Airplane mode can save battery because it disables or limits wireless radios that would otherwise search for networks, maintain connections and transmit data. The effect is often most noticeable where cellular coverage is weak, because a phone may work harder trying to find or hold a usable signal.
Airplane mode is not a secret low-power switch for the whole phone. Its main job is to turn off wireless transmissions required for flight rules, although modern phones often let users re-enable Wi-Fi or Bluetooth afterward. Any battery savings come from reducing radio activity.
Weak signal can be expensive
A phone’s cellular modem does not use the same amount of power in every situation. When a strong nearby signal is available, maintaining a connection can be relatively efficient. In a basement, remote area, moving train or other weak-coverage environment, the device may repeatedly search for service, change towers or transmit at higher power. That extra radio work can consume more energy even when the screen is off.
Airplane mode stops the cellular search. That is why switching it on can make a noticeable difference during a long period with no usable service. It can also reduce background network activity because apps can no longer send or receive cellular data while the radio is disabled.
Why the savings are not always dramatic
The display, processor, camera, GPS use, gaming and video playback can consume substantial power too. If the phone already has excellent reception and is sitting mostly idle, disabling cellular service may save less than expected. Likewise, if Wi-Fi is turned back on after airplane mode is enabled, the phone still has an active wireless connection and apps may continue syncing.
Bluetooth can also be re-enabled on many devices. Modern low-energy Bluetooth accessories are designed to use modest power, so their contribution may be small compared with a bright screen or heavy cellular data transfer.
When it makes practical sense
Airplane mode is most useful as a battery-saving tool when the phone cannot get reliable service and the user does not need to be reachable by cellular network. It can also be useful when someone deliberately wants to stop background communications for a while. The key idea is situational: airplane mode saves energy by removing radio tasks, not by making every part of the phone more efficient.