Loss of communications between aircraft and ATC may occur for a variety of reasons, some technical and others resulting from mismanagement of the man-machine interface. Losses of communications can vary considerably in length; it is, however, those with an impact on day-to-day ATC functions which have drawn attention to the problems and led to studies for their resolution.
The term "PLOC", an acronym for "prolonged loss of communications", has come into use in civil aviation to describe this phenomenon, while the term "COMLOSS", an abbreviation of "communications loss", is preferred by the military.
One early communications-loss problem was known as the "sleeping receiver". Aircraft radio receivers fell silent and were reactivated only when the pilot pressed the "transmit" key. Initially, in 1998, these events were reported and investigated by only a small number of airlines, supported by the UK CAA. Investigation into the subject found that a susceptibility of certain receiver types to the use of transmitter frequency offset for ATC sectors with more than one ground transmitter - a practice in the UK at the time - was the cause of at least some instances of this and the receiver types involved were modified or replaced.
Since the terrorist attacks of 11 September 2001, PLOC events have become a much more sensitive issue because of security concerns. Any aircraft silence lasting more than a few minutes is often treated as a potential security risk. A Safety Improvement Initiative was launched by the EUROCONTROL Safety Team, addressing safety issues such as call-sign confusion, blocked transmissions, radio interference, standard phraseology and PLOC from an operational perspective.
In 2002, EUROCONTROL's Communications Domain took over the investigative task from UK Safety Regulation Group. The UK database of incidents was transferred to EUROCONTROL and extended to enable the logging of any PLOC events reported by civil or military controllers and aircrews.
The purpose of the database is to quantify PLOC phenomena across Europe and scrutinise the reports in search of common elements in order to progressively identify new PLOC incident profiles.