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PilotSchool Knowledge Base

Aircraft Transponder Operations

Introduction

Aircraft transponders are critical electronic devices used to enhance communication between aircraft and Air Traffic Control (ATC). They play a pivotal role in air traffic management by broadcasting identification, altitude, and other pertinent data to ground systems and other aircraft. Understanding the operations of various transponder modes, correct code settings, and operational procedures is essential for pilots to ensure aviation safety and compliance with flight regulations.

Transponder Modes: C, S, and ADS-B

Mode C

Functionality: Mode C transponders transmit a four-digit squawk code along with pressure altitude information. This data assists ATC in maintaining separation between aircraft by providing altitude data shown in 100-feet increments on the controller’s radar screen.

Mode S

Functionality: Mode S transponders not only carry Mode C capabilities but also transmit a unique 24-bit ICAO aircraft address, which allows for selective interrogation by ATC systems. This mode supports additional functionalities such as the Traffic Collision Avoidance System (TCAS) by providing detailed information including speed and heading.

Applications: Predominantly used in commercial aviation, Mode S enhances communication between aircraft and facilitates easier traffic management by ATC.

ADS-B

Functionality: Automatic Dependent Surveillance–Broadcast (ADS-B) does not require radar interrogation to transmit its data. It automatically broadcasts an aircraft’s identity, GPS position, altitude, and velocity to ground stations and other aircraft.

Types:

  • ADS-B Out: Continuously transmits information to ATC and nearby equipped aircraft.
  • ADS-B In: Receives information broadcasts from other aircraft.

Difference from Mode S: Unlike Mode S, ADS-B relies on satellite navigation to determine and transmit precise aircraft location data, offering a more accurate and comprehensive picture of airborne traffic.

Transponder Code Settings

Common Transponder Codes

Aircraft transponders use standardized squawk codes for specific situations. These codes provide information to ATC about the nature of the flight or any issues aboard the aircraft:

  • 7500: Unlawful interference or hijacking.
  • 7600: Loss of radio communications.
  • 7700: General emergency.
  • 1200: Visual Flight Rules (VFR) flights for civilian aircraft in certain airspace regions.
  • 2000: For civil Instrument Flight Rules (IFR) flights, typically reserved for use in Class G airspace in Australia.
  • 3000: Used for civil flights in controlled airspaces such as classes A, C, and D, or IFR flights in Class E airspace.
  • 4000: Applied to civil flights operating in Class G airspace beyond 15NM offshore, not engaged in special operations or search and rescue (SAR).

For a detailed list and mnemonic tips for remembering these codes, refer to List of transponder codes - Wikipedia and Squawk Codes List and 3 Ways to Remember Them - Pilot Institute.

Memorization Techniques for Emergency Codes

Mnemonics and rhymes are useful tools:

  • “7-5, he can’t drive” (7500: hijacking).
  • “7-6, get the radio fixed” (7600: communication failure).
  • “7-7, gone to heaven” (7700: general emergency).

Transponder Settings: When to Use ON/ALT, STANDBY, or OFF

ON/ALT Mode

  • When to Use: Engaged during taxi, take-off, flight, and landing. This setting ensures continuous broadcasting of the aircraft’s identity and altitude information to ATC, allowing for active monitoring and collision avoidance.

STANDBY Mode

  • When to Use: Used while the aircraft is on the ground before take-off, and after landing before reaching the gate. The STANDBY mode reduces radio frequency congestion by halting the transmission of altitude data while not under active air traffic control.

OFF Mode

  • When to Use: Applied when the aircraft is parked, undergoing maintenance, or in situations where the transponder is not required, thus conserving power and preventing unnecessary transmissions.

Conclusion

Effective use and understanding of aircraft transponders, including the operations of Mode C, Mode S, and ADS-B, are necessary for maintaining safe airspace management. Properly setting and operating transponder codes, alongside knowledge of mode functions, greatly enhances safety and compliance with flight regulations. Ensuring these devices function correctly during critical flight phases reduces the risk of miscommunication or airspace confusion and contributes significantly to overall aviation safety.