A new rule will end the hundreds of mode-S installation exemptions currently in effect. Beginning next March 1, Part 121 and 135 operators will no longer be exempted to fly without a mode-S transponder. No new exemptions will be granted after that date, though mode-A or -C transponders can continue to be used until they can no longer be repaired, after which they will have to be replaced with a permanent mode-S unit.