Minimum crossing altitude
Encyclopedia
In aviation
, a minimum crossing altitude, or MCA, is the lowest altitude
at which a navigational fix
can be crossed when entering or continuing along an airway
that will allow an aircraft to clear all obstacles while carrying out a normal climb to the required minimum en route IFR altitude
(MEA) of the airway in question beyond the fix.
The definition given here concerns primarily United States
airspace
; procedures and practices may vary in other countries.
The normal climb values used for determining MCAs in the United States are: 150 feet per nautical mile from mean sea level (MSL) to 5000 feet MSL; 120 feet per nautical mile from 5000 feet to 10,000 feet MSL; and 100 feet per nautical mile at 10,000 feet MSL or above.
For example, see the illustration above. In this case, a segment of an airway ending at fix ABC has a MEA of 5200 feet MSL, and the minimum obstacle clearance altitude
(MOCA) required to clear an obstacle six nautical miles from the fix within the next segment is 6620 feet MSL. The total increase in altitude from 5200 feet to 6620 feet over that distance is 1420 feet. At a normal rate of climb (120 feet per nautical mile at this altitude), the maximum altitude that can be gained over that distance is 720 feet, which is 700 feet below the required MOCA. Thus, a MCA for fix ABC of 5900 feet MSL (700 feet above the MEA of the previous segment) will be published for fix ABC for traffic continuing into the airway segment concerned from that fix.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
, a minimum crossing altitude, or MCA, is the lowest altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
at which a navigational fix
Fix (position)
In position fixing navigation, a position fix or simply a fix is a position derived from measuring external reference points.The term is generally used with manual or visual techniques such as the use of intersecting visual or radio position lines rather than the use of more automated and accurate...
can be crossed when entering or continuing along an airway
Airway
The pulmonary airway comprises those parts of the respiratory system through which air flows, conceptually beginning at the nose and mouth, and terminating in the alveoli...
that will allow an aircraft to clear all obstacles while carrying out a normal climb to the required minimum en route IFR altitude
Minimum enroute altitude
Minimum en route altitude , alternately spelled as Minimum enroute altitude, is the lowest published altitude between radio navigation fixes that assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes.The definition given here is that of the...
(MEA) of the airway in question beyond the fix.
The definition given here concerns primarily United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
airspace
Airspace
Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere....
; procedures and practices may vary in other countries.
Overview
Airways are normally designed such that an aircraft moving from one segment with one MEA to another segment with a higher MEA can safely begin a normal climb (see below) to the higher MEA upon crossing the fix that divides the two segments and still remain well clear of obstacles. When obstacles along the airways are such that a normal beginning at the fix defining an airway segment is not adequate to provide proper obstacle clearance, a minimum crossing altitude (MCA) is published for the fix indicating the minimum altitude at which the fix must be crossed when entering that specific airway segment in order to make it possible to safely climb to the MEA while remaining clear of obstacles. You must be at or above the MCA by the time you reach the intersection so a climb should be established prior to reaching the intersection.The normal climb values used for determining MCAs in the United States are: 150 feet per nautical mile from mean sea level (MSL) to 5000 feet MSL; 120 feet per nautical mile from 5000 feet to 10,000 feet MSL; and 100 feet per nautical mile at 10,000 feet MSL or above.
Minimum obstacle clearance altitude
Minimum 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...
(MOCA) required to clear an obstacle six nautical miles from the fix within the next segment is 6620 feet MSL. The total increase in altitude from 5200 feet to 6620 feet over that distance is 1420 feet. At a normal rate of climb (120 feet per nautical mile at this altitude), the maximum altitude that can be gained over that distance is 720 feet, which is 700 feet below the required MOCA. Thus, a MCA for fix ABC of 5900 feet MSL (700 feet above the MEA of the previous segment) will be published for fix ABC for traffic continuing into the airway segment concerned from that fix.