Minimum safe altitude

Minimum safe altitude(MSA) is a concept used in planning and executing aircraft flights. It is an altitude which allows adequate vertical clearance from nearby terrain and manmade obstacles, and allows proper navigational functions.

Types of flight

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

• Above ground level