Minimum descent altitude

An instrument approach, or instrument approach procedure (IAP), is a series of predetermined maneuvers for the orderly transfer of an aircraft under instrument flight conditions from the beginning of the initial approach to a landing or to a point from which a landing may be made visually. These approaches are approved in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or enroute obstacle clearance criteria apply.

There are three types of IAPs: precision approach (PA), approach with vertical guidance (APV), and nonprecision approach (NPA). A PA uses a navigation system that provides course and glidepath deviation. Examples include precision approach radar (PAR), instrument landing system (ILS), and GBAS landing system (GLS). An APV also uses a navigation system for course and glidepath deviation, just not to the same standards as a PA. Examples include baro-VNAV, localizer type directional aid (LDA) with glidepath, LNAV/VNAV and LPV. An NPA uses a navigation system for course deviation but does not provide glidepath information. These approaches include VOR, NDB and LNAV. NPAs are flown to a minimum descent altitude (MDA).

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