If no MCA is specified, what is the lowest altitude for crossing a radio fix?

When no Minimum Crossing Altitude (MCA) is specified for a radio fix, the correct procedure is to cross the fix at the Minimum Enroute Altitude (MEA) for the segment of the route being flown. The MEA ensures that an aircraft maintains the required obstacle clearance along the route as well as adequate navigation signal reception.

Crossing a fix at the MEA helps pilots ensure safe vertical separation from terrain and obstacles, while also ensuring that they remain within the coverage of navigation aids. This is particularly crucial in mountainous areas or regions with extensive obstructions. By adhering to the MEA, pilots can also maintain compliance with airspace regulations and thereby enhance the safety of flight operations.

Other altitudes, such as the Minimum Reception Altitude (MRA), the Minimum Obstruction Clearance Altitude (MOCA), or the maximum safe altitude, do not serve the same purpose in this context and may not guarantee the necessary safety margins or navigation reliability when crossing a radio fix. Therefore, crossing at the MEA is the preferred practice when no MCA is specified.