If you ever forget to put your phone in flight mode, the plane won't crash right away. In the worst case scenario, the pilot could hear jamming noises. This could then lead to the pilot no longer being able to correctly understand radio signals that are important to him or her.

If you forget to enable airplane mode while flying in or out of the UK, your phone will continuously attempt to connect to ground-based cell towers at maximum power. Because the plane is moving at 500 mph and at high altitude, the phone "sees" multiple towers but cannot maintain a stable handshake, causing it to transmit a high-energy "search" signal. This creates two primary issues: first, it causes interference with the aircraft's sensitive communication and navigation equipment, which pilots may hear as an annoying "buzzing" or "clicking" sound in their headsets—similar to placing a phone too close to a speaker. While modern planes are shielded against this, it remains a distraction during critical phases like landing in low visibility. Second, it causes extreme battery drain on your device and can potentially lead to massive roaming charges if your phone accidentally connects to an "aeronautical" or "maritime" satellite network. In the UK, while forgetting is not a criminal offense, it violates airline safety policy. In extreme cases of non-compliance after being asked by crew, you could be met by airport police upon landing, as the Civil Aviation Authority (CAA) takes any interference with flight deck safety very seriously to ensure the "Swiss Cheese" model of safety remains intact.