Oceanic operations for first-timers: NAT HLA, CPDLC, ADS-C and contingency

In short

• In the North Atlantic, NAT HLA extends from FL285 to FL420 and requires a specific operator approval; NAT Doc 007 is the reference manual.
• CPDLC is text communication with ATC; ADS-C sends the aircraft’s position automatically, by contract. The GOLD manual (Doc 10037) covers both.
• Contingency per Doc 4444: turn at least 30° to a 5 NM offset, a level offset by 500 ft, broadcast on 121.5 MHz and exterior lights on.

Pilots coming from domestic narrowbody flying are used to radar for most of the flight, VHF always at hand and ATC calling when something looks wrong. In the middle of the ocean, much of that disappears. ATC knows where you are because the aircraft reports it, the conversation is by text and aircraft fly on closely spaced parallel routes.

This is an introduction. Detailed procedures vary by region and are updated often. Use the documents listed at the end, always in their current edition, together with your operator’s manuals.

Why the ocean is different

• No radar on most of the route. Separation depends on reported positions and calculated time or distance, not on a controller watching a screen.
• Long-range communication, by HF voice, which is noisy, or by datalink.
• Precision navigation. The airspace is designed for aircraft that hold their route very accurately. A gross navigation error puts you on someone else’s track.
• Far from everything. Alternates hundreds of miles away affect fuel, depressurisation and medical emergencies.

NAT HLA: North Atlantic airspace

The North Atlantic is the busiest oceanic airspace in the world and the usual reference. According to ICAO NAT Doc 007, the NAT HLA (North Atlantic High Level Airspace) extends from FL285 to FL420 inclusive, in the oceanic areas defined in the document. To fly there, the operator needs a specific approval from its State, in addition to meeting navigation and RVSM requirements.

Much of the traffic uses the Organized Track System (OTS), published daily according to the winds: one set for the eastbound night flow and another for the westbound day flow. Before entering oceanic airspace, the aircraft receives an oceanic clearance with route, level and, in some cases, a Mach number.

CPDLC and ADS-C

CPDLC (Controller-Pilot Data Link Communications) is the exchange of text messages between pilot and controller: clearances, level requests, re-routes and reports. It reduces dependence on HF and misunderstandings. The golden rule is the same as for voice: read the whole message, make sure you understand it, and only then accept.

ADS-C (Automatic Dependent Surveillance – Contract) makes the aircraft send its position and intent automatically, according to “contracts” set up with ATC: periodically, on events (such as passing a waypoint or deviating from the route) or on demand. That is how ATC knows where you are without radar.

The reference for both is the ICAO GOLD manual, Doc 10037. In the North Atlantic, NAT Doc 007 describes the datalink requirement for most of the airspace between FL290 and FL410, with the exemptions published in the document itself.

SLOP: the strategic offset

Because aircraft navigate so precisely, two aircraft on the same track at adjacent levels end up exactly on top of each other. SLOP (Strategic Lateral Offset Procedure), described in ICAO Doc 4444, allows flying offset to the right of the route centre line, up to 2 NM, to spread the risk. The offset is always to the right, and in the North Atlantic NAT Doc 007 encourages its use as standard practice.

Contingency: when you can’t maintain your clearance

Depressurisation, engine failure, a medical emergency: sometimes you must leave the route or level before you can get a new clearance. ICAO Doc 4444 (PANS-ATM), in its special procedures for in-flight contingencies in oceanic airspace, defines a standard procedure. The current version, summarised:

1. Leave the route by turning at least 30° to the right or left, and establish a parallel, same-direction track offset by 5 NM.
2. To choose the side, consider your position relative to other routes and the track system, the direction to your alternate, terrain and the SLOP offset you were flying.
3. Once established on the offset, climb or descend to a level that differs by 500 ft from those normally used. If unable to maintain level, minimise the rate of descent.
4. Tell and be seen: inform ATC as soon as practicable, broadcast position and intentions on 121.5 MHz (or 123.45 MHz as a backup), turn on exterior lights, keep the transponder on and watch for traffic visually and on TCAS.

The older version, which many pilots still remember, used a 90° turn and a 15 NM offset. It has been replaced. If your study material still says 15 NM, it is out of date.

Weather deviation without a clearance

If you must deviate around weather and cannot get a clearance in time, Doc 4444 also defines what to do. Communicate your intentions, move away from the track system if possible and, for deviations of up to 5 NM, stay at your level. For larger deviations, once 5 NM from the centre line, adjust your level according to the route direction:

When returning to within 5 NM of the centre line, return to your cleared level. This table is a classic technical interview question: understand the logic instead of memorising it.

Habits that prevent navigation errors

• Check the FMS route against the oceanic clearance, waypoint by waypoint, with both pilots.
• Watch latitude/longitude waypoints: one wrong digit takes the aircraft somewhere else.
• Do a position check shortly after each waypoint and follow the navigation on a plotting chart or its electronic equivalent.
• Read back every CPDLC clearance to yourself before accepting and executing it.

FAQ

References

• ICAO. Doc 4444, PANS-ATM, special procedures for in-flight contingencies in oceanic airspace and the strategic lateral offset procedure (SLOP).
• ICAO. NAT Doc 007, North Atlantic Operations and Airspace Manual.
• ICAO. Doc 10037, Global Operational Data Link (GOLD) Manual.
• ICAO. Doc 9613, Performance-based Navigation (PBN) Manual.