dailynewsnblog
The Federal Aviation Administration’s (FAA) Code of Federal Regulations states, “The pilot in command of an aircraft is directly responsible for, and is the final authority as to, the operation of that aircraft.” This is a particularly vital distinction when push comes to shove – during emergencies, for instance. However, the reality of flight under normal circumstances is less clear-cut, and in practice, pilots are bound by extensive rules, regulations, and, importantly, the clearances provided by air traffic control (ATC).
This even extends into remote airspace, such as that above vast oceans like the North Atlantic. Notably, in such cases, rules and instructions are as important as ever. Here, pilots must adhere to strict procedures governing their speed, even when it may be desirable to accelerate or slow down for optimal performance or to make up for lost time, say. So, why is this?
Safely Governing Aircraft Without Conventional Radar
Where aircraft travel over remote areas and extensive open ocean, conventional means of controlling such airspace become impractical. Indeed, very little in the way of infrastructure below hampers controllers’ ability to guide pilots safely through the skies above. All of a sudden, continuous radar coverage like that often seen over continental airspace is no longer available, meaning another way to sort planes and ensure they are in the right place at the right time is needed.
The North Atlantic offers a useful example. Historically the busiest oceanic airspace in the world, approximately 460,000 planes crossed the North Atlantic in 2012, according to Skybrary. That is more than 1,256 per day on average, with the number rising in the years since. What is more impressive is that this massive operation took place and continues to take place where conventional radar surveillance is unavailable for much of the journey and where communications have historically been more limited than over land.
Catch what other flight trackers miss
Emergency squawks, holds, NOTAMs — live signals, no signup.
Open tracker
Catch what other flight trackers miss
Emergency squawks, holds, NOTAMs — live signals, no signup.
Open tracker
So, instead of ATC being able to rely on the usual means, predictability has become an integral part of organizing aircraft. Within this, speed is a key element. Simply put, by assigning each jet a specific cruise speed, where they will be and when can be interpreted far more easily. For pilots, that means being given what is known as a true Mach number.
How Mach Number Technique Maintains Separation
ATC uses this as part of a wider system to maintain separation between planes, or in plain terms, to ensure that no more than one jet is too close to one place at one time. There are two main ways of doing this: either through horizontal separation, by spacing planes at the same flight level, or via vertical separation, by placing them at different heights.
Where the Mach number technique — which gives pilots a set speed, as touched on above — fits in is horizontal separation. More specifically, this relates to longitudinal separation. Essentially, that means spacing between jets moving in the same direction, at the same height, on the same track (or path), exactly like those used to organize the busy airspace over the North Atlantic.
By assigning a specific Mach number to successive aircraft following the same track, ATC can help maintain spacing between them. This also reduces the need for constant radio check-ins in theory and allows airspace to be used much more efficiently. So long as pilots maintain this, then, predicting where they will be and when becomes possible for controllers. Because of this line of thinking, any developments that could alter such predictions must be relayed by pilots.
The Three-Minute Rule Over The Atlantic
Predictions themselves are conveyed as estimated times of arrival (ETAs) at certain waypoints. For the North Atlantic, tracks are published twice a day to account for factors such as prevailing weather and wind conditions. Having established routes, controllers can then rely on jets to follow the same path and maintain their assigned Mach to arrive at these waypoints, broadly in line with their anticipated ETAs.
The reality can be different, though, and it is when ETAs shift that pilots must act. That is not to say accelerate or decelerate. Rather, when reporting on positions by voice, any three-plus minute changes in ETA at a waypoint must be conveyed to ATC. Were one aircraft to unexpectedly change speed, the risk of losing longitudinal separation would increase, and for instance, with several jets in a row, the ramifications could be serious.
Also in the North Atlantic, a Mach variation of 0.02 or more must also be reported. All told, the idea is that ATC can paint the most accurate picture of the whereabouts of what might be hundreds of planes at a time, even without continuous radar coverage. Again, it is this lack of continuous radar coverage that makes following procedures by the book so important.
ATC Clearance Required For Speed Changes
In any case, if adjustments are needed, it is generally up to ATC to approve and coordinate them. Hence, pilots do not have full authority over their speed whilst crossing oceanic airspace. That does not mean slight changes cannot be made at all, though.
Turbulence, weather, or even aircraft performance may prompt flight crews to seek adjustments. In any case, there are procedures allowing certain deviations without clearance. Namely, the Strategic Lateral Offset Procedure (SLOP) allows aircraft to be taken up to two nautical miles (3.7 kilmeters) right of their assigned track on oceanic crossings and is standard practice worldwide. Similarly, very minor adjustments in altitude are also permitted.
Catch what other flight trackers miss
Emergency squawks, holds, NOTAMs — live signals, no signup.
Open tracker
Catch what other flight trackers miss
Emergency squawks, holds, NOTAMs — live signals, no signup.
Open tracker
For speed, however, any deviation from the assigned speed of Mach 0.02 or greater is considered significant and requires coordination with ATC. For context, that equates to just 15.3 miles per hour (24.7 kilometers per hour), so in relation to a widebody’s typical cruising speed of around Mach 0.85, it is, realistically, minimal. Given ATC’s reliance on the numbers to determine when jets will hit their waypoints, deviations outside of this can make for a risky game. This is especially the case when it comes to airspace like that over the North Atlantic, where conditions are complex anyway when things are running smoothly.
The North Atlantic Tracks
As mentioned, the North Atlantic represents the busiest oceanic airspace in the world. The only cross-border connection outside of the Middle East and Asia to rank among the top ten busiest in 2025, for example, was between New York John F. Kennedy International Airport (JFK) and 
London Heathrow Airport (LHR). This alone accounted for four million seats over the year and served as just one of dozens upon dozens of regular routes to rely on the airspace.
Needless to say, the number of aircraft crossing from Europe to North America and vice versa at any given time can be vast, particularly in the area of high-level airspace between 28,500 and 42,000 feet (8,687 and 12,802 meters). According to ICAO’s latest manual covering the area, separation between jets within this could, in theory, be as little as 15 nautical miles (27.8 kilometers) laterally, 14 nautical miles longitudinally, and 1,000 feet (305 meters) vertically to maximize capacity.
Add in the obvious historical issues around radar coverage, as well as weather patterns like jet streams that have to be navigated, and the complexity of the task at hand for ATC starts to make sense. This is also the context for why pilots flying such routes must not stray from any set Mach numbers without prior authorization. That said, in positive news for ATC and pilots alike, technology has begun to allow procedural updates in recent years.
New Technology Is Loosening The Rules
Throughout 2024, procedures across North Atlantic oceanic airspace shifted notably. Before this, crews were given an oceanic clearance from ATC incorporating their route, altitude, and speed. Since then, they have generally been allowed to enter the space based on a flight plan filed beforehand, unless told otherwise. This means flexibility around speeds has increased somewhat, and that is down to improved technology.
Modern data link and surveillance systems, particularly Automatic Dependent Surveillance Contract (ADS-C) and Controller-Pilot Data Link Communications (CPDLC), have helped enable Operations Without An Assigned Fixed Speed (OWAFS), a procedure in place since 2019. So in practical terms, jets are no longer universally required to operate at a fixed Mach number.
When traffic allows, control centers like Gander on the Canadian side and Shanwick in the UK can allow crews to operate in line with their own flight plans and so at normal operating speed. That said, deviations of 0.02 Mach or more must still be relayed by pilots in such cases. Alternatively, if conditions do not allow it, specific parameters are provided to follow based on the Mach number technique, much as they have been for decades.
