Isn't there a difficulty option that toggles whether you actually have to land or not to complete a mission? I don't use it myself but I seem to remember seeing it.

Have you analyzed the reasons for your crashes? There are probably more than one but some are no doubt more common than others. When I was learning to land properly I discovered that airspeed was one of my major errors. You need to come in as slow as possible without stalling. After a long flight from the battle one can get impatient and approach the runway at too high a speed and then try to cut it quickly after getting too close to the runway.

One important question: Are you using the flaps? Landing without them is crazy. The IL2 has a stall speed of around 160 without flaps but with flaps fully lowered to the landing position the stall speed falls to about 140. This gives you far smoother landings and reduces the risk of breaking the landing gear on impact.

The other major issue for me was lining up for the runway, and this is still my main challenge. Give yourself a lot of space for it, i.e. don't do a 90 degree turn towerd the runway 100 meters in front of it. Furthermore, if you see you're not going to line it up well enough, don't hesitate to throttle up and come around again. Even more importantly, remember that you can land off the runway with only slighty more difficulty if there aren't hills or trees in the immediate vicinity.