would control the LSALT. Further, whenever contour intervals are used to determine the highest terrain, then the highest terrain must be assumed to be at the level of the next higher contour.
8.2.3.4 Obstacles shown on Australian produced WACs are generally limited to those having a height of 360 ft (110 m) or more. Therefore, whenever these charts are used for the determination of LSALTs an obstacle allowance of 360 ft must be made. Further, due to this allowance, caution must be exercised when a published tower/mast appears at first to be the critical obstacle. With the addition of the 360 ft obstacle allowance it is possible that the corrected highest terrain within the navigation tolerance area may be higher than the tower/mast first identified as the critical obstacle.
8.2.4 Altitude Tolerance
8.2.4.1 Where the corrected highest terrain (refer paragraphs 8.2.3.3 and 8.2.3.4) in the navigation tolerance area for a particular route or route segment is not above 500 ft, and there are no obstacles above 500 ft, the LSALT must be 1,500 ft.
8.2.4.2 In Australia, where the corrected highest terrain or obstacle in the navigation tolerance area for a particular route or route segment is above 500 ft, the LSALT must be 1,000 ft higher than the corrected terrain or obstacle.
8.2.4.3 For operations outside Australia where the highest terrain or obstacle in a particular route or route segment is:
(a) above 500 ft but not above 15,000 ft, the LSALT must be 1,000 ft higher than the highest terrain or obstacle, except that where the topographic reliability as shown in the WAC for any portion of the navigation tolerance area for the route or route segment is not ‘A’ (that is, compiled from accurate topographic maps and surveys), the LSALT must be 2,000 ft higher than the highest terrain in the navigation tolerance area;
(b) above 15,000 ft, the LSALT must be 2,000 ft higher than the highest terrain or obstacle in the navigation tolerance area.
8.2.4.4 The altitude calculated in accordance with the above must be rounded up to the next highest 100 ft increment.
8.2.5 Navigation Tolerance Area Construction—Examples
Overlapping nav-aid coverage on route segment with each coverage half length of segment or more or coverage of destination nav-aid half length of segment or more
Figure 8‑4: Overlapping nav-aid coverage on route segment with each coverage half length of segment or more or coverage of destination nav-aid half length of segment or more
Coverage of destination nav-aid less than half-length of segment
Figure 8‑5: Coverage of destination nav-aid less than half-length of segment
No nav-aid at departure aerodrome, nav-aid at destination with coverage such that 10.3° lines intersect 15° lines from departure aerodrome
Figure 8‑6: No nav-aid at departure aerodrome, nav-aid at destination with coverage such that 10.3° lines intersect 15° lines from departure aerodrome
No nav-aid at departure aerodrome, nav-aid at destination with coverage such that 10.3° lines do not intersect 15° lines from departure aerodrome
Figure 8‑7: No nav-aid at departure aerodrome, nav-aid at destination with coverage such that 10.3° lines do not intersect 15° lines from departure aerodrome
Nav-aid at departure aerodrome, no nav-aid at destination—segment distance not exceeding 275 NM (10.3° diverges 50 NM in 275 NM)
Figure 8‑8: Nav-aid at departure aerodrome, no nav-aid at destination—segment distance not exceeding 275 NM (10.3° diverges 50 NM in 275 NM)
Nav-aid at departure aerodrome, no nav-aid at Destination—segment distance exceeding 275 NM
Figure 8‑9: Nav-aid at departure aerodrome, no nav-aid at Destination—segment distance exceeding 275 NM
Route segment without nav-aids — segment distance not exceeding 186 NM (15° diverges 50 NM from track in 186 NM)
Figure 8‑10: Route segment without nav-aids — segment distance not exceeding 186 NM (15° diverges 50 NM from track in 186 NM)
Route segment without nav-aids—segment distance exceeding 186 NM
Figure 8‑11: Route segment without nav-aids—segment distance exceeding 186 NM
As for Figure 8 5, but 10.3° lines expand to maximum distance of 50 NM either side of track—in no case is the maximum width of basic area inside 5 NM buffer are greater than 50 NM either side of track
Figure 8‑12: As for Figure 8‑5, but 10.3° lines expand to maximum distance of 50 NM either side of track—in no case is the maximum width of basic area inside 5 NM buffer are greater than 50 NM either side of track
Route segment with intermediate reporting point (no positive fix) basic area for A-B is ADEF and for B-C is GDCF
Figure 8‑13: Route segment with intermediate reporting point (no positive fix) basic area for A-B is ADEF and for B-C is GDCF
Route segment with change of direction at D.R position — coverage of destination nav-aid not less than length of second sector of route segment
Figure 8‑14: Route segment with change of direction at D.R position — coverage of destination nav-aid not less than length of second sector of route segment
Route segment with change of direction at D.R position—coverage of destination nav-aid less than length of second sector of route segment
Figure 8‑15: Route segment with change of direction at D.R position—coverage of destination nav-aid less than length of second sector of route segment
Route segment with change of direction at DME/azimuth aid fix—coverage of destination nav-aid not less than length of second sector of route segment (NB: diagram shows DME at A; DME fix area would be different if DME at another position)
Figure 8‑16: Route segment with change of direction at DME/azimuth aid fix—coverage of destination nav-aid not less than length of second sector of route segment (NB: diagram shows DME at A; DME fix area would be different if DME at another position)
Route segment with change of direction at DME/azimuth aid fix—coverage of destination nav-aid less than length of second sector of route segment (NB: diagram shows DME at A. DME fix area would be different if DME at another position)
Figure 8‑17: Route segment with change of direction at DME/azimuth aid fix—coverage of destination nav-aid less than length of second sector of route segment (NB: diagram shows DME at A. DME fix area would be different if DME at another position)