An aircraft's flight planned route contains a succession of waypoints. The controller may clear the aircraft to fly in a way that skips some of them. This technique is called "direct routing". It often saves time and fuel and is therefore normally welcomed (and sometimes requested) by pilots. This is why ICAO Doc 4444 states "Subject to airspace constraints, ATC workload and traffic density, and provided coordination can be effected in a timely manner, an aircraft should whenever possible be offered the most direct routing.". While there are many benefits to the use of direct routing, controllers should always carefully examine the implications of this option, as it has the potential to cause issues as well.
As a means of conflict solving direct routing is similar to vectoring as both methods are used to achieve horizontal separation by altering the track of an aircraft. However, there is a significant difference from navigational point of view - an aircraft flying on a direct route is maintaining own navigation while a vectored one is not.
Direct routing is arguably the technique that most favoured by both pilots and controllers. The former consider that it would enable them to reach their destination earlier while the latter consider they provide more efficient service. The benefits of direct routing include:
Delay compensation. If a flight has experienced delay for whatever reason (late departure, adverse weather avoiding, etc.), clearing it to fly on a direct route can compensate this, at least in part.
Increase of flight efficiency due to reduced fuel burn.
Reduction of flight time. This is especially helpful in emergency situations where reaching the chosen landing aerodrome earlier can be crucial.
Conflict solving. Similar to vectoring, a direct route may change the track of a flight, thereby resulting in achieving the necessary horizontal separation or providing additional time to achieve vertical separation. However, unlike vectoring, direct routing shortens the flight path, thus achieving both safety and efficiency.
Neutrality. If used after vectoring, the direct routing can compensate the extended flight path (and time) so that a zero net effect is achieved.
Easier conflict calculation. If an aircraft flies on a direct route, its direction will remain the same and speed fluctuation will be kept to a minimum in most cases. Naturally, for climbing/descending aircraft or under specific weather circumstances, this statement will not be true.
The aircraft continues to fly using own navigation. This is especially beneficial in case of [[loss of radio communication]] as the flight path will be more predictable.
Things to Consider
While clearing an aircraft to fly on a direct route is normally welcomed by pilots (and thus, a technique preferred by controllers) there are some situations and circumstances that may need to be considered before issuing such a clearance, e.g.:
A direct routing often (though not always) causes the aircraft to leave the flight planned path. If the change affects the next sector(s) or unit(s), this should be coordinated. Omiting this step may yield undesired consequences, such as surprised colleague, the need to urgently develop a new plan, loss of separation, infringement of SUA, etc.
If the direct routing takes the IFR flight away from the ATS route, it is up to the controller to ensure that terrain clearance is achieved
A direct routing clearance might actually increase fuel burn. For example, if the aircraft arrives earlier than expected, it may need to hold. This is normally done at a level that is lower than the cruise level (therefore the fuel consumption will not be optimal).
Clearing multiple flights on direct routings may have a negative impact on the applicable flow control measures. Since capacity planning (and sector management) assume that aircraft will follow their flight planned routes, direct clearances may result in too many aircraft arriving too early, thus resulting in sector (and controller) overload.
Direct routing may change the aircraft path significantly. This may result in the creation of new conflicts.
A direct routing can cause state aircraft to enter airspace for which they do not have diplomatic clearance to do so.
Compared to vectoring, direct routing is a technique that is limited to the waypoints in the flight plan. This means that controller intervention is limited to a small number of new trajectories and there is no guarantee that any of them will be appropriate. While it is possible to negotiate with the crew to fly to a point (or points) that is not included in the flight plan, this should be done with caution and the way of rejoining the planned trajectory must be specified.
When an arriving aircraft is cleared to proceed direct to a published waypoint on the STAR, the speed and level restrictions associated with the bypassed waypoints are cancelled. If the controller needs the aircraft to adhere to such restrictions, this must be explicitly specified. Restrictions after that point will remain in force unless cancelled.
The controller may use several phrases to issue a direct routing clearance:
CLEARED TO (point) DIRECT
CLEARED DIRECT (point)
WHEN ABLE PROCEED DIRECT (point) (Note: In this case, the moment when the aircraft turns toward the specified point is chosen by the pilot)
RESUME OWN NAVIGATION [DIRECT] (point) (Note: This phrase is used when vectoring is complete)