Filing the plan has its own conventions. Only a flight riding an organised track along its entire length, oceanic entry to oceanic exit, may abbreviate the route in Item 15 as 'NAT' followed by the track letter; join or leave a track anywhere in the middle and the flight is a random route aircraft whose full route details must be spelled out, with the track letter abbreviating nothing. The planned Mach number and flight level should be specified at the last domestic reporting point before oceanic airspace or at the track commencement point, every later change of Mach or level gets its own position and the next significant point, and the accumulated estimated elapsed time to each oceanic FIR boundary goes in Item 18 after the EET/ indicator. The plan also declares what the aircraft and crew can do, from RNP and RVSM to data link and NAT HLA approval, because separation in the region is issued against filed capability.
When the navigation degrades
Navigation failures over the ocean are rare, and rarely loud. The manual warns that their effects can be subtle or progressive, a gradual degradation that no single alarm announces, which is why the flight crew's frequent comparison checks matter even though the installations carry comparator and warning devices. Operations manuals give the crew a definition of failure to work with: a red warning light, a self-diagnosis indication, or an error over a known position exceeding the value the operator has agreed with its authority. Inertial systems drift, the manufacturers claiming under 2 NM per hour and modern laser-gyro units doing better than 1, and abnormal inertial indications after take-off have one hard consequence: analyse them on the spot, and never continue into oceanic airspace with unresolved navigation errors or a misaligned platform. Any degradation of navigation capability is reported to ATC immediately.
GNSS fails differently: the satellites are outside the aeroplane, and what is usually lost first is not accuracy but the ability to check it. The radio-navigation course taught the receiver's integrity monitoring, five satellites to detect a fault and six to exclude one; when that monitoring is unavailable, the manual has the crew compare the GNSS position against another long-range system, or, in a GPS-only fit, against a position extrapolated from the last verified fix with airspeed, heading and estimated winds. Disagreement beyond 10 NM means adopting the navigation-failure procedures, and the same 10 NM bound applies to the position uncertainty the receiver displays after a failed-satellite alert.
The three-versus-two arithmetic decides how hard fault-finding is. With three independent systems, identifying the defective one is straightforward: the odd one out loses. With only two systems disagreeing in oceanic airspace, the manual offers a ladder of emergency cross-checks: read the malfunction codes; obtain a fix, with the weather radar's range marks and bearing lines against an identifiable landmark, or ADF bearings from a long-range NDB converted to true using the variation at the aircraft, or a VOR if in range, converted using the variation at the VOR, remembering that VORs in Canada's Northern Domestic Airspace may be oriented to true north already; call a nearby aircraft on VHF and compare spot wind or groundspeed and drift; and, as a last resort, compare the flight-plan wind for the dead-reckoning position with what the systems claim. If the faulty system still cannot be identified, the accepted procedure is to fly halfway between the cross-track differences for as long as the uncertainty lasts.
Unable to continue as cleared
When an aircraft cannot continue in accordance with its ATC clearance, the first move is administrative: obtain a revised clearance whenever possible before initiating any action. Only when that cannot be done do the contingency procedures begin, and they are flown until a revised clearance arrives. The initial action is geometric: leave the cleared track by turning at least 30 degrees right or left, to intercept and maintain a parallel, same-direction track offset by 9.3 km (5 NM). The direction of that turn is not a coin toss; it weighs the aircraft's position relative to any organised track system, the direction and levels of flights on adjacent tracks, the direction to an alternate airport, any strategic lateral offset already being flown, and terrain clearance.
The rest of the initial drill keeps the aircraft seen and heard: fly a level and offset where other traffic is less likely, watch for conflicting traffic both visually and on ACAS with RA mode left on, turn on the exterior lights, keep the transponder on and squawk 7700 as appropriate when able, advise ATC of any deviation as soon as practicable by voice or CPDLC, and use the distress or urgency signal, MAYDAY or PAN PAN, preferably spoken three times. Nearby aircraft are alerted by broadcasting at suitable intervals on 121.500 MHz, with the inter-pilot air-to-air frequency 123.450 MHz as backup: identification, the nature of the distress, intentions, position with the route or track code, and flight level.
Then comes the vertical decision. If possible the assigned flight level is maintained until the aircraft is established on the 5 NM offset; if that is not possible, the rate of descent is minimised as far as operationally feasible. Once established, the crew either descends below FL 290 and sets up a 150 m (500 ft) vertical offset from the levels normally used, the option the manual points at where a predominant traffic flow or parallel track system will be crossed, or stays up and establishes the same 500 ft offset, which becomes 300 m (1,000 ft) above FL 410, in each case proceeding as the situation requires or as a subsequently obtained clearance directs. The judgement factors for diverting without a clearance include the parallel track system itself, the potential for user-preferred routes alongside, the nature of the contingency, and the weather waiting at lower levels.
Losing the radios
The general rules for a radio communications failure come from ICAO Annexes 2 and 10 and each State's AIP; what the NAT adds is a regional overlay shaped by its physics. HF over the ocean lives on the ionosphere, and ionospheric disturbances born of solar activity cluster near the magnetic poles, the northern one currently sitting at roughly 87N 150W, so flights through the North Atlantic and northern Canada can find HF degraded on some frequencies, on all of them, or across the whole region at once. The escalation is patient: try alternative HF frequencies first, then ask another radio station in the cooperating network to relay to the intended one.
When VHF and HF have both failed, the general provisions run in order of equipment: use CPDLC to reach the controlling authority when so equipped; use SATVOICE to the responsible facility via the short codes published in State AIPs; otherwise raise any ATC facility or another aircraft on VHF and ask them to relay. The inter-pilot frequency 123.450 MHz may carry relayed position reports through another aircraft, and because the whole region monitors it, 121.500 MHz may be used to establish initial contact with that aircraft, never for the regular traffic itself, before transferring to the inter-pilot frequency. A crew with a two-way ATS communications failure should broadcast regular position reports on 123.450 MHz until communications return, and a total failure, VHF, HF, CPDLC and SATVOICE all gone, gets the transponder answer: 7600 with Mode C. An HF equipment failure discovered before departure has its own administrative path: coordinate with the initial oceanic centre for any HF relief waiver and record it in Item 18 of the flight plan.
Where the compass gives up
Approach the magnetic pole and the compass quietly resigns: the horizontal field strength falls, and the manual's guidance, carried in its earlier editions, put numbers on the surrender. Below 6,000 nanotesla the magnetic compass can no longer be considered reliable; below 3,000 nanotesla it is unusable. North-west of Greenland lies an area of compass unreliability, adjoining Canadian airspace includes areas where the compass is unusable outright, and the en-route charts for the North Atlantic and polar areas draw both. The precaution is knowing what this does to the fallback chain: the compass cannot back up the inertial systems there. Basic inertial navigation itself needs no special procedures in these areas, manufacturers offer their own solutions to the remaining problems, and those solutions should not involve charts and manual measurement of direction. Before operating in the area of compass unreliability, the operator and pilots are responsible for checking with their State authorities whether specific regulatory approval or training is required.