Minimum safe altitude
Encyclopedia
Minimum safe altitude 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.
IFR flights
There are two restrictions on altitude which are important to IFR flight planning:
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 chart
s, 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.
IFR flightsInstrument flight rulesInstrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
There are two restrictions on altitude which are important to IFR flight planning:
- Minimum reception altitudeMinimum reception altitudeIn aviation, minimum reception altitude is the lowest altitude on an airway segment where an aircraft can be assured of receiving signals from navigation aids like VOR or NDB. Typically, the greater the distance between navigation aids, the higher the MRA...
, 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. - Minimum obstacle clearance altitudeMinimum obstacle clearance altitudeMinimum Obstacle Clearance Altitude, or MOCA, is the lowest published altitude in effect between fixes on VOR airways, off-airway routes, or route segments that meets obstacle clearance requirements for the entire route segment. Within the United States, this altitude also assures acceptable...
, 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
MSL
- Organizations :* Major Series Lacrosse* Major Soccer League* Malaysia Super League* MBCgame Starleague * Metal Storm Limited, an Australian modern armament company...
on IFR planning charts, and are thus the altitudes which would be indicated on the aircraft's barometric altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...
.
VFR flightsVisual flight rulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
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 pilotagePilotage
Pilotage is the use of fixed visual references on the ground or sea by means of sight or radar to guide oneself to a destination, sometimes with the help of a map or nautical chart. People use pilotage for activities such as guiding vessels and aircraft, hiking and Scuba diving...
, 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
is usually accomplished with the use of sectional chart
Sectional Chart
In United States aviation, a sectional chart, often called sectional for short, is a type of aeronautical chart designed for navigation under visual flight rules.-Overview:...
s, 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.