Airplane mode means you stay on but you can't receive or send signals. Can save power, can also start quickly, the harm is that people call you to find that you are off there is no harm, but direct charge or bad for the phone, because direct charge will make the battery heat, bad for the phone. Universal charging will damage the battery. But this kind of damage is negligible! Flight mode does not connect to the mobile phone signal, the better point is smaller, if the boot charging, there will be benefits