I am not sure if you have to overclock för 250mhz. I have not checked it up. You need a program that uses terminal to execute though. Maybe CpuFreqUI will allow you to do this without overclocking. But i am fairly certain that you need the custom kernel for this. I have configured Alarmed (install with app manager) to switch this for me. The flight mode is a preconfigured setting in alarmed.. so you would only need to select it.. and then choose daily.. and what time it should go into flight mode.