Light aircraft, which constitute the bulk of Visual Flight Rules (VFR) traffic, are not normally fitted with Airborne Collision Avoidance System (ACAS), but increasingly, many are fitted with transponders. These are often required for flight in controlled airspace where Transponder is a vital safety tool, and are also useful for indicating emergency situations. Outside controlled airspace, altitude encoding transponders provide a valuable disclosure of aircraft altitude to both ATC and ACAS-equipped aircaft. Even a non altitude-encoding transponder is better than no transponder.