What are the standard emergency squawk codes?

7500 for hijack, 7600 for radio failure, and 7700 for general emergency.

What is the difference between Mode A, Mode C, and Mode S transponders?

Mode A transmits the selected code, Mode C adds altitude reporting, and Mode S provides aircraft identity and additional data.

How is a transponder code selected and entered?

The code is set using four selectors, each ranging from 0 to 7, forming a four-digit octal code.

What is the purpose of the standby (STBY) position on a transponder?

STBY powers the transponder but prevents it from replying to interrogations, typically used on the ground or when changing codes.

What is the ICAO aircraft address in Mode S operation?

It is a unique 24-bit code hardwired into the Mode S transponder, identifying the aircraft globally.

Transponder Codes and Operation

Medium4 min readRadio Navigation

Moderately Examined

Why this matters

Correct transponder operation ensures ATC can identify and track aircraft, especially in busy or emergency situations. Misuse or misunderstanding can lead to loss of radar contact, misidentification, or compromised safety.

Transponder codes, or squawk codes, are four-digit numbers set by pilots to identify aircraft to air traffic control (ATC) using Secondary Surveillance Radar (SSR). The transponder responds to ground or airborne interrogations by sending coded signals, including aircraft identity and altitude, depending on the selected mode. Understanding transponder operation and the use of emergency squawk codes is essential for safe and efficient ATC communication.

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In depth

Explanation

Transponder Codes and Operation Explained

Transponders play a crucial role in modern air traffic management by providing ATC with aircraft identification and altitude information. The transponder operates in several modes:

Mode A: Transmits the four-digit squawk code selected by the pilot, uniquely identifying the aircraft on radar.
Mode C: Sends both the squawk code and the aircraft's pressure altitude, enabling altitude readout on ATC displays.
Mode S: Adds advanced features, including a unique 24-bit ICAO aircraft address (hard-coded), aircraft identification, and the ability to exchange data with ground stations and TCAS-equipped aircraft.

Transponder codes are four digits, each ranging from 0 to 7 (octal), allowing for 4096 unique combinations. These codes are set using rotary selectors on the control panel. Standard codes include:

7500: Hijack
7600: Radio failure
7700: General emergency

The transponder panel typically features OFF, STBY (standby), ON, ALT (altitude reporting), and TEST positions. The IDENT button, when pressed, sends a special position identification (SPI) pulse, highlighting the aircraft on ATC radar for about 25 seconds.

SSR ground stations interrogate aircraft at 1030 MHz; the transponder replies at 1090 MHz. Mode S transponders can also respond to TCAS interrogations, enhancing collision avoidance capabilities.

Key Points for ATPL Transponder Operation

4096 possible codes, valid in Modes A, C, and S
Codes are four-digit octal numbers (digits 0-7)
Mode S transponders are required for IFR in much of Europe and provide both legacy and advanced data
The SPI/IDENT function is activated only by pilot input
The aircraft's ICAO address is unique and factory-set in Mode S transponders
Emergency squawk codes (7500, 7600, 7700) must be memorized
Transponder replies to both ground SSR and airborne TCAS interrogations

The essentials

Key Points

Transponder codes (squawk codes) are four-digit octal numbers (0-7 per digit), giving 4096 combinations.

Modes A and C transmit the code and, in Mode C, pressure altitude; Mode S adds aircraft address and flight data.

SSR ground stations interrogate at 1030 MHz; transponders reply at 1090 MHz.

Emergency codes: 7500 (hijack), 7600 (radio failure), 7700 (emergency).

The IDENT button sends a special position identification pulse for 25 seconds.

Mode S transponders are required for IFR in much of Europe and contain a unique ICAO address.

Transponders also respond to TCAS interrogations for airborne collision avoidance.

Watch out

Exam Traps & Typical Mistakes

Confusing the function of Mode A, Mode C, and Mode S—remember, Mode S includes all previous functions and adds unique address/data.

Assuming the SPI/IDENT pulse is automatic; it must be manually activated by the pilot.

Believing emergency codes are only for specific modes—7500, 7600, and 7700 apply regardless of mode.

Thinking the Mode S ICAO address is pilot-selectable; it is factory-programmed and unchangeable by the crew.

Mixing up the interrogation/reply frequencies (ground: 1030 MHz, aircraft reply: 1090 MHz).

Test yourself

Example Exam Questions

Question 2Medium

What is the primary function of the IDENT button on a transponder?

Question 3Medium

How many unique squawk codes can be selected on a standard SSR transponder?

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