airplane mode= it doesn't let your phone to search for wifi or phone service thus it is saving battery life
from internet
"Airplane mode disables the same hardware functions each device. This includes:
Cellular: Your device will stop communicating with cell towers. You won’t be able to send or receive anything that depends on cellular data, from voice calls and SMS messages to mobile data.
Wi-Fi: Your phone will stop scanning for nearby Wi-Fi networks and attempting to join them. If you’re already connected to a Wi-Fi network, you’ll be disconnected.
Bluetooth: Airplane mode also disables Bluetooth, a wireless communication technology most people associate with wireless headsets. But Bluetooth can be used for many other things, including keyboards and mice.
GPS: Airplane mode also disables GPS-receiving functions, but only on some devices. This is a bit confusing and inconsistent. In theory, GPS is unlike all the other technologies here — a device with GPS turned on is only listening to GPS signals it receives, not transmitting any signals. However, some aircraft regulations do not allow the use of GPS-receiving functions for some reason.
Airplane mode can help even when you’re on the ground. It’s an excellent way to save battery power on your device. The radios on a device use a large amount of power, communicating with cell towers, scanning for nearby Wi-Fi networks and attempting to connect, waiting for incoming Bluetooth connections, and occasionally checking your location via GPS.
Turn airplane mode on and all the radios will be disabled. Bear in mind that this will block incoming phone calls and SMS messages on a phone, but it can be a great battery-saving tip if you really need that last bit of juice. It’s especially useful on a tablet when you’re just using your tablet as an offline eReader anyway"
http://www.howtogeek.com/194421/what-does-airplane-mode-do-and-is-it-really-necessary/