Minimum safe altitude

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.

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 specifically defines it as follows in §119 of Part 91 of the Federal Aviation Regulations (FAR):

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

For a published procedure, the greater of these two altitudes is the altitude which must be adhered to during that segment, and is called the Minimum Enroute Altitude (MEA). However, on an IFR en route chart, an asterisk appears next to the MOCA if there is also a MRA on that part of the route that uses a significantly higher altitude. One may choose to fly the lower MOCA altitude, but must keep in mind navaid reception is unlikely. One may do this if utilizing GPS and does not require navaids to navigate and would like the lower altitude.

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.

On Canadian IFR High- and Low-Level charts, area minimum altitudes (AMA) are published for quadrangle areas, which provide a buffer beyond the VFR maximum elevation figure. The AMA is the lowest off-airway altitude to be used under instrument meteorological conditions (IMC) that will provide a minimum vertical clearance of 1,000 feet (AGL), or in designated mountainous terrain 2,000 feet above all obstacles located in the area specified, rounded up to the nearest 100 foot increment.

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