Transponders transmit a pulsed reply when interrogated with particular radio pulses. By giving ATC a four-digit transponder code plus, in Mode C, the airplane's altitude, they let controllers keep airplanes separated and prevent midair collisions.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a plane's transponder system meant to prevent?
A plane's transponder system is meant to prevent midair collisions. It listens for ground radar's 1030 MHz interrogation and replies at 1090 MHz with a unique code, barometric altitude, and, in Mode S, a 24-bit identifier. These replies give air traffic control the identity, range, and altitude needed to keep airplanes separated. The same replies feed TCAS, an airborne collision-avoidance system that interrogates nearby transponders, computes range, bearing, and altitude for every intruder, and, when a threat of MAC appears, issues a Resolution Advisory maneuver that prompts pilots to climb or descend so the conflicting aircraft pass safely. ADS-B lets aircraft share their positions with each other, boosting situational awareness and helping prevent midair collisions without waiting for ground radar. Thus, from ATC's radar screen to the cockpit's TCAS display, the transponder's continuous flow of data is the link that keeps the national airspace system coordinated and collision-free.
Can private planes turn off the transponder?
Private aircraft can turn off both ADS-B and transponder at any moment. Pilots are trained to turn the transponder on, yet a Mode A/C transponder is disabled, and ADS-B Out is disabled, because it is integrated in, or wired to, the anti-collision strobe which is also disabled. GA operators equipped with UAT transceivers squawking 1200 and flying below FL180 have the option to select anonymous mode. This permission allows the pilot to turn off the aircraft transponder inside exempted airspace. Primary radar can still track aircraft without transponder, but the FAA can suspend certificates for operating aircraft without activated transponder or ADS-B Out transmission outside those exempted areas.
What happens when a plane turns off its transponder?
When a plane turns off its transponder, it does not vanish from every screen. Civilian ATC mainly relies on secondary radar, so you will completely disappear from those consoles. Yet the flight can still be identified, monitored and traced by controllers using primary radar even if your transponder is not working or is turned off. Planes are still visible on primary radar until they get below the radar's coverage ability, so total disappearance only occurs when altitude and distance drop beneath that limit.
Controllers will immediately notice the loss: the controller alerts the pilots that the transponder signal has been lost if the plane is in contact with an air traffic controller. Turning off your transponder makes it more likely that ATC will lose you on the radar scope, and ATC cannot guarantee separation anymore based on secondary radar if the transponder is switched off or fails. In that event, ATC must revert to primary radar or non-radar separation, spacing aircraft through procedural routes instead of precise data.
The same action will be requested for safety reasons: air traffic control will tell you to turn it off because they may be getting erroneous readings if your transponder is broken and reporting the wrong code or wrong altitude. Whether done by pilots turning the knob to off or by equipment failure, controllers can still identify aircraft using primary radar, but workload rises and certainty falls until the device is restored.
What are the airplane transponder requirements?
14 CFR 91.215 regulates when and how a transponder must be installed and used. In Class A, B, and C airspace, and in the 30 nautical-mile (55.56 kilometers) Mode C veil around a Class B primary airport, an operable Mode C transponder with automatic altitude-reporting equipment is compulsory up to 10,000 feet (3048 meters) MSL. Above 10,000 feet (3048 meters) MSL in Class E airspace over the 48 contiguous states and the District of Columbia, and at all altitudes at and above FL 180, a Mode C or Mode S unit must automatically reply in 100-foot (30.48-meter) increments unless the aircraft is at or below 2,500 feet (762 meters) AGL. Any airspace that mandates a transponder also demands a Version 2 ADS-B Out system. Below 18,000 feet (5,486.4 meters) within the U.S. ADS-B rule airspace this is satisfied either by a 1090ES Mode S transponder or by UAT equipment, while at and above FL 180 only a 1090ES Mode S transmitter is accepted. Balloons, gliders and aircraft originally certificated without an engine-driven electrical system are exempted, as are operations entirely outside controlled airspace and below 10,000 feet MSL (3048 meters) beyond the 30-nm (55.56 kilometers) Mode C veil.
Is an airplane transponder check required for maintenance? Yes. FAR 91.413 requires every ATC transponder and its automatic pressure-altitude encoder to be tested and inspected within the preceding 24 calendar months by an FAA Certified Repair Station. The same interval applies to the altimeter and static system if flight under IFR is contemplated. The check must verify proper reply on the assigned Mode A code, correct Mode C altitude reporting within 300 feet (91.44 meters) of the pilot's indicated altitude, and, for Mode S units, satisfactory 1090ES ADS-B Out performance. Only after the transponder is found to meet the performance and environmental requirements of TSO-C74b, TSO-C74c, or TSO-C112, as appropriate, will it be returned to service.
