Lowest safe altitude

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In aviation (particularly in air navigation) lowest safe altitude (LSALT) is an altitude that is at least 1,000 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.

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Australian definition

For example, if there is an obstacle at 200 feet above terrain of 2500 feet, LSALT is 2700 feet (obstacle height) + 1360 feet (clearance height), or an LSALT of 4060 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)

FAA definition

In the United States in particular, the Federal Aviation Administration calls this concept the Minimum Safe Altitude (MSA), and specifically defines it as follows in § 119 of Part 91 of the Federal Aviation Regulations (FAR):

  1. Anywhere: an altitude allowing a safe emergency landing without undue hazard to person or property on the ground;
  2. Over Congested Areas: an altitude of 1,000 feet above the highest obstacle within a horizontal distance of less than 2,000 feet;
  3. Over Populated Areas: an altitude of 500 feet AGL;
  4. Over Open Water or Sparsely Populated Areas: an altitude allowing for a linear distance greater than 500 from any person, vessel, vehicle, or structure;
  5. 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.

IFR flights

There are two restrictions on altitude which are important to IFR flight planning:

  1. Minimum reception altitude, an altitude which must be maintained across a flight segment (i.e. between two navigation radio transmitters) in order to assure reception of the required radio signals at all portions of that segment.
  2. Minimum obstacle clearance altitude, an altitude which provides a predetermined vertical clearance from known obstacles within a predetermined corridor along the specified flight segment.

For a given flight segment, the greater of these two altitudes is the altitude which must be adhered to during that segment.

These altitudes are listed as "MSL" on IFR planning charts, and are thus the altitudes which would be indicated on the aircraft's barometric altimeter.

VFR flights

Since VFR flights are not necessarily conducted on straight lines between ground-based radio navigation transmitters, the altitude restrictions for IFR flights (above) are not applicable. Instead, a VFR flight can be conducted using pilotage, watching landmarks to determine position and desired direction. In this situation, the minimum reception altitude becomes moot, and the over-riding concern is for obstacle clearance. Pilotage in the United States is usually accomplished with the use of sectional charts, which show the ground with considerable accuracy, both for terrain levels and for man-made objects. The charts are marked with grids, and at the center of each grid square a number shows the elevation (MSL) of the highest obstacle within that grid. Thus a pilot is alerted of how high he must fly while traversing that grid to assure clearance of all possible obstacles. Then it is up to the pilot to select a cruising altitude which will provide the required clearance above those obstacles.

The pilot is required to provide clearance based on several factors, one of which is the amount of human congestion below. If there is little or no human activity below, he may fly at an altitude which provides safety in the event of engine failure (i.e. no specified minimum altitude, but he must be high enough to make a successful landing if the engine fails). If there is a low density of human activity or construction in the area, he must fly 500 feet above it. If there is a congested area below, he must fly 1000 feet above it, or 2000 feet horizontally away from the area or obstruction. These requirements are contained in USA FAR Part 91.119, which governs flight activity.

On sectional charts, man-made obstacles less than 200 feet in height may not be shown.

See also


Notes

  1. ^ Aeronautical Information Publication Australia GEN 3.3 3

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