would be cool to have that also in planning mode (then compared to the altitudes I have planned on the legs), since it helps a lot to plan my way through valleys when mountain flying (right now I always have to use the flight or sim mode)

Totally agree Fl8planner, I have asked Sky Demon for it a couple of times.  Just a look at the 2 images below shows how useful it can be.

This was an actual situation, when on a low cloud day we wanted to get in to Gap Tallard.  Without switching in and out of Flying mode to get the Terrain Safe display, we would not have found the route we did.

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Great idea, but I cannot imagine an algorithm that can handle a complete route when planning legs with different altitudes.

Proposal: tapping on a leg in planning mode switches to terrain safe display. Planned altitude for that leg is the reference altitude for the terrain safe display.

I think the only way we could do this would be to not try and be that clever. We'd have a manual option under the Mapping -> Terrain window which allowed you to put in one altitude, and the TerrainSafe would then activate for that altitude.