Definition
The lowest altitude at certain fixes at which an aircraft must cross when proceeding in the direction of a higher minimum en route altitude (MEA). It ensures that an aircraft climbing toward a higher MEA segment has reached an altitude that provides required obstacle clearance before crossing the fix into that next segment.
Plain English
If the next part of your route requires a higher altitude for terrain or obstacle clearance, the minimum crossing altitude is the altitude you must already be at by the time you reach the fix where that next segment begins. You can't wait until after the fix to climb -- you have to be there by then.
Context Anchor
Seen on instrument flight rules en route charts and procedures, usually at a specific named point where the route ahead requires a higher altitude.
Derivation
Minimum comes from Latin minimus, meaning “smallest.” Altitude comes from Latin altus, meaning “high.” Together, the phrase points to the smallest allowed height at the moment you cross a specified point.
Why Pilots Care
It prevents the aircraft from dropping below safe altitude or losing navigation capability when the published route gets higher.
Intuition Check
Minimum crossing altitude does not mean the lowest altitude allowed anywhere along the route. It means the lowest altitude allowed at one specific crossing point, in the charted direction of flight.
Example Sentence 1
The chart showed an MCA of 8,000 feet at PALMR intersection eastbound, so I started my climb early to be level at 8,000 before reaching the fix.
Example Sentence 2
The chart showed a minimum crossing altitude of 4500 feet at the fix to allow a safe climb into the higher route altitude.