Separation (air traffic control)

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Staggering , also separation (English: separation , traffic resolution ), is a method of air traffic control . This is understood to mean the steering of aircraft with the aim of creating a sufficient safety distance between the individual aircraft. The separation is carried out by air traffic control units on the ground or by the pilots themselves.

Graduation obligation

Not every aircraft is staggered by air traffic controllers. This depends on whether the aircraft is in a controlled or uncontrolled airspace and whether the flight I nstrumenten- F lug- R ules (IFR) or visual flight rules is performed (VFR) (see this. Airspace structure ).

In general, air traffic control has a staggering obligation for

  • All flights in airspace classes A and B
  • IFR flights to IFR flights in airspace classes C, D and E.
  • IFR flights to VFR flights in airspace class C
  • IFR flights to SVFR flights
  • SVFR flights to SVFR flights, provided this is stipulated by the responsible aviation authority in the respective country (In Germany: no staggering obligation).

Removing staggering

For flights that are subject to a graduation obligation, one type of graduation may only be lifted if another is fulfilled.

Radar staggering

Radar staggering is a way of ensuring safe distances between aircraft at all times . It is used by air traffic controllers in control centers. As the name suggests, the use of radar graduation is only possible if the controller's working position has a radar image of his airspace .

Radar staggering is a lateral staggering form.

The radar separation equals 5 NM ( N autische M rush or nautical mile) . In regions in which available radar systems have sufficient accuracy, this distance can be reduced to 3 NM. In the final approach is also a further reduction to 2.5 NM can be made. These values ​​are to be understood as a minimum, i.e. H. if a greater distance is necessary due to the necessity of wake vortex staggering , this is considered the minimum required distance.

In general, the following values apply in Germany and Austria for radar separation between two aircraft:

  • The distance at the same height is 3  NM near the airfields and 5  NM on the route ( lateral staggering ).

Vertical graduation

Vertical separation, like radar separation, is used by air traffic controllers. It is one of the procedures of conventional air traffic control .

The height distance is below flight level (flight level FL) 290 1000 ft , ft above flight level 290 2000

Reduced vertical graduation minima ( RVSM )

If an aircraft meets specified technical requirements, it is possible in RVSM airspace to reduce the vertical separation between flight levels 290 and 410 to 1000 ft between two aircraft with RVSM equipment.

This serves to use the airspace more efficiently.

Vertical separation during ascent / descent phases

In order to ensure vertical separation even during the climb or descent of two aircraft, it is possible, in addition to the clearance to a different altitude, to instruct both aircraft involved in a climb / descent rate so that there is no risk of falling below the separation at any time.

If a pilot can no longer maintain this rate for aerodynamic reasons, he is obliged to inform the air traffic controller immediately.

Wake vortex staggering

Categories

Wake vortices are air turbulences caused by aircraft which, depending on their strength, can be dangerous for the aircraft behind.

The strength of a wake vortex depends largely on the weight of the aircraft, but also aerodynamic aspects play a role.

The ICAO therefore classifies aircraft into the following wake vortex categories:

  • L (Light): Aircraft with a maximum take-off weight of 7,000 kg
  • M (Medium): Aircraft with a maximum take-off weight of more than 7,000 kg but less than 136,000 kg
  • H (Heavy): Aircraft with a maximum take-off weight of 136,000 kg or more (excluding Airbus A380-800)
  • J (Super): Airbus A380-800

Above flight level 100, the Airbus A380 is also treated as an aircraft in the heavy category.

application

Wake vortex staggering must be used between two aircraft if one of the following conditions applies:

  • The following aircraft is flying directly behind the aircraft in front at the same altitude or less than 1000 ft lower in the same direction
  • The following aircraft is crossing directly behind the aircraft in front at the same altitude or less than 1000 ft lower
  • Two aircraft are using the same runway or parallel runways less than 760 m apart
  • Two aircraft use parallel runways with a distance of at least 760 m and the flight path of the aircraft in front is crossed at the same height or less than 1000 ft lower
  • Two aircraft use intersecting runways and the flight path of the aircraft in front is crossed at the same altitude or less than 1000 ft lower

Wake vortex staggering does not have to be used:

  • When the area in which wake vortices occur is not crossed by any aircraft
  • If the pilot of the aircraft behind waives additional separation
  • When the wake vortex staggering obligation is delegated to the pilot

Graduation minimums

Since different weight categories also have different susceptibility to wake vortices, the following minimum distances have been established between the respective weight classes:

  • Light behind medium: 5 NM
  • Light behind Heavy: 6 NM
  • Light behind Super: 8 NM
  • Medium behind Heavy: 5 NM
  • Medium behind Super: 7 NM
  • Heavy behind Heavy: 4 NM
  • Heavy behind Super: 6 NM

These distances are to be understood as minima, i.e. H. they must also be observed if the corresponding radar graduation minimum indicates a lower value.

Time-based graduation

The distance-based staggering leads to unnecessarily large distances, especially in certain weather conditions, for example with strong headwinds (which reduce the speed of the aircraft), which limits the capacity of an airfield.

At London Heathrow Airport , for example, approx. 80% of the delays and a relevant proportion of flight cancellations at arrivals are due to the weather.

Wake vortices dissolve much faster in strong headwinds, which is why capacity is wasted due to the usual staggering process.

Since spring 2015, air traffic control has therefore been using a system that staggered the distance between the aircraft not metrically but chronologically. The process is also known as TBS ( Time-based separation ). Arrival delays have since decreased by around 50%.

Independent graduation

Personal responsibility under VFR

Pilots who operate under VFR rules are obliged, depending on the airspace flown, to independently ensure their separation. If the VFR is checked , the separation is restored by the air traffic control unit. This usually happens when VFR pilots are in areas where airplanes can also operate under IFR rules, e.g. B. Arrival and departure areas of airports.

Personal responsibility under IFR

The air traffic controller can transfer the separation obligation to the pilot (pilot) during a flight under IFR rules , if the pilot agrees. Before he is given responsibility for the delegation of separation, the air traffic control unit determines in advance whether the pilot can see the other aircraft. During the daytime (between sunrise and sunset ) and when visibility is good (locally defined by aviation weather services ), the pilot is then allowed to stay below the minimum distance at his own risk , which increases the number of aircraft in the airspace ( control zone of the airfield , airway, etc.) can be. This can be seen in the control zones of large commercial airports during rush hour .

Example of a transfer of responsibility

A fictitious, but entirely possible scenario of a transfer of responsibility for the separation from the air traffic control unit (FVKSt) to the pilot could take place over the radio as described here.

Name of the flight
AUA540 is an Austrian Airlines flight, pronounced: Austrian Five Four Zero
communication
Typical communication process between the FVKSt and the pilot:
... the FVKSt gives traffic information and asks for visual contact ...
  • FVKSt: AUA540, traffic, Airbus 380, one o'clock, one zero miles, crossing right to left, one thousand feet above, fast moving, report traffic in sight.
... the pilot confirms receipt of the call ...
  • Pilot: AUA540, looking out.
... if the pilot does not see the traffic, the FVKSt updates the report about the traffic after a certain time ...
  • FVKSt: AUA540, mentioned traffic, twelve o'clock, five miles.
... when the pilot sees the Airbus, he calls ...
  • Pilot: AUA540, traffic in sight.
... the FVKSt asks whether a delegation is possible ...
  • FVKSt: AUA540, are you able to maintain own separation to mentioned traffic?
... the pilot confirms ...
  • Pilot: AUA540, affirm.
... the FVKSt hands over responsibility to the pilot ...
  • FVKSt: AUA540, maintain own separation.
Individual speaking groups can be combined if necessary in order to reduce the frequency load.

Security / problem

IFR

It is difficult for a pilot to estimate the graduation of heights with oncoming aircraft, and there is very little time available. Line machines or jets fly towards each other at around 900 km / h, i.e. at around 250 m / s (5 km are flown through by both in 10 seconds). The time from the sighting to a possible collision is only a few seconds with excellent visibility (50 km or more) and is reduced to zero with poor visibility. This is especially extremely dangerous when

  • the airspace is "full" and staggerings are reduced to a minimum,
  • other traffic is not known at all ( extremely unlikely under IFR ),
  • other traffic cannot or can only be recognized poorly (incorrectly or incorrectly set instruments - transponders, defective radar devices, etc.),
  • Traffic is believed to be in a different position than is actually believed (incorrect or incorrect information from pilots, incomplete or incorrect position reports from other pilots, etc.),
  • you believe you are in a different position than the real one due to faulty on-board instruments or sensors (for example: iced inlets of the altimeter, GPS malfunctions),
  • contradicting information is available, but you can recognize it in a short time, what is wrong and what is right (frequent cause),

and thus one may consider oneself in a false sense of security. Pilots operating under IFR rules rely primarily on their instruments and air traffic control centers. So you don't keep looking for other aircraft in the airspace. In poor visibility conditions (at night, in clouds), this is often not possible and also not expedient, as the monitoring of the instruments has priority under IFR. In order to avoid collisions, all scheduled or transport aircraft (at least if approved in Europe or the USA) have a TCAS , a collision warning and avoidance system, on board. The TCAS operates completely independently of ground stations and evaluates the transponder signals from other aircraft for possible dangers. If necessary, it sounds the alarm, calculates a possible evasive maneuver and suggests this to the pilot.

VFR

VFR pilots usually have to keep an eye out for other aircraft. However, since VFR pilots are mostly on the move with aircraft that achieve a relatively low cruising speed in contrast to jets, there is also more time to record and avoid collisions in accordance with the prescribed avoidance rules .

The following conditions make it more difficult to grasp (view) and increase the risk of collision, since it visually merges with the background:

  • Weather phenomena,
  • Flights next to, above and below clouds,
  • Flights against the sun,
  • Blend effects or reflections of clouds, layers of air and ground conditions (lakes, water, desert),
  • Heavily structured terrain (cities, densely populated areas, etc.),
  • Flights in mountainous terrain at the same or lower altitude of the surrounding peak levels,
  • Aircraft with camouflage paint (military) as well
  • Failed (not switched on) position lights and strobes .
General

In the case of an aircraft that is approaching the pilot, it is difficult to assess whether it is approaching at the same altitude, and thus on a collision course, or completely harmlessly 1000 ft (about 300 m) lower or higher. As a rule of thumb, an aircraft that appears level with the horizon line flies at the same altitude as your own aircraft.

Avoidance (rather VFR)

Under difficult flight conditions as well as assuming possible foreign traffic in the air, one can draw attention to oneself with the following methods:

passive
  • Compliance with general / all rules under VFR and IFR
  • Comply with local air traffic restrictions and rules
  • Use of a transponder with height coding
  • Rather light colors / paintwork on the top of the aircraft and dark on the bottom (contrast to the surroundings)
  • Use and observation of the TCAS
active
  • Observation of the airspace (95% of the time, 5% instruments and navigation - under VFR)
  • Increased attention in areas of high activity (glider and paragliding areas, airfields, etc.)
  • Switching on the landing lights (as well as any other lighting)
  • Attempt to contact you, preferably on the frequency of the air traffic control unit or the nearest airport and report your position
  • Serpentine flight (an object that flies wide serpentine lines is easier to recognize than one that is flying straight ahead) or an orientation curve (360 °) or brief tilting and turning so that blind spots (below or above the aircraft) can also be seen.
  • Avoidance of flights at low altitude (taking off from the landscape / structure)

Consequences of insufficient graduation

If the graduation is not adhered to, be it due to pilot or controller errors, the consequences can range from near collisions to collisions, even if other safety systems, such as e.g. B. TCAS should prevent this.

Serious accidents as a result of insufficient graduation

are usually collisions in the air, which are usually fatal for everyone involved (exception usually only possible for aircraft with a parachute / ejector seat).

Web links

Individual evidence

  1. ^ Anonymous: Time Based Separation at Heathrow. September 22, 2016, accessed on August 28, 2019 .
  2. ↑ Time Based Separation. Retrieved August 28, 2019 .