I just started to play RC planes again after 4 years leaving it behind. Anyway recently, I bought a cheap 27 mhz AM RTF plane, everything is fine except for the radio system. ya I know cheap ones.... but I managed to fly it 2 times before it crashed, and now it's fully repaired. The problem was just the radio system, after it went more than 200ft away the radio starts to get interference. I checked the electronics and found out that the Tx crystal doesnt match the Rx crystal! The Tx is 27.277 mhz and the Rx is 26.722 mhz. I'm just wondering how the radio system could still work even with different crystals. I need some advice on wether to change the crystals and match them or not, or is there some other way to improve my reception electronically? I am looking forward to buy a new radio system but I just don't want that plane to be a waste.

If the crystals are exactly as marked, the difference is 27.277-26.722 or 555KHz, about what the IF frequency should be. It is usually 455 KHZ (assuming this is an AM radio) which may fall within the design specs on that particular radio. They may use an oddball IF frequency and, if so, this may be okay. Transmitter crystals and receiver crystals are not on the same frequency, usually dual conversion is 11.7 MHZ different and single conversion 455KHZ different.

My mistake, the Tx is 27.225 and the Rx is 26.770. So that means it's a single conversion. I'll just leave the crystals then. Now I just need to solve the interceptions. Thank you.

It's hard to say for sure, but if the plane is small, it is likely that the radio is only good out to several hundred feet. This could be around the 200 feet you mentioned under the right conditions.