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From
your friends at ATCMonitor.com. © 2006 - Reproduction without
permission is prohibited.
If you have ever spent any time sitting in your vehicle during rush hour traffic while other motorists anxiously attempt to merge onto the freeway at high rates of speed, you have a good idea of how hectic an air traffic controller's job could become.
Everyday Air Traffic Control (ATC) has the task of merging hundreds of aircraft into the bottleneck that leads to the runway for landing.
In fact, the day to day problems faced by ATC are usually related to the volume of air traffic demand placed on the system or area that they manage. The increasing demand of air traffic placed on major airports, such as Atlanta Hartsfield-Jackson International airport, are
putting more demand on air traffic controllers. The job of ATC requires the ability to not only balance the workload of increased capacity of traffic, but also the ability to manage and direct it.
One of the ways that ATC handles their rush hour traffic is by directing aircraft so that they form a convoy of one single file line as they progressively transition from the en-route phase of flight to the approach phase and finally touchdown on the runway.
For instance, each time an aircraft is on final approach to land, the controller has to factor a certain amount of time for that aircraft to complete its approach, touchdown on the runway, and taxi clear of the active runway before another aircraft can land.
In order for the controllers to manage this abundance of traffic they create what's called a "sequence" or rather a process of establishing the order of arrival at a constraint
point (sometimes called a gate), such as an airway intersection, an Initial Approach Fix (IAF) or a specific runway.
In most cases ATC will create a sequence time factor to accommodate all of the aircraft on approach.
For example, the sequence might be +2+2, meaning that an aircraft lands and 2 minutes later another one lands, and 2 minutes after that another, and so on.
This sequence time could also vary depending on the amount of traffic inbound for landing and what type of performance capabilities they have.
In order for ATC to properly sequence aircraft of different size and performance, several methods are needed. One of those methods is called "Vectoring".
The ATC Manual of Operations (MANOP) defines a vector as "A heading issued to an aircraft, for the purpose of providing navigational guidance by means of radar."
To vector an aircraft, ATC gives the pilot a heading to turn towards; it's as simple as that.
However, the art of vectoring 20 or 30 aircraft into a sequence to arrive on time at any given airport is not so simple.
Vectoring could occur for any number of reasons. Pilots could be issued a vector to avoid restricted airspace, weather, other traffic, etc.
However, the main reason it's used is to assist pilots in transitioning from the en-route phase of flight to the approach phase when two or more aircraft are approaching the same position and need adequate spacing in order to continue the approach.
This sequencing also affects departures as well as arrivals from the gate.
If too many aircraft are inbound and improperly sequenced, then the aircraft that are holding for departure will also be delayed.
Another method used by ATC to control the sequencing of aircraft is speed.
The question arises of whether the controller should utilize vectoring or speed or both in the sequencing of aircraft inbound on approach.
There is some debate about which one of these methods is best. In fact, every controller is different and utilizes different methods, or a combination of those methods.
However, this greatly depends on the amount of traffic and the type of traffic entering the approach phase of
flight . For example, if twenty aircraft are approaching an approach fix that roughly have the same performance, such as say a Boeing 747, either a vector or a speed increase/reduction will need to be in place in order to keep them properly separated.
However, with so many aircraft in the vicinity, it seems as though slowing some of them down while speeding some of the up would suffice as a solution to keep them properly separated and remain inbound for the runway.
Sometimes speed increase or reduction is the better option when trying to coordinate to maintain sector uniformity.
In modern aircraft, the Flight Management System (FMS) (a picture of a
Honeywell FMS from a Boeing 777 is below) is capable of recalculating speed changes and the effects that it will have on fuel burn and delay a lot faster than calculating the prediction of where vectoring might lead the aircraft.
Even though the process of utilizing speed and vectoring is sometimes a required necessity for ATC, there is one thing that most airlines will agree upon and that equates to vectoring for sequencing uses too much fuel.
In fact, most airlines have suggested that they would prefer to be slowed down versus being vectored off the beaten path to the runway.
Since the aircraft on approach will eventually have to slow down anyway to land, most air carrier's factor in the costs of speed reduction into their cost of operation.
For example, an airliner who has been vectored just 8 miles off course will suffer a delay of one minute.
While that time seems insignificant, it can add up to over $35,000 per aircraft year in fuel
costs.
It is an inevitable part of ATC that in high congestion areas, aircraft will be vectored.
Where there is a choice, however, and provided the route segment is sufficiently long, air carriers suggest that they would generally prefer speed control.
This is due to the fact that most airlines operate using a cost index methodology to determine the optimum cruising speed in order to conserve fuel.
Moreover, a study by Eurocontrol
research facility recently stated that one suggestion to improving the debate over speed versus vectoring would be to provide the aircraft with the technology to space themselves from each other.
Their philosophy would indicate that if air crews could maintain their own sequence in spacing utilizing their own flight management system it would be more cost effective for the air carrier.
However, even with FMS capability there is still the element of variation in vectoring aircraft because they have variation in maneuvering time and the ability to capture the new course.
These days, there is currently other technology in place that may provide this same solution.
Currently across the nation there are several major airports deploying Area Navigation (RNAV).
With more and more aircraft flying Global Positioning Systems (GPS) and RNAV routes, the future of greater precision point-to-point flying should improve which would reduce the need for such variations of maneuverability.
This in turn should also not only reduce time delays and aircraft fuel burn, but also the need for adjustments in vectoring and speed.
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