In which mode is the mode S transponder unable to send replies?

In Standby mode, the mode S transponder is unable to send replies. This is because in this mode, the transponder is effectively inactive with regard to responding to interrogations. It maintains a minimal level of readiness but does not engage fully in communication. When in Standby mode, the transponder will not actively transmit data or replies to interrogations from ground equipment or other aircraft. This is often a precautionary measure to minimize radio frequency emissions and maintain functionality for critical systems while the aircraft is not actively engaged in flight operations.

Understanding the characteristics of each mode is crucial, as it impacts situational awareness and communication efficiency during different stages of flight. Active mode allows for normal operational communication, while Normal mode also supports standard air traffic control interactions. Emergency mode, on the other hand, is designed for critical communication, allowing the transponder to send alerts and information on the aircraft's status in a dire situation.