Lost communication—often referred to as a "NORDO" (no radio) situation—is one of the most demanding scenarios in aviation. For air traffic controllers and pilots, the sudden inability to exchange instructions, reports, or acknowledgements can quickly escalate into a safety risk if not managed with disciplined, pre-coordinated procedures. While rare, these events require a calm, methodical response grounded in international standards and local protocols. This article provides a comprehensive overview of the procedures, best practices, and regulatory framework for managing lost communications with aircraft, covering both controller and pilot actions across different flight phases and environments.

Understanding the Causes and Implications of Lost Communications

Before diving into specific procedures, it is important to recognise why communications fail and what the operational consequences can be. Radio communication failures can stem from a wide variety of causes:

  • Equipment malfunction – Failure of the aircraft's VHF/HF radios, audio panels, or antennas; similar failures in ground-based transmitter/receiver equipment.
  • Frequency interference or settings error – Incorrect frequency selection, stuck microphones, or adjacent-channel interference can block both transmission and reception.
  • Power or electrical issues – Loss of aircraft electrical power, tripped circuit breakers, or depleted backup batteries.
  • Range and propagation – Particularly over oceanic or remote areas where HF radio may be unreliable, or when flying below radar and radio line-of-sight.
  • Pilot or controller error – Failure to monitor the correct frequency, inadvertent frequency change, or misinterpretation of radio procedures.

Regardless of the cause, the immediate effect is a loss of positive control information exchange. Controllers can no longer issue clearances, vectors, or traffic advisories; pilots cannot acknowledge instructions or report position. This creates a need for pre-defined, predictable actions that all parties can execute without real-time coordination.

Regulatory Basis and Global Standards

Lost communication procedures are defined and harmonized by the International Civil Aviation Organization (ICAO) in Annex 2 – Rules of the Air and Procedures for Air Navigation Services – Air Traffic Management (PANS-ATM, Doc 4444). Individual states may publish more detailed guidance in their own regulations, such as the FAA Order JO 7110.65 in the United States or the UK CAP 493 (Manual of Air Traffic Services).

The fundamental principle behind all lost comms procedures is predictability. When two-way voice communication is lost, pilots are expected to follow a prescribed set of actions based on the last received clearance, the route filed in the flight plan, and the meteorological conditions. Controllers, in turn, assume that the pilot is following these rules and apply separation accordingly.

For deeper reference, pilots and controllers should consult the following authoritative sources:

Controller Actions: Initial Verification and Assessment

When a controller suspects that communications have been lost with an aircraft, the first step is to confirm the failure rather than immediately declare an emergency. The controller should systematically eliminate other possibilities.

Step 1 – Attempt to Re-establish Contact

  • Call the aircraft on the current working frequency, clearly stating the aircraft call sign and asking for a readback.
  • Instruct the aircraft to change to a pre-agreed secondary frequency (often the sector’s alternate or next sector frequency) and attempt contact there.
  • If the aircraft is equipped with a secondary radio (e.g., VHF and HF, or radio 1 and radio 2), request that the pilot switch via a different method, such as a transponder code change or a notice broadcast on the ground frequency.
  • For aircraft within radar coverage, observe if the pilot responds to a transmission using a transponder ident (IDENT) or by squawking a specific code.

Step 2 – Verify Ground Equipment and Coordination

  • Check with adjacent controller positions to see if the same frequency is working for other aircraft.
  • Contact technical maintenance (if available) to verify that the ground transmitter and receiver for that frequency are operational.
  • If the sector uses a shared frequency or a remote site, consider a possible site outage.

Step 3 – Determine the Aircraft’s Capabilities

Knowing the aircraft’s equipment and the pilot’s likely response plan aids in predicting its future actions. Controllers should consider:

  • Is the aircraft IFR or VFR? (Most lost comms procedures differ significantly by flight rules.)
  • Is the aircraft in radar coverage? In oceanic or remote airspace, no radar data may be available.
  • Can the aircraft use CPDLC (Controller Pilot Data Link Communications) if equipped? Modern airliners often have backup text-based communication independent of voice.
  • Does the aircraft have satellite voice capability (e.g., Iridium, Inmarsat) that can be used for emergency contact?

Lost Communications Procedures for Controllers

Once it is confirmed that voice communication failure exists, controllers must implement the standard lost communication checklist. The exact sequence may vary by facility, but the following core elements are universal.

IFR Aircraft

For an aircraft operating under Instrument Flight Rules, the controller assumes the pilot will follow the regulatory lost comms route (often referred to as the “AVEF” mneumonic: by route, altitude, time/forward fix, or the filed alternate). In practice, this means:

  • The pilot will continue along the last assigned route or, if being radar vectored, will proceed directly to the last assigned fix via the most direct route and then follow the filed route.
  • The pilot will maintain the last assigned altitude for the expected period, then climb to the planned cruising altitude as filed.
  • The pilot will begin descent at the estimated time of arrival (ETA) for the destination airport and execute an instrument approach at that airport, with a missed approach if not in visual conditions.

Therefore, the controller’s job is to separate the NORDO aircraft from all other traffic under the assumption that it is following that profile. Key actions include:

  • Continue to monitor radar returns and transponder codes (7600 is the standard lost comms squawk).
  • Issue traffic advisories to other aircraft that might conflict with the expected route and altitude of the NORDO aircraft. If possible, vector other aircraft away or restrict their altitude changes.
  • Coordinate with adjacent sectors or Area Control Centers (ACCs) along the aircraft's planned route to ensure they are aware of the situation.
  • If the aircraft is in radar contact, attempt to use secondary communications methods: light gun signals (for terminal areas) or asking other aircraft to relay messages on a discrete frequency (e.g., 123.45 MHz or company frequency).
  • If CPDLC or satellite voice is available, attempt to establish contact through those means; many modern jets can receive datalink messages that are not dependent on voice radio.

VFR Aircraft

For a VFR flight, the pilot’s procedures are simpler: the pilot should squawk 7600, maintain VFR, and land as soon as practicable at the nearest suitable airport. Controllers should:

  • Clear the airspace around the NORDO aircraft if it is in radar view, ensuring other VFR and IFR traffic know its approximate position.
  • Attempt light gun signals if the aircraft is within visual range of the tower.
  • Advise other aircraft to look out for the NORDO aircraft and report its position.
  • Provide any available information (e.g., position, heading) to search and rescue if the aircraft cannot be located or does not land within expected fuel endurance.

Oceanic / Remote Airspace (No Radar)

In oceanic airspace, standard voice communication may be via HF or satellite. Lost comms here is more complex because there is no radar to confirm the aircraft’s position. Controllers rely on procedural separation based on time and altitude. The pilot will follow the same AVEF rules, but controllers must:

  • Ensure that other aircraft in the same oceanic track or adjacent tracks are separated by standard procedural separation minima (usually 10 minutes longitudinal or 1000 ft vertical).
  • Use CPDLC (ATS datalink) if the aircraft is equipped – many airlines require CPDLC for oceanic operations.
  • Attempt to re-establish contact via a different frequency or via a relay using a nearby aircraft (a common practice in oceanic operations).
  • Coordinate with adjacent oceanic centers (e.g., Gander, Shanwick, Oakland) to track the NORDO aircraft.
  • If contact remains lost, implement the Lost Communications Contingency Plan which may involve slowing or diverting other traffic.

Pilot Procedures When Communications Are Lost

Pilots are trained to handle radio failure as part of their initial and recurrent training. The following is a summary of the standard actions expected by air traffic control, based on ICAO and FAA regulations.

Immediate Steps

  • Try to re-establish contact: switch to the last frequency, try a different radio, select the alternate communication system (e.g., HF for VHF failure, or vice versa).
  • Squawk 7600 on the transponder – this code universally indicates “radio failure” (or “lost communications”).
  • If in visual meteorological conditions (VMC), the pilot should continue VFR and land as soon as practical at the nearest suitable airport.
  • If in instrument meteorological conditions (IMC) or planning to fly in IMC, follow the lost comms procedures for IFR flight.

IFR Lost Comms Procedure – The “AVEF” Principle

ICAO Annex 2 and FAA 14 CFR 91.185 prescribe a specific sequence. The pilot must operate the aircraft on the:

  • A – Last assigned route or, if vectored, direct to the last assigned fix.
  • V – Maintain the last assigned altitude for the expected period (e.g., 20 minutes) then climb to the planned cruising altitude as filed.
  • E – Descend to the approach altitude at the ETA for the destination airport (or the appropriate fix).
  • F – Execute an instrument approach at the destination airport. If the approach cannot be completed (e.g., weather below minima), execute a missed approach and hold at the missed approach fix until the expected time, then proceed to the alternate airport as filed.

These procedures are designed to be fully predictable to air traffic control. It is critical that pilots do not deviate from this plan unless they re-establish communication or visualize the airport in VMC.

Using Transponder and Light Gun Signals

In terminal areas, pilots should be familiar with light gun signals from the tower. While very rare today, they remain a backup method. The signals are standardized:

  • Steady green – Cleared to land.
  • Flashing green – Cleared to taxi / return for landing.
  • Steady red – Give way to other aircraft and continue circling.
  • Flashing red – Airport unsafe, do not land.
  • Flashing white – Return to start point.
  • Alternating red and green – Exercise extreme caution.

Pilots acknowledge light gun signals by rocking their wings (day) or flashing landing lights (night).

Special Considerations: CPDLC, SATVOICE, and Relays

Modern aircraft are increasingly equipped with digital communication systems that function independently of voice radio. Controllers and pilots should use these as primary backup when voice fails.

  • CPDLC (Contextual) – Many aircraft can send and receive text messages via datalink. Controllers can send a “(frequency) contact (frequency) on (frequency)” message if the aircraft is logged in.
  • Satellite Voice (SATVOICE) – Airlines often have a company frequency available through satellite. Controllers can coordinate with the airline’s dispatch to relay instructions.
  • VHF Relay via Other Aircraft – A common technique when the NORDO aircraft is still capable of receiving but not transmitting. A nearby aircraft can be instructed to relay a message (e.g., “AAL123, request you climb to FL360.”). This requires that the NORDO aircraft can hear the relay.

After the Incident: Documentation and Debrief

Once communications are restored or the aircraft lands safely, controllers and pilots should complete appropriate reports. For air traffic control, this may involve an ATS incident report (such as FAA Form 7230-4 or equivalent). The report should detail the timeline, the steps taken, and any lessons learned. For pilots, filing a NASA Aviation Safety Reporting System (ASRS) report or equivalent in other states is encouraged to improve safety data.

Lost comms events are excellent training opportunities. Many simulator sessions and controller on-the-job training scenarios include NORDO situations to ensure proficiency.

Conclusion: Preparedness is Key

Lost communications procedures are a cornerstone of aviation safety. By adhering to standardized, predictable actions, both controllers and pilots can manage these emergencies without real-time coordination. The key elements are clear:

  • Verify the loss thoroughly before escalating.
  • Use all available backup communication methods (transponder, light gun, CPDLC, SATVOICE).
  • Follow the prescribed route, altitude, and timing rules (AVEF for IFR).
  • Communicate the situation early to other sectors and aircraft.
  • Document and debrief after the event to improve future responses.

As technology evolves, tools like datalink and satellite voice will further reduce the impact of lost voice communications. However, the foundational human procedures—understanding, predicting, and coordinating—will always remain essential. Every controller and pilot should review their local lost comms procedures regularly, because when the radios go silent, preparation is the only link that remains.