Several key things define whether an aircraft shows on Flight Radar and any other tracking site:

1. These sites do filter some aircraft and choose not to display them.
2. A plane may choose to turn off their transponder. Military aircraft in particular are exempt from many civil rules and regulations which would see most aircraft having an active transponder and of course their operational requirements will dictate they often do. That said, ultimately, any plane CAN disable it if they wish.
3. Flight tracking hardware work with older transponders. Old transponders require a spinning RADAR dish to pinpoint a bearing and can not be triangulated using static receivers. While most civil aircraft use newer transponders, again, the military is often exempt.

Several key things define whether an aircraft shows on Flight Radar and any other tracking site:

1. These sites do filter some aircraft and choose not to display them.
2. A plane may choose to turn off their transponder. Military aircraft in particular are exempt from many civil rules and regulations which would see most aircraft having an active transponder and of course their operational requirements will dictate they often do. That said, ultimately, any plane CAN disable it if they wish.
3. Flight tracking hardware used to feed these sites doesnt work with older transponders. Old transponders require a spinning RADAR dish to pinpoint a bearing and can not be triangulated using static receivers. While most civil aircraft use newer transponders, again, the military is often exempt.