I believe the aircraft cannot land on the ground as well as land on the water,
doing both things.
From what I know up to now (unless there is a new mod I am unaware of) this function was not made into
the IL2-1946 (yet).
As far back as when the PBY-"Black-cat" was being made, this was not possible either, that is why the wheels
were just on the airplane for historical-reasons (in the skins) but they did not function or were used. The airplane can only land on the water.
With so many breakthrough developments and new ideas already created for this game in the past years
(for example; Helicopters, modern Jets, vertical-takeoff+landing jets, HD-skins, radar, etc. etc.) one day someone will
figure out how to include this "special code" into these aircraft.
I have no idea how, but one day it may happen.