NAT Operating Procedures and Contingencies

Before the ocean

Longitudinal separation over the ocean is arithmetic on reported times, so the first pre-flight item is the clock: the pre-flight procedures for any NAT HLA flight must include a UTC time check, with the designated master clock synchronised to UTC or GNSS, because many aircraft master clocks can only be reset on the ground and every estimate and report of the crossing will reference that one time source. Next comes the paper spine of the flight, the master document: the master flight plan, checked for date, checked against the filed flight plan so the routing agrees, its en-route times and groundspeeds reviewed, and annotated with the estimated times over each waypoint in consistent symbols. Verification always runs the same direction: read from the FMS back to the master document, because reading from the document into the FMS breeds seeing what we expect to see.

The third ritual is the plotting or orientation chart, paper or electronic flight bag: the error-review groups rate its routine use an excellent aid against lateral errors, it becomes a critical aid in partial or total navigation failure, and the currently cleared route is what gets plotted on it. The inertial systems demand their own care: an IRS must be accurately aligned with the actual ramp position inserted before alignment, because initialisation errors introduced on the ground cannot be adequately recovered in flight, whatever the indications suggest; both pilots run silent independent checks of position and groundspeed-zero during pre-flight. A GNSS long-range fit brings the satellite availability prediction programme: predicted loss of position coverage on any part of the route means re-routing, delaying or cancelling, and in the classic figures a continuous predicted RAIM or fault-exclusion outage of more than 51 minutes in the NAT HLA had the same effect. Waypoint loading is a two-person operation in sequence and independently, one inserting, the other recalling and confirming against source, never one merely watching the other; the ramp position plus at least the first waypoints go in at the gate, and the first en-route waypoint deserves the most care. These, together with the route-against-equipage check, the currency of the track message with its TMI, the weather and alternates along the crossing, are the factors weighed before commencing the flight.

The strategic lateral offset

The strategic lateral offset procedure exists because modern navigation is too good: everyone sits exactly on the centreline unless told otherwise, so the procedure spreads them out. Offsets are flown right of the centreline only, up to a maximum of 2 NM, and offsets to the left are prohibited. The syllabus keeps the classic three-position picture, centreline, 1 NM right or 2 NM right, and the current manual refines it: aircraft able to fly offsets in tenths of a nautical mile should do so, crews should select their position randomly, and aircraft without automatic offset capability fly the centreline. The procedure is standard operating procedure for the entire NAT region, needs no ATC clearance and no advice to ATC, runs from cruising level to top of descent, may begin at the oceanic entry point and ends with a return to the centreline before the exit point unless ATC says otherwise. Two footnotes carry weight: voice position reports are based on the cleared waypoints, never the offset positions, an overtaking aircraft may offset to spare the other wake turbulence, coordinating on 123.45 where needed, and no offsets are flown below FL 285 in the Reykjavik and Bodo areas.

The separation standards stand on navigation specifications with a naming quirk the exam loves. The specification history ran ahead of the vocabulary: the approval created for the Pacific in the 1990s was labelled RNP 10 before the performance-based navigation framework defined its terms, and under that framework, since it requires no on-board performance monitoring and alerting, it is really an RNAV specification. Re-badging a worldwide approval base was judged not worth the disruption, so RNAV 10 retains the RNP 10 designation, as the navigation manual records. For the North Atlantic the rule of entry is that both aircraft and operator must be RNP 10- or RNP 4-approved by the State of the Operator or the State of Registry as appropriate, and RNP 10 is the minimum navigation specification for the application of 93 km (50 NM) lateral separation, the figure implemented where those approvals are exploited; within today's organised tracks, tighter performance-based communication and surveillance standards buy separations tighter still, 42.6 km (23 NM) laterally between suitably equipped aircraft.

Flying level in RVSM

The vertical plane runs on 300 m (1,000 ft) between FL 290 and FL 410, and its discipline is the altimeter drill. Immediately before entering the NAT HLA, a final confirmation at level flight: the two primary altimeters within 200 ft of each other, or the lesser value the flight manual sets, the standby altimeter noted, and the readings recorded with the time for use in any later contingency. Entering and flying the airspace, one automatic altitude-control system stays operative and engaged throughout the cruise, and the primary altimeters are cross-checked at every waypoint, at intervals not exceeding 60 minutes, and on reaching each new level; a difference beyond 200 ft means the altimetry is considered defective and ATC must be informed. Approaching any cleared level the vertical speed comes back to 1,500 ft per minute at most, ideally less, so the level is neither overshot nor undershot by more than 150 ft and no ACAS alert is provoked in the aircraft above or below.

Two reports guard the system. Because indirect communications leave room for ATC loop errors, a level flown that the controller does not expect, flight crews not using CPDLC or ADS-C always report to ATC immediately on leaving the current cruising level and on reaching any new cruising level. And whatever the equipment, flight crews should report every deviation of 90 m (300 ft) or more from the cleared level: the monitoring agency's vertical statistics are built from exactly these reports. The loss of one or more primary altimetry systems, or of all automatic altitude-control systems, is reported to ATC as soon as identified.

The two-system entry ticket makes the failure cases a staircase, and the manual walks each step for an aircraft equipped with two long-range navigation systems. One system fails before take-off: the crew and operator weigh delaying for repair, planning instead via the special routes and corridors that accept one system, or planning outside the high level airspace. One fails before the boundary: land short or turn back, divert through the corridors, or take a re-clearance above or below the NAT HLA. One fails after the boundary is crossed: the flight normally continues on the current clearance, with the crew assessing the remaining system's performance and the distance still to run, consulting ATC on the most suitable action, obtaining a re-clearance before any deviation, and opening a careful monitoring programme, frequent compass cross-checks, the remaining system's record watched, even visual sighting of other aircraft and their contrails or a VHF call to nearby traffic for drift, groundspeed and wind.

And the bottom step: the remaining system fails after entering the NAT HLA. Notify ATC immediately, use the sighting and VHF procedures, keep a special look-out with maximum use of exterior lights, and if no ATC instructions arrive within a reasonable period, consider climbing or descending 500 ft, broadcasting the action on 121.500 MHz and advising ATC as soon as possible; the same drill applies when the last system merely shows degradation, or when two systems diverge widely and the defective one cannot be told. Even a total navigation-computer failure leaves the raw inertial outputs, and the drill becomes the plotting chart's finest hour: steer mean true tracks extracted from the chart, and plot the position at intervals of not more than 15 minutes, adjusting heading to regain track.

Contingencies and weather

The general contingency machinery belongs to this module's first lesson, and its concept survives every variant: whenever operationally feasible, offset the assigned route by 5 NM and change level, by 500 ft below FL 410 or 1,000 ft above, from those normally used, with the current edition favouring a descent below FL 290 where the diversion will cross the flow. The direction of the initial turn weighs the direction to an alternate aerodrome, terrain clearance, the levels allocated on adjacent routes or tracks, and any strategic lateral offsets adopted by nearby traffic.

Weather gets its own procedure, built on the same bones. The request comes first, with priority available on the words WEATHER DEVIATION REQUIRED or PAN PAN; ATC either clears the deviation or declares inability, names the conflicting traffic and asks intentions. When no clearance can be obtained, the pilot deviates away from the organised track system if possible, broadcasts identification, level, position and intentions on the frequency in use and on 121.5 with 123.45 as backup, watches for traffic visually and on ACAS, and lights the aircraft. The levels then follow the geometry: within 5 NM of the cleared track, stay at the assigned level; deviating more than 5 NM, at about 5 NM from track initiate a 300 ft level change whose direction depends on the track flown and the side of the deviation: eastbound, 000 to 179 magnetic, descend 300 ft when deviating left and climb 300 ft when deviating right; westbound, 180 to 359, the mirror image, climb left and descend right. Returning, the aircraft is back at its assigned level within about 5 NM of the centreline, and the search for a proper clearance never stops.

Before the ocean

The strategic lateral offset

Check you understood

Navigation specifications

Flying level in RVSM

When a navigation system dies

Contingencies and weather