Which transponders are prohibited from being tuned from the FMS?

Transponder 1 and transponder 2 are prohibited from being tuned from the Flight Management System (FMS) because they are critical components of the aircraft's avionics and are typically intended to be manually operated to ensure they are set correctly for flight operations. This procedure mitigates the risk of accidental changes to transponder settings that could affect communication with air traffic control and the aircraft's ability to transmit its location and pertinent flight data.

Transponder 1 and transponder 2 serve as primary transponders, and their manual tuning provides pilots with direct control over these systems, ensuring that the appropriate codes for airspace and flight operations are being utilized. Maintaining manual control offers a level of redundancy as well, as relying solely on automated systems for critical functions can lead to vulnerabilities.

In contrast, the other transponders may be designed with different operating protocols, allowing them to be tuned through the FMS safely and effectively. Having designated systems where manual override is required emphasizes the necessity for pilot engagement and situational awareness, especially in environments with heavy traffic or specific ATC requirements.