For the time being, a Mode C transponder is your key to regularly accessing Class A, Class B, TFR and ADIZ airspace. Under most circumstances, losing altitude squawk capability means ATC isnt going to let you into those places, although you can continue flying elsewhere. Thats one reason many operators have installed a second, back-up Mode C transponder, in case the primary box fails. But that Mode C transponder is destined to be supplemented by-maybe replaced by-the FAAs new air-traffic surveillance system, ADS-B, or automatic dependent surveillance-broadcast. As plans currently stand, well all need this technology installed in our aircraft to access various airspace types by 2020, when the FAAs final rule mandates the latest and greatest ATC system, NextGen, is implemented. Putting aside the new systems relative merits and implementation timetable, the time will come when at least ADS-B Out equipment will be required for a lot of places we want to go.