You can't send SMS when the phone is in airplane mode.

Flight mode, also known as aviation mode, means that the mobile phone must be turned off during flight to avoid the influence of the transmission and reception of mobile phone signals on aircraft flight. Some mobile phones have this function, which can turn off the signal transceiver of SIM card. And it is realized by some software. For example, the system tool of S60 software has aviation mode, which can turn off the function of SIM card, so that the mobile phone is in a state of not transmitting and receiving signals, without affecting other functions of the mobile phone (generally, the mobile phone cannot operate without SIM card).