For air traffic control, communication is fundamental. Controllers give pilots instructions, clearances and information, and pilots read them back and report their position or intentions.
Most of the time, this communication works without anyone giving it a second thought.
But what happens when an aircraft suddenly stops answering?
First, There Is No Reason to Panic
An aircraft not responding to a controller does not automatically mean that something serious has happened.
There are many possible explanations.
The pilot may have selected the wrong frequency. The radio volume may have been turned down. A frequency change may have been misunderstood or missed. The aircraft may be experiencing a radio problem, or the controller’s transmission may simply not have been heard.
Sometimes communication is restored within seconds.
The first reaction is therefore usually straightforward: try again.
The controller will call the aircraft again, possibly several times.
If there is still no response, other possibilities are explored.
Trying to Re Establish Contact
Controllers have several ways of trying to reach an aircraft.
Another frequency may be used. The previous ATC unit may be contacted to check whether the aircraft is still listening on its former frequency.
Other aircraft can also help. A controller may ask another pilot to try to contact the aircraft and relay a message.
Because an aircraft at altitude may be able to hear another aircraft that ATC cannot reach directly, this can sometimes solve the problem surprisingly quickly.
The international emergency frequency 121.500 MHz, commonly referred to as 121.5 or Guard, can also be used to attempt contact.
Meanwhile, ATC continues monitoring the aircraft’s position and flight path using the surveillance information available to the controller.
Sometimes a Phone Call Solves the Problem
With smaller aircraft, particularly VFR traffic, there is another possibility that has become increasingly practical: the telephone.
If a pilot experiences a radio problem and has the appropriate telephone number available, they may be able to call the control tower or relevant air traffic services unit directly from a mobile phone.
It may sound surprisingly simple, but in suitable circumstances it can work very well.
A telephone call is not a replacement for the aircraft’s radio equipment or the established procedures for radio communication failure. Mobile coverage is not guaranteed, and using a phone may not always be practical or appropriate during flight.
But when circumstances allow it, direct telephone contact can provide controllers with extremely useful information. The pilot can explain the problem and their intentions, while ATC can establish what the aircraft is going to do.
For a VFR aircraft close to an airport, turning an apparently uncertain situation into one where the pilot and controller can communicate again can make a considerable difference.
The Transponder Can Tell ATC Something
Even when normal voice communication has been lost, the aircraft may still be visible to ATC.
The aircraft’s transponder can also provide an important indication of what is happening.
The internationally recognised transponder code for a radio communication failure is:
7600
Selecting transponder code 7600 indicates a radio communication failure.
If the pilot selects 7600, the controller immediately has additional information: the aircraft is experiencing a communication failure.
But not every aircraft that stops responding immediately selects 7600.
At first, ATC may simply have an aircraft that is no longer answering.
That uncertainty is one of the reasons controllers begin trying to re establish communication quickly.
What Does the Pilot Do?
Pilots also have procedures for communication failures.
Exactly what happens depends on factors including whether the aircraft is flying under visual or instrument flight rules, the phase of flight, the airspace and the circumstances of the failure.
For an IFR flight, established procedures provide a framework for what the aircraft should do if communication cannot be restored.
That is important for ATC.
Even without being able to speak to the pilot, controllers are not necessarily dealing with an aircraft behaving randomly. They can anticipate certain actions based on the flight plan, previous clearances and applicable procedures.
ATC can then protect other traffic accordingly.
What If It Happens During an Approach?
A communication failure becomes particularly interesting when an aircraft is approaching an airport.
The controller may have other aircraft arriving, aircraft waiting to depart and vehicles operating on or near the runway.
Normally, all of this is coordinated using radio communication.
If one aircraft can no longer communicate, controllers must take that into account when managing everything around it.
Other traffic may be delayed or repositioned to maintain a safe and predictable situation.
At a controlled airport, visual signals from the control tower can also be used in certain circumstances. Traditional light signals can communicate basic instructions to an aircraft when radio communication is unavailable.
They are rarely needed in normal airline operations, but they remain part of aviation procedures.
Does Losing Radio Contact Mean an Emergency?
Not necessarily.
A short loss of communication can have a very simple explanation and may be resolved almost immediately.
But an aircraft that remains unresponsive requires increasing attention.
Controllers do not know immediately why communication has been lost.
Is it a radio problem? Is the pilot listening on the wrong frequency? Is there an electrical problem? Or is something more serious happening on board?
ATC therefore works with the information available, attempts to restore communication and ensures that other traffic remains safely separated.
The longer an aircraft remains unresponsive, the more significant the situation can become and the more coordination may be required.
Losing Communication Does Not Mean Losing Control
From the outside, an aircraft no longer answering ATC may sound alarming.
But aviation procedures are designed with exactly these situations in mind.
Controllers try different ways of restoring communication. Pilots have procedures to follow if contact cannot be re established. Surveillance can allow ATC to continue monitoring the aircraft, and other traffic can be managed around it.
And sometimes, particularly with a smaller VFR aircraft, something as ordinary as a telephone call can help restore communication.
The radio may have gone silent.
That does not mean that everyone suddenly stops knowing what to do.