Your transmitter uses a battery. Unfortunately, someday it will have low voltage and it will not be enough to transmit a signal. You obviously don't want that to happen. Check your transmitter voltage as soon as you turn it on. Voltages over 10.5V should be enough for your flight. Some advanced transmitters will beep as soon as they have low voltage, and this means that you have to land your craft as fast as you can.
When the voltage is lower than 10V, you should land your craft as soon as you can and power up your transmitter with another battery.