What is the difference between a great circle route and a rhumb line on a transatlantic flight?

A great circle route is the shortest path between two points on a sphere, arcing toward the nearest pole—on a standard Mercator map it appears curved, but it is geometrically shorter. A rhumb line crosses all meridians at a constant angle and appears straight on a Mercator chart, but it is actually a longer path for most long-haul routes. Transatlantic flight planning uses great circle geometry as the baseline distance calculation.

How does the North Atlantic Organized Track System (NAT OTS) work and who publishes it?

The NAT OTS is a set of daily, lettered parallel tracks across the North Atlantic published jointly by Shanwick, Gander, New York, and Santa Maria Oceanic Control Areas. Two track sets are issued each day—one for eastbound traffic (designed to ride the jet stream) and one for westbound traffic (designed to avoid headwind cores). Each track message specifies the waypoints, usable flight levels, valid Mach number range, and validity times.

Do all North Atlantic flights have to use the organized tracks, or is random routing allowed?

Random routing outside the OTS is permitted for North Atlantic flights. Operators may file a self-constructed route if it better suits their city pair or if OTS tracks are not advantageous. However, random routes must still comply with NAT MNPS equipment and authorization requirements, RVSM rules for the applicable flight levels, and the same oceanic clearance, position reporting, and Mach number technique procedures that apply to OTS flights.

Great Circle Routing, Track Messages, and the North Atlantic Organized Track System

When an aircraft flies from New York to London, the shortest path on the surface of the Earth is not a straight line on a standard flat map—it is a great circle route, an arc traced by a plane that passes through the center of the Earth and intersects the two endpoints. Understanding why this matters, how it is structured operationally in the North Atlantic, and how dispatchers work with the daily Organized Track System (OTS) track message is essential knowledge for anyone earning an Aircraft Dispatcher certificate and for anyone responsible for real-world transatlantic flight planning.

The North Atlantic corridor is the busiest oceanic airspace in the world, with hundreds of flights crossing daily in each direction. Because it lies largely beyond radar coverage and VHF radio range, special procedures govern how aircraft are separated, routed, and monitored. The combination of great circle geometry, wind optimization, and structured track routing is the backbone of North Atlantic dispatch planning.

Great Circle Routes: The Geometry of the Shortest Path

A great circle is any circle whose center coincides with the center of the Earth and whose circumference lies on the Earth's surface. The equator is a great circle. Every meridian of longitude is half of a great circle. The shortest distance between any two points on a sphere is always the arc of the great circle connecting them—not a line of constant latitude or a rhumb line.

On a Mercator projection (the familiar rectangular world map), a great circle appears as a curved line bowing toward the nearest pole. This is counterintuitive but geometrically correct: a New York–London great circle route actually passes near southern Greenland and Iceland, far north of a simple east-west rhumb line. A rhumb line, by contrast, crosses all meridians at the same angle and appears straight on a Mercator chart, but it is longer than the great circle route for most long-haul city pairs.

On a polar stereographic chart or a Lambert conformal conic chart—both of which approximate great circles as straight lines—the geometry becomes much more intuitive. These projections are standard for high-latitude oceanic planning. Dispatchers working with oceanic route planning software use great circle calculations automatically, but understanding the underlying geometry explains why transatlantic routes appear to arc northward on standard maps.

The North Atlantic Organized Track System (NAT OTS)

While great circle geometry identifies the shortest distance, operational factors—primarily the North Atlantic jet stream, which can exceed 200 knots at cruise altitude—mean that the absolute shortest path is rarely the most fuel-efficient. Strong tailwinds can reduce block time and fuel burn dramatically; strong headwinds do the opposite. The NAT OTS is a daily system of organized tracks structured primarily around these winds, along with traffic flow management and separation requirements.

The NAT OTS is jointly managed by the Shanwick Oceanic Control Area (UK/Ireland), Gander Oceanic Control Area (Canada), New York Oceanic Control Area (USA), and Santa Maria Oceanic Control Area (Portugal/Azores). Each day, these authorities analyze upper-wind forecasts and publish a set of parallel tracks—typically labeled Alpha through Zulu (not all letters used)—that span the North Atlantic. Tracks are designed so that aircraft flying within them receive organized, predictable separation.

Westbound and Eastbound Track Sets

Two separate track sets are published each day: one for the eastbound flow (Europe-bound, primarily during overnight/morning hours, taking advantage of the jet stream) and one for the westbound flow (North America-bound, typically during afternoon/evening hours, routing to avoid headwinds). The tracks are not the same in both directions. Eastbound tracks are designed to capture the jet stream core; westbound tracks route aircraft away from it, often at different latitudes or flight levels.

The Track Message: Reading and Using the OTS

The operational heart of NAT OTS planning is the track message, formally known as the NAT Track Message. It is issued twice daily (one for eastbound, one for westbound) and is distributed via ARINC or through national NOTAM systems. FAA-H-8083-16 (Instrument Procedures Handbook) and the AIM Chapter 5 both address the procedures governing oceanic operations, including the use of organized track systems.

A typical track message specifies, for each lettered track:

TMID (Track Message Identification): a unique identifier for the message itself
Track identifier letter (e.g., Track Alpha, Track Bravo)
Route of flight: defined by a series of latitude/longitude waypoints, expressed in degrees North and West, in the standard 5-degree North Atlantic grid
Flight levels: the range of usable flight levels for that track (e.g., FL310–FL390)
Mach number constraints: the valid Mach number range, since lateral separation on NAT tracks depends on all aircraft maintaining nearly the same true airspeed
Entry point: the oceanic entry fix (e.g., MIMKU, BURAK, 57N020W)
Exit point: the oceanic exit fix connecting back to the domestic route structure
Validity time: the UTC time window during which the track is active

Dispatchers and flight operations officers use the track message to select the most advantageous track for each flight, balancing wind advantage, flight level availability for the aircraft's weight, and any routing restrictions.

Random Routing vs. Organized Tracks

Not every North Atlantic flight must use the OTS. Aircraft may file random routing—a self-constructed route outside the organized tracks—if the operator determines it more efficient or if OTS tracks do not fit the city pair. Random routing is permitted but requires that the route be filed at least 30 minutes before entry into the Oceanic Control Area, uses the same position reporting requirements, and must still comply with the North Atlantic Minimum Navigation Performance Specifications (MNPS) and RVSM requirements applicable to the flight level selected. Random routing is common for city pairs that the OTS does not efficiently serve—for example, a flight from Dallas to Madrid might find OTS tracks poorly aligned and opt for random routing through the southern NAT.

NAT MNPS and Equipment Requirements

To operate within the NAT MNPS airspace (generally FL285–FL420), aircraft must be equipped with at least two long-range navigation systems (LRNS)—historically inertial navigation systems (INS), now typically FMS with GPS or dual IRS—and operators must hold an MNPS authorization. RVSM authorization is also required for flight at or above FL290 in the NAT. These requirements exist because the oceanic environment has no radar backup; navigation accuracy is the sole guarantee of separation. Dispatchers must verify both the aircraft's equipment and the operator's authorizations before planning a NAT routing.

Why It Matters for Dispatchers

Under 14 CFR Part 121, an aircraft dispatcher shares legal responsibility with the pilot-in-command for the safety and legality of every flight. In the oceanic context, this means the dispatcher must:

Select or evaluate the optimal track considering winds, track availability, and flight level
Verify ETOPS (Extended Operations) authorization if applicable—most NAT widebody flights are ETOPS operations, requiring specific maintenance, equipment, and alternate airport planning
Identify equal-time points (ETPs) and ensure adequate diversion airports exist within the required range for the ETOPS authorization time
File the correct oceanic clearance request in the proper format and within the required pre-departure clearance window
Ensure the aircraft holds valid MNPS, RVSM, and any required PBCS (Performance Based Communication and Surveillance) authorization
Account for the minimum fuel requirements including contingency, alternate, and final reserve fuel for the oceanic segment

The AIM Chapter 5 (Preflight and Enroute procedures) discusses oceanic and international operations including filing procedures, the use of organized track systems, and position reporting requirements. Dispatchers working transatlantic operations should treat the AIM Chapter 5 material alongside company operations specifications and ICAO Doc 4444 procedures as the operational standard.

Key Numbers and Rules

NAT MNPS airspace: FL285–FL420 between 27°N and the North Pole in the North Atlantic
Two LRNS required for MNPS airspace operations
Track message validity: eastbound valid approximately 0100–0800 UTC; westbound approximately 1130–1900 UTC (exact times published in each message)
Mach number technique: ATC assigns Mach numbers to maintain longitudinal separation; operators must not deviate without clearance
Oceanic clearance: must be obtained before entry—typically requested 30–40 minutes prior to oceanic entry fix
Random routing: permitted but subject to same MNPS, RVSM, and reporting requirements
Position reporting: required at each designated waypoint (oceanic and 10°W/30°W lines) by voice HF or SELCAL, or via ACARS/datalink where approved

Common Test Traps

Rhumb line vs. great circle confusion: Test questions often ask which route is shorter. The great circle is always the shortest distance between two points on a sphere; the rhumb line is NOT shorter even though it looks straight on a Mercator chart.
Track letters vs. direction: Eastbound and westbound tracks are published separately. Track Alpha eastbound is not the same as Track Alpha westbound—the sets are completely independent each day.
MNPS equipment vs. authorization: The aircraft must have the right equipment AND the operator must hold a specific MNPS authorization in their operations specifications. Equipment alone is insufficient.
OTS is not mandatory: Examiners test whether students know that random routing is legal. The OTS is a structured option, not an absolute requirement, but MNPS and RVSM rules still apply to random routes in that airspace.
Mach number technique purpose: Students sometimes think Mach number restrictions are only about speed or efficiency. The primary purpose is longitudinal separation—maintaining safe distance between aircraft on the same track at the same altitude without radar backup.