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I continue to read definitions of MEA as the minimum altitude across the entire segment to ensure obstacle clearance and navigation/comm signal reception. It also states that this is the lowest altitude across the entire segment.

The way I interpret this is that as I fly towards the segment I must be at or above the MEA when I enter the segment to ensure that minimum across the entire segment. Given this why is there a need for MCA (minimum crossing altitude)? The MCA definition requires that minimum altitude at the segment boundary. Why would the MEA not be sufficient?

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MCA is for the fix and specifies the minimal altitude at which a normal climb can clear all obstacles.

MEA is the for between fixes and assures acceptable navigation signal coverage.

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source wikipedia.

The MCA can be lower than the MEA of the adjoining route so MEA is required so you won't crash into the next mountain and MEA can also be lower than MCA so it's required so that the pilot can know to climb before reaching the fix so he knows he can get over the next obstacle safely without needing to circle or zigzag to buy time for the climb.

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