Loss of communication may be transitory or prolonged. It most often occurs because of inadvertent mis management of aircraft equipment by flight crew.
The term Prolonged loss of communication (PLOC) has no official definition but was used by some people in the period 1995/2005 to describe lengthy periods of loss of communication which were believed to be attribtable to technical causes but, as hard evidence was examined, also became widely attributable to pilots failing to communicate with serviceable equipment.
The term COMLOSS is used by some military agencies to refer to significant losses of communication for any reason.
In the USA, aircraft that have lost the ability to communicate via radio, are sometimes referred to as NORDO (no radio).
Whether brief or prolonged, loss of communication has obvious flight safety significance. Potentially dangerous outcomes include the following:
A pilot is unable to receive (and therefore to follow) a new clearance, leading to loss of separation and perhaps an AIRPROX;
A pilot is unable to pass important information to ATC;
Loss of communications may be interpreted as a security threat and result in military interception;
Flight Crew not altering the previous frequency on the pre-select position on a COM radio used for primary ATC communications until two way communications have been established on the new frequency.
Flight crew awareness of the likely frequency sequence en route can be helpful. In the event that a loss of communication is discovered, reference to radio navigation charts, which show ACC and TMA frequencies, can provide one to try.
Aircraft Operator SOPs which require 'contact calls' to be made to ATC at a regular interval appropriate to the radio environment in the event that there is no other exchange during that time period.
The pilot copies a radio frequency incorrectly, changes frequency before the error can be corrected and forgets to check in.
The pilot copies a frequency change correctly but fails to actually change frequency or changes to the wrong frequency.
The pilot misses a frequency change instruction because of a blocked transmission, radio interference or because it is not given until the aircraft has already left coverage of the frequency in use.
The ATCO passes a frequency change as the part of a multi-part clearance. The pilot reads back the new frequency correctly and changes frequency without waiting for the remainder of the clearance.
Ensure that flight crews, cabin crews and ground engineers are aware of the loss of communications issue through publicity.
Ensure that SOPs for copying, setting and cross-checking frequency changes, and for loss of communication are sound, and that they are followed by all pilots.
Install radio anti-blocking devices if appropriate.
Ensure the carriage and pre-flight functional checking of at least one spare headset.
Install radios with a pre-select frequency capability.
Do not switch immediately to the next sector frequency following read back of controller’s instruction. Ensure confirmation of your read back is received.
Always follow standard procedures for copying, setting and cross-checking RTF frequencies. As soon as a loss of communication is suspected, check radio equipment settings and audio panel settings and carry out a radio check.
If any part of a message for you is garbled or unclear, request confirmation or clarification.
Always use headsets during times of high RTF loading. Always wear a headset when members of the flight crew are involved in other tasks and may not be monitoring the RTF.
If the squelch control is adjusted to reduce the effect of interference, take care to ensure that transmissions from ATC or other aircraft are not cut out.
Always report any radio interference experienced whether or not it affected safe operation.
Make use of other aircraft to relay messages when operating at extreme range or when poor propagation is suspected.
If there is no suitable frequency on which to initially re-establish communications, then 121.5 MHz can be used. This frequency should also be selected if it is impossible to re-establish communications on any frequency so that any transmission from intercepting military aircraft might be heard.
Do not pass RTF frequency changes as part of a multi-part clearance.
Do not follow any frequency change with another instruction such as a heading or level change.
Listen carefully to read-back of RTF frequency changes and immediately correct any error.
On observing or being informed of radio interference, arrange for transfer of affected aircraft to another RTF frequency.
Report any radio interference through the appropriate reporting channels.
If loss of communication is suspected, attempt to contact the aircraft by other means, including relay through other aircraft, through the previous operating agency/RTF frequency and through the operator, who may be able to contact the aircraft by other means, e.g. SELCAL or ACARS.
If attempts to restore two-way communications with the aircraft are unsuccessful, inform the appropriate military authorities in accordance with national procedures and thereafter keep them informed of any action taken by the ATS unit as well as any further intended action.
When contact is not quickly established, do not delay issuing precautionary clearances to potentially conflicting aircraft because of an assumption that contact will soon be re-established.
Accidents and Incidents
The following events include Loss of Communications as a contributory factor:
TBM8, Birmingham UK, 2011 (On 12 January 2011, a privately operated Socata TBM850 light aircraft on a flight from Antwerp to Birmingham lost radio contact with ATC whilst in IMC on a non precision approach to runway 15 prior to the issue of a landing clearance and prior to checking in on the ATC TWR frequency. It continued the approach to obtain the required visual reference before landing over the top of a DHC8-400 aircraft which had lined up ready for take off in accordance with ATC instructions. No damage or personal injury resulted from the close proximity.)
A332 / A333, en-route, North West Australia, 2012 (On 31 March 2012, after the implementation of contingency ATC procedures for a period of 5 hours due to controller shortage, two Garuda A330 aircraft which had been transiting an associated Temporary Restricted Area (TRA) prior to re-entering controlled airspace were separately involved in losses of separation assurance, one when unexpectedly entering adjacent airspace from the TRA, the other when the TRA ceased and controlled airspace was restored. The Investigation did not find that any actual loss of separation had occurred but identified four Safety Issues in relation to the inadequate handling of the TRA activation by ANSP Airservices Australia.)
A319, en-route, Nantes France, 2006 (On 15 September 2006, an Easyjet Airbus A319, despatched under MEL provision with one engine generator inoperative and the corresponding electrical power supplied by the Auxiliary Power Unit generator, suffered a further en route electrical failure which included power loss to all COM radio equipment which could not then be re-instated. The flight was completed as flight planned using the remaining flight instruments with the one remaining transponder selected to the standard emergency code. The incident began near Nantes, France.)
B738 / E135, en-route, Mato Grosso Brazil, 2006 (On 29 September 2006, a Boeing 737-800 level at FL370 collided with an opposite direction Embraer Legacy at the same level. Control of the 737 was lost and it crashed, killing all 154 occupants. The Legacy's crew kept control and successfully diverted to the nearest suitable airport. The Investigation found that ATC had not instructed the Legacy to descend to FL360 when the flight plan indicated this and soon afterwards, its crew had inadvertently switched off their transponder. After the consequent disappearance of altitude from all radar displays, ATC assumed but did not confirm the aircraft had descended.)
A320, en-route, Denver CO USA, 2009 (On 21 October 2009, an Airbus 320-200 being operated by Northwest Airlines on a scheduled passenger flight from San Diego to Minneapolis-St Paul, with the Captain as PF, overflew its destination at cruise level in VMC at night by more than 100 nm, after the two pilots had become distracted in conversation and lost situational awareness. They failed to maintain radio communications with a series of successive ATC units for well over an hour. After a routine inquiry from the cabin crew as to the expected arrival time, the flight crew realised what had happened and re-established ATC contact after which the flight was completed without further incident.)