Bought this aircraft couple days ago. Its going great and I have been able to teach myself the FMC. My problem is the last 4 landings resulted in the wheels collapsing . The plane sinks into the runway with smoke and flames. I am sure I extended the wheels. Is there a locking mechanism that I have overlooked? Been in flight sim several yrs and never had this problem.Thanks in advance for any help you can give.Frank

The usual reason for this is a hard landing. Three in the green is the lock indication, there's no other mechanism. Speed and attitude are important... but, if you don't think the landings are so hard as to warrant the crash trucks I suggest you share your speed and IVSI (sink rate) just before "it" happens. There's lots of 737 experience in this forum.I broke one the other night, trying to make a single engine night landing at a high airport (KELP El Paso)... it wasn't pretty.