Anybody who experienced this problem on fsx acceleration with any type of piston engine,being it fixed wing or rotor,might find helpful that there is a solution which is not as bothering as to uninstall completely the game.
The problem itself is caused by an unknown acceleration pack's file,so if you do uninstall(not repair) the acceleration pack,and install it again you should get rid of the problem.
It did work for me,and I tried everything possible from deleting the fsx.cfg file,to delete the fltsim.flt file itself,but the only thing that worked is this.Give it a try,it just takes few minutes and you won't loose any of your addons during the process.
I hope that whis will be useful to you.