What does the Airbus A320 transponder do and how is it used?
On the Airbus A320 and in aviation flight simulators, the transponder replies to surveillance interrogations with the aircraft’s assigned four-digit code, identity and pressure altitude; Mode S also supports TCAS coordination. To use it, enter the assigned squawk, select AUTO or ON, enable altitude reporting, and press IDENT only when ATC asks.
What information does the A320 transponder send?
The A320 transponder allows air traffic control and other suitably equipped aircraft to identify and track the aeroplane. A typical A320 has two transponder units, with one selected for use and the other available as a backup.
| Transponder capability | Information provided |
|---|---|
| Mode A | The assigned four-digit squawk code |
| Mode C | Squawk code plus encoded pressure altitude |
| Mode S | Selective aircraft identification, altitude and additional surveillance data, including support for TCAS coordination |
Pressure altitude is referenced to standard pressure, not the local QNH shown on the pilot’s altimeter. ATC equipment applies the appropriate correction, so changing the altimeter setting will not fix a missing altitude report.
On suitably equipped A320 variants, the transponder installation also contributes to ADS-B Out. Detailed add-ons may obtain the Mode S flight identification from the entered flight number or simulator aircraft identity rather than from the four-digit code panel.
How do you set the A320 transponder in a flight simulator?
To set it, select one transponder, enter the assigned four-digit squawk, enable altitude reporting and choose the appropriate operating mode.
- Power the aircraft and locate the panel. The ATC/TCAS controls are on the centre pedestal, although the precise keypad or rotary-knob arrangement varies between A320 generations and add-ons. Our guide to the main A320 cockpit panels will help if the controls are difficult to find.
- Select one transponder. Choose system 1 or 2 according to the simulated operator’s procedure. Only one is normally active; select the other if the first unit has failed.
- Enter the squawk code. Use the code issued by ATC and confirm all four digits in the display. Transponder codes use only the digits 0 through 7, so the panel correctly rejects 8 and 9.
- Enable altitude reporting. If the simulated panel has a separate ALT RPTG control, switch it on. Other A320 implementations include altitude reporting automatically when the transponder is operating.
- Select AUTO or ON. AUTO is the normal choice in a faithfully modelled A320. ON can be required by a particular procedure or when simplified simulator ATC does not recognise the aircraft’s automatic mode.
- Set TCAS separately. Select TA/RA for departure when the applicable checklist calls for it. This affects collision-avoidance advisories; it does not change the squawk code.
- Use IDENT only when requested. When ATC says “squawk ident”, press IDENT once. It adds a temporary identification indication to the controller’s display and may produce little or no obvious cockpit feedback.
The exact timing belongs in the normal cockpit flow. Our A320 checklist sequence places these checks in context, while the complete A320 simulator workflow shows how transponder setup fits around engine start, taxi and departure.
Which squawk code should you enter?
Enter the code assigned by ATC; if no code has been assigned, use the regional VFR or conspicuity code specified for that airspace. 1200 is common for VFR flying in the United States and Canada, while 7000 is used in many European contexts, but neither is universal.
7500indicates unlawful interference.7600indicates radio communication failure.7700indicates a general emergency.
Do not enter a special-purpose code casually, particularly during a multiplayer session. For training, use it only in a controlled scenario where the simulated ATC service expects it.
Should the A320 transponder be in AUTO or ON?
Use AUTO in a fully modelled A320 unless the applicable procedure or simulator integration specifically requires ON.
- AUTO uses the aircraft’s air/ground logic to manage airborne and surface transponder behaviour. This is normally the realistic A320 selection.
- ON commands the selected transponder to operate without relying on automatic air/ground switching. It can solve compatibility problems with basic simulator ATC or an external ATC connection.
- STBY prevents normal operational replies. Use it when required during initial setup, shutdown or abnormal procedures—not after ATC expects to track the aircraft.
Some simplified aircraft models display AUTO in the cockpit but expose it as standby to the simulator. If ATC repeatedly reports that the transponder is off, try ON or the simulator’s generic altitude-reporting mode after confirming that electrical power and the selected unit are correct.
How do the A320 transponder and TCAS differ?
The transponder makes the A320 visible to surveillance systems, while TCAS uses transponder replies from nearby aircraft to detect collision threats.
They share an ATC/TCAS control panel on many A320s, but they are separate functions. TA provides traffic advisories; TA/RA can also issue vertical resolution advisories. An active squawk does not prove that TCAS is in TA/RA, and selecting TA/RA cannot make an unpowered or standby transponder visible to ATC.
Why can’t simulator ATC see the transponder?
Simulator ATC usually loses the aircraft because the transponder is in standby, altitude reporting is disabled or the aircraft add-on is not synchronising its panel state with the simulator.
- No aircraft return: Check electrical power, the selected transponder unit and the AUTO or ON position.
- Correct code but no altitude: Enable ALT RPTG where provided. Do not change QNH in an attempt to repair the report.
- Code keeps changing: AI assistance, an automated copilot or built-in ATC may be managing the transponder. Disable automatic transponder handling if you want manual control.
- AUTO still appears as standby: Use ON or the simulator’s generic ALT mode; this usually indicates an aircraft-to-ATC synchronisation limitation rather than incorrect Airbus procedure.
- No TCAS traffic: Confirm TA or TA/RA is selected and that the simulator’s traffic source supplies compatible transponder data. TCAS can remain empty even when your own squawk is working.
- IDENT appears to do nothing: This is often normal. Its useful indication is primarily on the controller’s display, not in the cockpit.