UA.I.B.K27: ATC transponder equipment prohibition.

ACS Area I — Regulations Task B: Operating Rules References: 14 CFRCode of Federal RegulationsCloseThe federal rulebook where Part 107, Part 89, Part 48, and other aviation rules live. parts 47, 48, 89, and 107, subpart B; ACAdvisory CircularCloseFAA guidance that explains acceptable ways to comply with rules or understand FAA procedures. 107-2; FAAFederal Aviation AdministrationCloseThe U.S. aviation regulator responsible for Part 107 rules, airspace, and pilot certification processes.-H-8083-25; FAAFederal Aviation AdministrationCloseThe U.S. aviation regulator responsible for Part 107 rules, airspace, and pilot certification processes.-G-8082-22

Key Concepts

What ATC Transponders Are (and Why This Matters to Remote Pilots)

An ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. transponder is part of the ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. Radar Beacon System (ATCRBSAir Traffic Control Radar Beacon SystemCloseSecondary surveillance radar system that uses aircraft transponders and ground interrogators.), also called secondary surveillance radar. ATCRBSAir Traffic Control Radar Beacon SystemCloseSecondary surveillance radar system that uses aircraft transponders and ground interrogators. uses three components—an interrogator on the ground, an aircraft transponder, and a radar display. It reinforces targets, speeds identification, and allows controllers to see unique squawk codes and altitude from equipped aircraft. ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized. builds on this surveillance, using satellite-based position updates at high accuracy. As surveillance tools, transponders and ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized. Out significantly enhance controller situational awareness; however, pilots must still visually scan for traffic. For aircraft that are equipped, the transponder’s altitude-reporting mode (Mode C or Mode S) should be ON and ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized. Out enabled any time the aircraft is on an airport movement area (taxiways/runways). Departures enable them before taxi and arrivals keep them on until clear of the movement area. Remote pilots should recognize that these procedures, codes, and altitude reports apply to aircraft equipped with ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. transponders—your sUASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it. will not be “seen” the same way by ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. surveillance, so do not assume ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. radar will provide separation from other aircraft. ^{[4] [1]}

Transponder Codes and Altitude Reporting: Core Facts

• 4096-code system and “00” rule: ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. uses a four-digit (4096 discrete) code set. Unless ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. specifically instructs otherwise, the last two digits of the selected code should read “00.” For example, “2100” is stated as “TWO ONE ZERO ZERO.” This is foundational knowledge for understanding what controllers expect on a squawk. ^[5]
• Mode C altitude: Automatic altitude reporting (Mode C) converts aircraft altitude to coded information in 100-foot increments and sends it with the transponder’s replies. If ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. detects a mismatch, they may instruct “STOP ALTITUDE SQUAWKTransponder code instructionCloseATC phraseology telling a pilot to set or use a specific transponder function or code., ALTITUDE DIFFERS (number of feet) FEET.” Because Mode C uses a standard 29.92 setting internally, a wrong altimeter setting affects the aircraft’s actual altitude but not the transmitted Mode C value. Installed equipment must be tested/calibrated per 14 CFRCode of Federal RegulationsCloseThe federal rulebook where Part 107, Part 89, Part 48, and other aviation rules live. §91.217; if ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. directs, turn off the altitude-reporting feature. For remote pilots, the key takeaway is that ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services.’s altitude awareness of cooperative aircraft depends on properly functioning Mode C/S equipment—something small UASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it. operations do not use to establish separation. ^[5]

Airspace Expectations: Where Transponders Are Part of “Equipage”

Airspace entry requirements for many manned VFRVisual Flight RulesCloseRules for flight using visual reference and minimum weather conditions. operations are defined by ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. clearance/communication and specific equipment, including two-way radio and a transponder with altitude reporting capability. In particular, the equipment list for certain controlled airspace classes includes “Two-way radio, transponder with altitude reporting capability.” Do not attempt to “solve” airspace access for a small UASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it. by thinking in terms of installing or operating a transponder; instead, understand that these are manned-aircraft equipage expectations. Your planning should be based on the airspace’s entry/communication requirements and ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. procedures, not on carriage of ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. surveillance equipment by the sUASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it.. ^[8]

Surveillance Designators You May See (Know What They Mean)

When flight plans are filed (manned operations), surveillance equipment is declared using designators. “A” = Mode A transponder (4096 codes), “C” = Mode A plus Mode C altitude, and various “Mode S” options (“S,” “E,” “L,” etc.) indicate aircraft identification, pressure-altitude, and enhanced capabilities such as extended squitter (ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized.). “N” means no surveillance equipment is carried or it is unserviceable. While small UASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it. do not file flight plans, recognizing these designators helps you interpret traffic and NOTAMNotice to Airmen / Notice to Air MissionsCloseOfficial notice about temporary conditions, hazards, or changes that can affect a flight./ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. context and reinforces that ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. separation is built around cooperative targets with declared transponder/ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized. capabilities. ^[7]

Practical implications for the test and real-world ops:

• ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. transponders and ADS-BAutomatic Dependent Surveillance-BroadcastCloseAircraft broadcast technology. Part 107 drones generally may not use ADS-B Out unless specifically authorized. Out are surveillance tools for cooperative aircraft; do not assume your sUASSmall Unmanned Aircraft SystemCloseA small drone plus the control and communication links used to operate it. is visible to ATCAir Traffic ControlCloseThe system that manages controlled airspace, traffic flow, and aircraft separation services. in the same way or that installing such equipment is how you gain access to controlled airspace. ^{[4] [8]}
• Expect crewed aircraft in busy airspace and on/near airports to have transponders ON with altitude reporting; maintain vigilant visual scanning and give way as required. ^[1]