Sector Skipping

Sector Skipping

Description

Sometimes situations occur where a flight passes through an ATS sector for a very short time (a few seconds up to a few minutes). This is called sector clipping. Sector skipping is a procedure that allows a sector that is part of a flight's sector sequence to opt out of that sequence. Thus, communication with the flight crew is transferred directly from the previous to the next sector.

Benefits

Fewer frequency changes and reduced frequency occupancy.

  • Reduced risk of loss of communicaiton due to e.g. poor communication (including readback) or equipment mishandling. 
  • Reduced pilot workload. Frequency changes add communication workload and may also interrupt other pilot tasks.
  • Reduced controller workload. While the workload for the previous and next sector remains the same regardless of whether or not skipping is used, the sector in between would need to perform at least two additional tasks if the flight is not skipped - establish contact and transfer the aircraft.

Example Situations

Very short clipping. This is the most common scenario. The aircraft will fly in the skipping sector for less than a minute.

Longer clipping close to the boundary. In this situation, the aircraft will fly in the skipping sector for several minutes. However, as this happens along the inter-sector boundary, the controllers may decide that no benefit would come from transferring the aircraft.

Re-entry. If the aircraft is supposed to enter the skipping sector for a few minutes and then enter the previous sector again, skipping would reduce both controllers' workload.

Vertical skipping. Sometimes thin vertical sectors are considered optimal to cope with traffic demand. Such a configuration would be appropriate if the prevailing traffic flows are overflights. Nevertheless, an occasional climbing or descending aircraft would appear. Depending on the circumstances, skipping a sector could prove beneficial. It shopuld be noted, however, that this is an inherently riskier situation, as vertical separation generally erodes much faster than the lateral. This is because while a controller would normally be content with aircraft being separated at the vertical minimum, they normally prefer to have some buffer of horizontal spacing.

The cruising level is FL 390. This means that the flight will likely pass through Sector 2 for a short period of time, climbing to FL 350 while entering the sector and to FL 370 while leaving it.

Risks

Sector skipping is a feature that, if properly used, benefits both safety and efficiency. However, there are some inherents risks that the controller needs to consider before using it:

  • Conflict overlooking. After performing a skip, the colour of the track and its label often changes to "unconcerned". This makes conflicts harder to detect.
  • No contact with an aircraft in the Sector. The controller remains responsible for the safety of traffic in their sector regardless of whether in contact with it or not. In case an unexpected situation happens (e.g. emergency, weather avoidance, deviation due to call sign confusion, etc.) there will likely be less time to react.
  • Misuse. While the sector skipping procedure is generally well described in the relevant local procedures (e.g. manual of operations), it needs to provide some room for controller discretion in order to be usable. Restrictions commonly include criteria when the procedure can be used (e.g. if the flight time is less than XX minutes or the distance within the sector is less than XX miles). In general, the longer the aircraft spends in the skipping sector, the longer the controller is responsible for an aircraft they do not have contact with. Therefore, skipping flights just because it is within the procedure parameters (as opposed to careful consideration for each and every case) is likely to ultimately lead to an occurrence.
  • Ambiguity. Sometimes the downstream sector may be unsure which sector they need to contact with a request for coordination (the previous or the skipping) as they may not be aware that the flight has been skipped.

Contributory Factors

The folllowing factors may lead to increased need for sector skipping or impact the associated risks:

  • Sector shape and size. This is the primary reason for sector clipping.
  • Free route airspace (FRA). FRA allows greated flexibility for flight planning. This sometimes comes at the cost of individual flights (or traffic flows) choosing routes that clip some sectors.
  • Increased traffic levels. Increased demand is often compensated with more sectors which in turn provides opportunity for more clippings.
  • "Unconcerned" track label colours. Suppressing skipped labels so that they do not draw attention is genereally accepted as beneficial as it allows the controller to focus on the other traffic. However, this also makes it more difficult to detect a potentially dangerous situation.

Defences

  • Sector design. Part of appropriate sector design includes taking account of sector clipping scenarios. While FRA makes it more difficult to eliminate the need for skipping, periodic review and adjustment of sector boundaries may reduce the issue.
  • Increased vigilance. Users of the procedure must remain aware that there is an aircraft in their sector, even though they are not in contact with it and it is in "unconcerned" colour.
  • Any change to the flight profile affecting the skipping sector must be communicated by the neighbouring sectors (previous and next). This does not necessarily mean that the aircraft must pass through the skipping sector in level flight. If climb/descent conditions are coordinated appropriately (e.g. the level to which the aircraft is climbing or descending as well as the vertical speed), skipping can be performed safely.
  • Appropriate procedure design. The conditions under which skipping is acceptable need to be crearly defined. This should include parameters (e.g. maximum clipping time or clipping distance) as well as who can perform the procedure. For example, the first or the last sector of an ATS unit (or both) may be forbidden from using it. If they are allowed to, measures must be implemented to minimise the risk of the adjacent unit not knowing which sector to contact in case a coordination is necessary.
  • System support. An appropriate indication in the track label could help the controller identify (or remember) that the flight is subject to skipping while at the same time not drawing too much attention to a relatively unconcerned flight. An example of this is using a distinct colour for part of the label.

In the example above, the flight is skipped and therefore the track label is presented in unconcerned colour to the skipping sector. However, to indicate that the aircraft might be of interest, yellow string "SKP" is displayed above the label. Clicking on that label would allow the controller to undo the skip.

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