Lowest safe altitude
Operational requirements CAR 174B, AIP GEN 3.3
The pilot in command of an aircraft must not fly the aircraft at night under VFR at a height of less than 1000 ft above the highest obstacle located within 10 nm of the aircraft in flight if it is not necessary for take-off or landing.
The area to be considered must be:
- the area specified for aircraft being navigated by means of a radio navigation system or
- within a radius of 10 nm from any point along the aircraft’s nominal track.
However, an aircraft which has positively determined by visual fix that a critical obstruction has been passed may descend immediately to a lower altitude, provided that the required obstacle clearance above significant obstructions ahead of the aircraft is maintained.
An aircraft must not be flown at night under VFR lower than the published lowest safe altitude or the lowest safe altitude calculated in accordance with this section except:
- during take-off and climb in the vicinity of the departure aerodrome
- when the destination aerodrome is in sight and descent can be made within the prescribed circling area of three nm radius of the destination or
- when being radar vectored.
Lowest safe altitude (LSALT) published on aeronautical charts AIP GEN 3.3
Grid LSALTs have been determined for ERC and TAC. On ERC-H, the grid for each LSALT is a square with the dimensions of four degrees of latitude by four degrees of longitude. On ERC-L and TAC, the grid squares comprise one degree of latitude by one degree of longitude. The grid LSALT is normally displayed in the centre of the grid square.
A pilot using grid LSALT for obstacle clearance is responsible for determining the allowance for navigation error that should be applied, considering the limitations of the navigation aids or method of navigation being used for position fixing. This navigation error allowance must be applied to the proposed track. The highest grid LSALT falling within the area covered by the determined navigation error must be used.
LSALT details for RNAV routes are shown in each grid square formed by the parallels and meridians. On the ERC-H, the grid is at 4 degree intervals, and at 1 degree intervals on the ERC-L and TACs (See also AIP GEN 3.3).
On IFR charts, some LSALTs on one-way air routes have an associated direction arrow. This arrow indicates that the LSALT is only applicable in the direction of the one-way route, and an LSALT has not been calculated for the opposite direction
A LSALT without a direction arrow on any air route indicates that the LSALT is the same in both directions. However, one-way routes should only be flown, in controlled airspace, in the direction indicated by the route designator box.
On ERCs, the LSALT figure is always attached adjacent to the distance ‘bubble’ of the route to which the LSALT applies. In areas of chart clutter, these LSALT figures may sometimes cross adjacent route tracks.
LSALT not published on aeronautical charts AIP GEN 3.2 and AIP GEN 3.3
The LSALT specified for a route segment is that for IFR procedures. Where an NDB or VOR mark the segment, the tolerances applicable to the NDB are used. Unreported obstacles up to 360 ft may exist in navigation tolerance areas. Unpublished LSALTs must be calculated using the following method:
- where the highest obstacle is more than 360 ft above the height determined for terrain, the LSALT must be 1000 ft above the highest obstacle or
- where the highest obstacle is less than 360 ft above the terrain, or there is no charted obstacle, the LSALT must be 1360 ft above the elevation determined for terrain except
- where the elevation of the highest terrain or obstacle in the tolerance area is not above 500 ft, the LSALT must not be less than 1500 ft.
If the navigation of the aircraft is inaccurate, or the aircraft is deliberately flown off track, or whenever there is failure of any radio navigation aid normally available, the pilot in command must ensure that the aircraft is flown not lower than 1000 ft above the highest terrain or obstacle within a circle, centred on the DR position, with a radius of five nm plus 20 per cent of the air distance flown from the last positive fix.
For routes defined by radio navigation aids or to be navigated by DR: the area to be considered must be within an area of five nautical miles surrounding and including an area defined by lines drawn from the departure point or en route radio aid, 10.3 degrees each side of the nominated track (where the track guidance is provided by a radio navigation aid), or 15 degrees each side of the nominal track (where no track guidance is provided) to a limit of 50 nm each side of the track, thence paralleling track to abeam the destination and then converging by a semicircle of 50 nm radius centred on the destination. On shorter routes, where these lines are displaced by less than 50 nm abeam the destination, they shall converge by a radius based on the lesser distance. Where the lines thus drawn at any time come within the coverage of an en route or destination radio aid the aircraft is equipped to use, they will converge by straight lines to that aid. The minimum angle of convergence which must be used in this case is 10.3 degrees each side of track (AIP GEN 3.3).
Rated coverage AIP GEN 1.5
The following ranges are quoted for planning purposes. Actual ranges obtained may sometimes be less than these due to facility and site variations (see ERSA FAC for individual stations). The localiser ranges are for those installations that have been nominated for position fixing at ranges beyond 25 nm.
| Aircraft altitude | Range |
|---|---|
| NDB (published in ERSA FAC) | |
| VOR and DME [alt] | |
| Below 5000 ft [alt] | 60 nm |
| 5000 ft to below 10,000 ft [alt] | 90 nm |
| 10,000 ft to below 15,000 ft [alt] | 120 nm |
| 15,000 ft to below 20,000 ft [alt] | 150 nm |
| 20,000 ft and above [alt] | 180 nm |
| Localiser | |
| At 2000 ft AGL within ±10° of course line | 25 nm |
| Below 5000 ft | 30 nm |
| 5000 and above ft | 50 nm |
How to calculate LSALT
How to calculate LSALT at night
460 ft + 1000 ft = 1460 ft + 1000 ft = LSALT 2460 ft
How to calculate LSALT at night – with additional unmarked obstacle
Assuming an obstacle is 360 ft beside marked obstacle
360 ft + 1000 ft = 1360 ft + 1000 ft = LSALT 2360 ft
How to calculate LSALT with short leg between NAVAID and NAVAID
How to calculate LSALT with long leg between NAVAID and NAVAID
How to calculate LSALT with short leg between No NAVAID and NAVAID
How to calculate LSALT with long leg between No NAVAID and NAVAID
How to calculate LSALT with short leg between No NAVAID and No NAVAID
How to calculate LSALT with long leg between No NAVAID and No NAVAID
