A Transponder Mandatory Zone (TMZ) refers to a specific area in the airspace where using transponder equipment is essential. All flights operating within these zones must have Secondary Surveillance Radar (SSR) transponders that can function on Modes A and C or Mode S. This requirement is set by the relevant authority overseeing the airspace. However, if alternative provisions are in place for that area, pilots may follow those guidelines instead. For pilots who need to enter a TMZ without a working transponder, special arrangements can be made with the controlling authority of the zone. A good example of this is the TMZs around Stansted.
