Effective and continuous communication between aircraft and air traffic control (ATC) is fundamental to the safe and orderly management of air traffic. However, there are occasions when this communication is lost for an extended period. Such events, known in civil aviation as Prolonged Loss of Communication (PLOC) and in military terminology as COMLOSS (communications loss), are taken extremely seriously due to their potential implications for safety and security.

Table of Contents

Prolonged loss of communication

Causes of Communication Loss

Loss of communication may occur for a variety of reasons, broadly divided into two categories:

Technical Factors
- Radio receiver malfunctions.
- Transmitter frequency interference or offset issues.
- Antenna or avionics system failures.
- Blocked or overlapping transmissions on congested frequencies.
Human Factors (Man–Machine Interface Issues)
- Crew workload leading to missed frequency changes.
- Misinterpretation or mismanagement of radio equipment settings.
- Failure to follow correct communication protocols or phraseology.

The duration of communication loss can vary from a few seconds to several minutes. While short interruptions may have little operational impact, prolonged interruptions severely hinder ATC’s ability to manage air traffic safely, raising the risk of separation loss, misunderstandings, or security alerts.

Historical Context

One of the earliest documented problems associated with communication loss was referred to as the “sleeping receiver” phenomenon. In such cases, certain aircraft radio receivers would effectively fall silent, only reactivating when the pilot pressed the transmit key.

Investigations in the late 1990s, particularly in the United Kingdom under the guidance of the Civil Aviation Authority (CAA), identified that some receiver models were susceptible to transmitter frequency offsets used in multi-transmitter ATC sectors. This practice, while operationally efficient, inadvertently caused some aircraft radios to “sleep” until transmission occurred. The receiver types involved were subsequently modified or replaced, reducing the frequency of such incidents.

Post–9/11 Security Sensitivity

Following the terrorist attacks of 11 September 2001, the issue of prolonged communication loss took on a new and urgent dimension. Aircraft that remain silent for even a few minutes may now be treated as potential security threats, prompting defensive measures that could include the launch of military Quick Reaction Alert (QRA) aircraft.

As a result, what was once seen primarily as a technical or operational issue has become an integral element of aviation security. The emphasis has shifted towards early detection, rapid recovery of communications, and preventing unnecessary escalations.

EUROCONTROL’s Safety Initiatives

To address these challenges, EUROCONTROL launched a Safety Improvement Initiative that broadened the scope of investigation beyond technical malfunctions to include operational and procedural issues such as:

Call-sign confusion – misheard or misused call-signs leading to missed instructions.
Blocked transmissions – simultaneous transmissions resulting in neither message being understood.
Radio interference – from environmental factors or external sources.
Non-standard phraseology – deviations from ICAO standard phraseology causing ambiguity.
Prolonged Loss of Communication (PLOC) – requiring specific monitoring and reporting protocols.

In 2002, responsibility for PLOC investigation formally shifted from the UK’s Safety Regulation Group to EUROCONTROL’s Communications Domain. A Europe-wide database was established, consolidating data from both civil and military air traffic services and operators.

Objectives of the PLOC Database

The primary aims of the centralised database are:

Quantification: To measure the prevalence of PLOC events across Europe.
Pattern Recognition: To identify recurring causes or contributing factors.
Profile Development: To classify incident types and develop targeted mitigation strategies.
Continuous Safety Monitoring: To support ongoing safety studies and ensure lessons learned are shared with stakeholders.

Through this systematic approach, EUROCONTROL and its partners aim to progressively reduce the incidence of PLOC events and strengthen resilience against both operational risks and security concerns.

Safety Implications

PLOC events present multiple layers of risk:

Operational Safety: Reduced ATC control, loss of situational awareness, and potential loss of separation between aircraft.
Security Concerns: Silent aircraft may trigger unnecessary military responses, heightening tension and risk of escalation.
Reputational and Economic Impact: Unplanned diversions, delays, or defensive responses carry cost and reputational consequences for airlines and air navigation service providers.

The aviation community therefore treats the issue as a shared responsibility between airlines, flight crews, ATC units, regulators, and international organisations.

Prolonged Loss of Communication (PLOC) is a complex phenomenon influenced by both technical and human factors. While early incidents often stemmed from equipment limitations, modern aviation faces the additional dimension of heightened security sensitivity.

Thanks to collaborative efforts by organisations such as EUROCONTROL, IATA, ICAO, and national regulators, significant progress has been made in identifying causes, sharing safety information, and reducing risks. Nonetheless, vigilance remains essential. Preventing PLOC requires continuous training, adherence to phraseology standards, robust equipment design, and proactive international cooperation to ensure that air traffic communication remains uninterrupted — and aviation remains safe and secure.