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6.6.8 Poor HF propagation conditions are the result of ionospheric disturbances. These are usually caused by sun-spot or solar flare activity creating bursts of charged particles in the solar wind which can spiral down around the Earth‟s magnetic lines of force and distort or disturb the ionised layers in the stratosphere which are utilised to refract HF radio waves. As with the Aurora Borealis, which is of similar origin, these ionospheric disturbances most commonly occur in regions adjacent to the Magnetic Poles. Since the Earth‟s North Magnetic Pole is currently located at approximately 80N 110W, flights through the North Atlantic and Northern Canada regions can, on occasion, experience resulting HF communications difficulties.
6.6.9 SATCOM Voice communications are unaffected by most ionospheric disturbances. Therefore, when so equipped, an aircraft may use SATCOM for ATC communications (See General Provisions 2 above).
6.6.10 If not SATCOM equipped, in some circumstances it may be feasible to seek the assistance, via VHF, of a nearby SATCOM equipped aircraft to relay communications with ATC (See General Provisions 3. & 4. above).
6.6.11 Whenever aircraft encounter poor HF propagation conditions that would appear to adversely affect air-ground communications generally, it is recommended that all pilots then broadcast their position reports on the air-to-air VHF frequency 123.45 MHz. Given the density of traffic in the NAT Region and the fact that in such poor propagation conditions ATC will be unable to maintain contact with all aircraft, it is important that even those aircraft that have been able to establish SATCOM contact also broadcast their position reports.
6.6.12 If for whatever reason SATCOM communications (direct or relayed) are not possible, then the following procedures may help to re-establish HF communications. Sometimes these ionospheric disturbances are very wide-spread and HF air-ground communications at all frequencies can be severely disrupted throughout very large areas (e.g. simultaneously affecting the whole of the NAT Region and the Arctic.). However, at other times the disturbances may be more localised and/or may only affect a specific range of frequencies.
6.6.13 In this latter circumstance, HF air-ground communications with the intended aeradio station may sometimes continue to be possible but on a frequency other than either the primary or secondary frequencies previously allocated to an aircraft. Hence, in the event of encountering poor HF propagation conditions pilots should first try using alternative HF frequencies to contact the intended aeradio station.
6.6.14 However, while the ionospheric disturbances may be severe, they may nevertheless only be localized between the aircraft‟s position and the intended aeradio station, thus rendering communications with that station impossible on any HF frequency. But the aeradio stations providing air-ground services in the NAT Region do co-operate as a network and it may, even then, still be possible to communicate with another aeradio station in the NAT network on HF and request that they relay communications. Efforts should therefore be made to contact other NAT aeradio stations via appropriate HF frequencies.
6.6.15 Nevertheless, as previously indicated, there are occasions when the ionospheric disturbance is so severe and so widespread that HF air-ground communications with any aeradio station within the NAT Region network are rendered impossible.
NORTH ATLANTIC MNPSA OPERATIONS MANUAL CHAPTER 6
NAT MNPS 35 Edition 2009
Rationale for Lost Communications Operational Procedures
Tactical ATC Environment
6.6.16 In a tactical ATC environment,, such as one in which Secondary Surveillance Radar and VHF voice communications are used, ATC has continuous real-time data on the position/progress of all relevant traffic and the intentions of any individual aircraft with which ATC may have lost communications can be inferred from that aircraft‟s filed flight plan. Hence, in such an environment, when voice communications with a single aircraft fail, the relevant published “lost comms procedures” normally require that aircraft to “land at a suitable aerodrome or continue the flight and adjust level and speed in accordance with the filed flight plan”. Communications blackouts affecting multiple aircraft, are not a feature of this type of VHF environment and hence in these circumstances, if required, ATC will be able to re-clear other traffic to ensure safe separations are maintained.
Procedural ATC Environment
6.6.17 However, in a (largely) non-radar environment such as the North Atlantic, ATC must rely significantly upon the HF Voice Position Reports communicated by each aircraft for position, progress and intent data. Communications equipment failures and/or poor propagation conditions can interrupt the provision of this information. Therefore, to mitigate against such occurrences in the busy NAT MNPS airspace, outside of VHF coverage ATC often employs strategic traffic planning and issues Oceanic Clearances which have been pre-co-ordinated with downstream OACs. Thereby ensuring that flights following such a pre-coordinated strategic oceanic clearance are guaranteed conflict-free progress to oceanic exit. By this means, safe NAT passage for flights continuing to adhere to such a received oceanic clearance, is ensured, even if no ATS communications are subsequently possible with any one, or even with all, of those strategically planned aircraft.
　
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