Prolonged Loss of Communication (PLOC)
Prolonged Loss of Communication (PLOC)
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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. Pilots and controllers also use the term "NORDO," an abbreviation of "no radio."
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.
Airlines Directory For Use During PLOC
Prolonged Loss of Communication (PLOC) or COMMLOSS has serious consequences on ATM. An Airlines Directory is maintained by EVAIR (EUROCONTROL Voluntary ATM Incident Reporting) for use during prolonged loss of communication events by the ANSPs. It includes the relevant contact details of the AOs to address the issue. The directory contains the AO’s name, ICAO code, dispatch office phone number and notes providing information on the airline AO’s dispatch office working hours and availability. The contact details are strictly restricted for use in operations and in cases of PLOC, and may be accessed only by operational staff.
ATS providers wishing to obtain access to the Airlines Directory shall contact the EUROCONTROL EVAIR team at evair@eurocontrol.int.
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