Code 7400 may be transmitted by unmanned aircraft systems (UAS) when the control link between the aircraft and the pilot is lost. Lost link procedures are programmed into the flight management system and associated with the flight plan being flown.

In the world of aviation squawk codes, 7400 is a specialized transponder code used specifically to signal an Unmanned Aircraft System (UAS) Lost Link emergency. This code was established to alert Air Traffic Control (ATC) that a large drone or remotely piloted aircraft has lost its command-and-control connection with the ground pilot. When a transponder is set to 7400, it tells controllers that the aircraft is now following a pre-programmed "lost link" flight profile, which usually involves loitering in a specific area or automatically returning to a launch site. In 2026, as commercial drone deliveries and large-scale UAS operations have become more common in shared airspace, this code is critical for maintaining "Separation Assurance." It distinguishes a lost-link drone from other emergencies like a radio failure (7600) or a general emergency (7700). For 2026 aviation buffs, knowing this code is a sign of deep technical knowledge, as it represents the intersection of traditional "manned" air traffic safety and the rapidly evolving world of autonomous and remotely piloted flight systems.