AIM Text
Due to increased airspeeds at 10,000 ft MSL or above, the published minimum enroute altitude (MEA) may not be sufficient for obstacle clearance when a turn is required over a fix, NAVAID, or waypoint. In these instances, an expanded area in the vicinity of the turn point is examined to determine whether the published MEA is sufficient for obstacle clearance. In some locations (normally mountainous), terrain/obstacles in the expanded search area may necessitate a higher minimum altitude while conducting the turning maneuver. Turning fixes requiring a higher minimum turning altitude (MTA) will be denoted on government charts by the minimum crossing altitude (MCA) icon (“x" flag) and an accompanying note describing the MTA restriction. An MTA restriction will normally consist of the air traffic service (ATS) route leading to the turn point, the ATS route leading from the turn point, and the required altitude; e.g., MTA V330 E TO V520 W 16000. When an MTA is applicable for the intended route of flight, pilots must ensure they are at or above the charted MTA not later than the turn point and maintain at or above the MTA until joining the centerline of the ATS route following the turn point. Once established on the centerline following the turning fix, the MEA/MOCA determines the minimum altitude available for assignment. An MTA may also preclude the use of a specific altitude or a range of altitudes during a turn. For example, the MTA may restrict the use of 10,000 through 11,000 ft MSL. In this case, any altitude greater than 11,000 ft MSL is unrestricted, as are altitudes less than 10,000 ft MSL provided MEA/MOCA requirements are satisfied.
Source: FAA Aeronautical Information Manual · current edition · paragraph 5-3-7.
The short answer
AIM 5-3-7 is FAA advisory guidance explaining the Minimum Turning Altitude (MTA), a higher altitude required where a turn over a fix, NAVAID, or waypoint at or above 10,000 ft MSL means the published MEA may not give obstacle clearance through the turn. MTA-restricted fixes are charted with the MCA icon (an “x” flag) and a note describing the restriction.
Research Notes
Common Questions
What is a Minimum Turning Altitude (MTA) and why does it exist?
Per AIM 5-3-7, because of increased airspeeds at 10,000 ft MSL or above, the published MEA may not be sufficient for obstacle clearance when a turn is required over a fix, NAVAID, or waypoint. An expanded area around the turn point is examined, and in some (normally mountainous) locations terrain or obstacles require a higher MTA during the turning maneuver.
How is an MTA depicted on the chart and what does the note look like?
AIM 5-3-7 states turning fixes requiring a higher MTA are denoted by the minimum crossing altitude (MCA) icon — the “x” flag — with a note describing the restriction. The note normally lists the ATS route to the turn point, the ATS route from it, and the required altitude; for example, MTA V330 E TO V520 W 16000.
When do I have to be at the MTA and when can I leave it?
Per AIM 5-3-7, pilots must be at or above the charted MTA not later than the turn point and maintain it until joining the centerline of the ATS route after the turn. Once established on that centerline, the MEA/MOCA determines the minimum altitude. An MTA may instead preclude a range of altitudes — e.g., restricting 10,000 through 11,000 ft MSL, leaving higher or lower altitudes unrestricted if MEA/MOCA are met.