My plane/heli seems to sink underground when I land. Or, my plane/heli crashes the instant I touch a landscape object.

Answer

This problem most often comes about when you have increased the visual scale of your aircraft, without making similar changes to physics parameters (such as the landing gear position). The landing gear are still set for a normal (scale 1.0) size airplane.

You can fix this by adjusting the landing gear (or skid) position yourself, to correspond to the new aircraft size. In the Aircraft menu, select "Edit (aircraft name)..." to open the editor. Then (for planes) click on the "fuselage" tab. Then, input new landing gear positions/dimensions. You may have to experiment to get just the right position. On the various pages of the plane editor, you can also adjust other physics parameters (wingspan, fuselage length, etc.) to more closely match the flight characteristics of a larger scale model.

If you are using helis, there are similar adjustments for the skid positions.