North Atlantic High Level Airspace (NAT HLA) Track System Procedures

The North Atlantic High Level Airspace (NAT HLA) is one of the busiest oceanic airspace regions in the world, handling hundreds of transatlantic flights each day between North America, Europe, and beyond. Because radar coverage is extremely limited or nonexistent over most of the open ocean, pilots and dispatchers must follow a rigid set of procedural rules that substitute for the real-time surveillance controllers rely upon over land. The Organized Track System (OTS)—a daily set of published routes optimized for favorable winds—is the backbone of NAT HLA operations, and understanding it is essential not only for ATP written-test success but for actual line operations in international air carrier service.

The primary FAA regulatory and advisory framework governing U.S. operators in the NAT HLA is Advisory Circular 91-70C, Oceanic and International Operations. It consolidates guidance on pre-departure planning, oceanic clearances, position reporting, Reduced Vertical Separation Minimum (RVSM) compliance, Required Navigation Performance (RNP), and contingency procedures. This article unpacks those requirements in the depth an ATP candidate and line pilot need.

The Organized Track System (OTS)

The OTS is a set of one-way parallel routes—called North Atlantic Tracks (NAT Tracks)—that shift every day to capture the jet stream for westbound traffic and avoid headwinds for eastbound traffic. The tracks are coordinated and published by the Shanwick Oceanic Control Area (covering the eastern portion of the NAT) and the Gander Oceanic Control Area (covering the western portion), with additional coordination from Santa Maria (Azores area) and New York Oceanic.

Tracks are labeled with single letters (e.g., Track Alpha, Track Bravo) and organized into two daily messages:

• Westbound OTS message — valid roughly 1130–1900 UTC, optimized for flights departing Europe toward North America.
• Eastbound OTS message — valid roughly 0100–0800 UTC, optimized for flights departing North America toward Europe.

Each track message specifies the waypoints (expressed as latitude/longitude or five-letter named waypoints), the usable flight levels, and the Mach number range. The published tracks are one-way only during their validity period. Flying a track in the wrong direction or at an unauthorized flight level is a serious procedural deviation.

Operators are not required to use the OTS; they may file a random routing (also called a user-preferred route or UPR) outside the organized track structure. However, random routings must still meet all NAT airspace entry requirements, including RNP, equipment, and clearance requirements.

Oceanic Clearance Requirements

Before entering the NAT HLA, the flight crew must obtain a discrete oceanic clearance from the appropriate oceanic control area. This is typically obtained via VHF radio approximately 30–40 minutes before the oceanic entry point (OEP), or via datalink (CPDLC/ADS-C) where available. In many cases, the clearance can also be obtained through the departure or domestic en route controller who relays it from the oceanic facility.

The oceanic clearance specifies the cleared track or route, entry fix, flight level, and Mach number. The crew must read back the full clearance and comply exactly. If the clearance differs from the filed route, the crew must amend the FMS accordingly before entry. Entering the NAT HLA without a valid oceanic clearance is prohibited.

A critical operational discipline is the Oceanic Entry Procedure (OEP): once the crew receives the oceanic clearance, they perform an oceanic entry check—verifying that waypoints are correctly loaded, that the Mach number matches the clearance, and that all navigation and communication equipment is functional. Both pilots independently verify the FMS route entry. This independent cross-check prevents gross navigation errors (GNEs), which are generally defined as lateral deviations of 10 NM or more from the cleared centerline.

Navigation and Equipment Requirements

The NAT HLA mandates specific navigation performance to maintain safe separation on the parallel track structure. The Required Navigation Performance (RNP) standard for the NAT HLA is RNP 4 or RNP 10 depending on the specific portion of airspace and flight level, although much of the high-density core area now requires RNP 4. RNP 4 means the aircraft must remain within 4 nautical miles of the centerline for 95% of flight time, with a containment value of 8 NM.

Operators must also comply with RVSM requirements throughout the NAT HLA. RVSM allows 1,000-foot vertical separation between aircraft at flight levels FL290 through FL410 (versus the previous 2,000-foot standard), dramatically increasing capacity. Aircraft must be RVSM-approved, and crews must be aware that any significant altimetry system error or autopilot failure that prevents maintaining assigned altitude within the RVSM tolerances must be reported to ATC immediately.

For communications, the NAT HLA requires aircraft to monitor and be capable of communicating on HF (High Frequency) radio because VHF range does not extend across the ocean. Where approved datalink services are available (CPDLC via ACARS and ADS-C), operators may use datalink as the primary means of communication. Selective Calling (SELCAL) capability is strongly recommended so that crews can rest without constantly monitoring the HF voice frequency.

Position Reporting

In the absence of radar, oceanic separation relies on procedural separation—primarily lateral (track separation), longitudinal (time or distance separation), and vertical separation. Position reporting is the mechanism that gives controllers confidence that aircraft are on their cleared routes and maintaining longitudinal separation.

At each significant waypoint (every oceanic waypoint on the cleared route), the crew must transmit a position report containing:

1. Aircraft identification
2. Position (waypoint name or lat/long)
3. Time over the position (UTC)
4. Flight level
5. Next waypoint and estimated time of arrival (ETA)
6. The waypoint after next (to enable controllers to project the route)

Position reports are made on the appropriate HF frequency assigned by the oceanic facility, or via CPDLC datalink. Failure to make a position report within a prescribed time (typically 40 minutes after the last expected report) triggers a communications search, which is extremely disruptive to oceanic operations and can result in TCAS advisories, heading changes for other traffic, and significant delays.

Mach Number Technique

A unique feature of NAT HLA operations is Mach Number Technique. Controllers assign a specific Mach number (e.g., M0.84) to each aircraft as part of the oceanic clearance. All aircraft in the track must maintain their assigned Mach number to preserve longitudinal separation. Unlike domestic IFR operations where pilots manage airspeed relatively freely, changing Mach number in the NAT HLA without ATC clearance is prohibited except in emergency situations. If wind conditions cause the actual Mach to deviate significantly, the crew must notify ATC and request a revised clearance.

Contingency Procedures

Despite meticulous planning, in-flight emergencies and abnormal situations do occur over the ocean. The NAT HLA has well-defined contingency procedures pilots must memorize:

• Loss of one engine or pressurization loss requiring descent: The pilot must immediately notify ATC (if able), turn 30 degrees off the assigned track (right of track if in the eastbound OTS, left if westbound — following the general published guidance), descend to a lower altitude, and broadcast intentions on 121.5 MHz as well as the assigned HF frequency.
• Weather deviation: Obtain ATC clearance before deviating if possible. If unable to contact ATC, the crew must broadcast position and intentions on 121.5 MHz and the appropriate oceanic HF frequency, offset no more than 15 NM from the cleared track, and return to the cleared route as soon as possible.
• Loss of navigation capability: Notify ATC immediately. The controller may apply increased separation standards appropriate to the aircraft's remaining navigation capability.
• Communications failure: Continue on the cleared route and flight level, maintain Mach number, and attempt contact on all available frequencies including 121.5 MHz and SELCAL.

Key Numbers and Rules

• RNP 4 — the standard navigation performance requirement for high-density NAT HLA airspace (±4 NM, 95% of flight time).
• RVSM — 1,000-foot vertical separation from FL290–FL410; aircraft must be RVSM-approved.
• Oceanic clearance — must be obtained approximately 30–40 minutes before the oceanic entry point.
• Gross Navigation Error (GNE) — generally defined as a lateral track deviation of 10 NM or more; must be reported.
• Mach Number Technique — assigned Mach must not be changed without ATC clearance.
• 121.5 MHz — must be monitored during oceanic flight; used for emergency broadcasts.
• Position reports — required at every oceanic significant waypoint; failure to report triggers a communications search after approximately 40 minutes.

Common Test Traps

• Track direction and validity periods: Candidates often confuse which OTS message applies to eastbound versus westbound traffic, or assume tracks are bidirectional. Each daily message is one-way only.
• RNP values: Confusing RNP 4 with RNP 10 or misapplying them. Know that RNP 4 is the more stringent (smaller error) standard increasingly required in the high-density core.
• Mach Number Technique is mandatory: Some candidates treat Mach as advisory. It is a clearance item — deviating without authorization is a procedural violation.
• Contingency turn direction: The 30-degree turn direction during an emergency or weather deviation depends on context; memorize the guidance in AC 91-70C, not a simplified rule.
• Clearance timing: Failing to obtain the oceanic clearance before the entry point — or accepting a clearance that differs from the filed route without updating the FMS — is a very common practical error that the exam exploits.