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Concept
Lowest safe altitude
Résumé
In aviation (particularly in air navigation), lowest safe altitude (LSALT) is an altitude that is at least 500 feet above any obstacle or terrain within a defined safety buffer region around a particular route that a pilot might fly. The safety buffer allows for errors in the air by including an additional area that a pilot might stray into by flying off track. By flying at or above this altitude a pilot complies with terrain clearance requirements on that particular flight leg.
The minimum LSALT is 1500 feet.
LSALT is 1360 feet above the highest terrain where any obstacle thereon is less than 360 feet above the terrain, or there is no charted obstacle.
LSALT is 1000 feet above the highest obstacle which is greater than 360 feet above the terrain............
For example, if there is an obstacle at 200 feet above terrain of 2500 feet, LSALT is 2500 feet (terrain height) + 1360 feet (clearance height), for an LSALT of 3860 feet; if there is an obstacle at 450 feet above terrain of 3600 feet, LSALT is 4050 feet (obstacle height) + 1000 feet (clearance height), or an LSALT of 5050 feet.
The differences in the clearance heights for obstacles is due to the potential for unreported obstacles up to 360 feet which are not marked on maps or charts.
The determination of which is the highest obstacle along the flight path depends on the method of navigation (radio navigation aid, dead reckoning or area navigation systems) and on the flight rules (instrument or night VFR)
In the United States in particular, the Federal Aviation Administration calls this concept the minimum safe altitude (MSA), and is defined within the Federal Aviation Regulations (FAR):
Anywhere: an altitude allowing a safe emergency landing without undue hazard to person or property on the ground;
Over Congested Areas: an altitude of 1,000 feet above the highest obstacle within a horizontal distance of less than 2,000 feet;
Over Unpopulated Areas: an altitude of 500 feet AGL;
Over Open Water or Sparsely Populated Areas: an altitude allowing for a linear distance greater than 500 feet from any person, vessel, vehicle, or structure;
Helicopters: If without hazard to persons or property on the surface, an altitude lower than in definitions 2, 3, and 4 above, provided in compliance with any routes or altitudes specifically prescribed for helicopters by the FAA.
Source officielle
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