Why do my legs fall through the ground?

Logically, I did everything right. Added 4 points on the legs of the aircraft, determined the upper surface of the earth

Answer 1

[ScottB]

"determined the upper surface of the Earth"

The actual terrain height at a point on the Earth is the WGS-84 ellipsoid plus the EGM-96 geoid height over ellipsoid (can be positive or negative) plus the terrain height above geoid (like dted data). What did you use to determine your upper surface?

[Ilya]

I used the top frame for contact

[Ilya]

The earth is a simple rectangle