Stacks are the number of pairings operating within a critical  period that remain unassigned.  Stacks may result from spikes in  the flight schedule during a given period or an abnormally high number  of requests for the same day off, such as Thanksgiving or Christmas Day.

        pairing stacks happen when   too many pairings are unassigned

        language stacks happen when   too many pairings requiring the same language are unassigned

Coverage Awards ensure work coverage, and takes precedence  over any bid preferences. The N-PBS Scheduler makes multiple passes through  the schedule, to ensure that all rules are met and all pairing stacks  fall within the limits set by Crew Planning. Coverage awards occur on  each pass, but the N-PBS Scheduler adjusts its settings for each pass  as it brings the stacks within acceptable limits.

Critical Period -  a critical period is a specific time period determined by how pairings  and rules interact. For example, a critical period may occur from 23:30  Dec 24th to 01:30 Dec 26th

Pairing Stack -  pairing stacks represent a set of mutually exclusive pairings (only one  of these pairings can be held by a Crewmember at a time); coverage awards  is triggered when pairings must be forced on Crewmembers to avoid too  many pairings remaining in open time during a critical period (based on  the maximum stack height setting)

Language Credit Stack  - language credit stacks represent all of the pairings that require  a given language; language credit stacks happen when more language-required  credit remains in open time than can be covered by the remaining language-qualified  people language credit stacks are based on the overall language-required  credit for the bid period, and are not associated with a specific critical  period

Language Pairing Stack  - language pairing stacks represent a set of mutually exclusive  pairings with a shared language requirement; language stacks happen when  more pairings requiring the same language remain in open time during a  critical period that can be covered by reserve (based on the maximum stack  height setting)

A pairing stack is ”triggered” for coverage awards during  a pass when the N-PBS Scheduler calculates that it must attempt to force  the stack on every remaining Crewmember to avoid having the final number  of unassigned pairings operating in a critical period exceed the maximum  value set by Crew Planning (usually the maximum number of pairings that  can be covered by reserve). Once a stack is triggered, the N-PBS Scheduler  needs to eliminate it through coverage awards.

To eliminate or reduce stacks, the N-PBS Scheduler needs  to award the pairings in the stacks by assigning them to junior Crewmembers,  despite those specific preferences that conflict with the pairings that  need to be awarded. The N-PBS Scheduler determines where it needs to start  awarding pairings from the stack, and any Crewmembers that fall below  this point receive a Coverage Award pairing (subject to legalities and  pre-awards). Coverage Award pairings are assigned to a block first, before  the N-PBS Scheduler starts to build the rest of a Crewmember’s block  using their bid. This pairing is marked as an unmovable event, like a  pre-award, training or vacation.

The N-PBS Scheduler tries to find  a pairing in the stack that either meets the Crewmember’s preferences  or conflicts with the least number of Avoid Pairings and Prefer Off bid preferences (in order of priority) and also satisfies the Crewmember’s award preferences. Basically, if the N-PBS Scheduler can’t meet all the bid preferences, it tries to award each Crewmember the pairing that matches as many bid preferences as possible.

Because pairings vary in length, and interact with non-overlapping  pairings due to legality rules, solving a stack for one particular critical  period may result in the emergence of a stack during another critical  period. Because of these interactions, on rare occasions it may appear  as if the system has violated seniority by, for example, a junior Crew  member holding a day off specifically requested by a more senior Crew  member. When, in fact, there was no way to combine the flight duties such  that the work was covered and rules regarding rest and max duty were respected.       

If a pairing awarded as Coverage violates a Prefer Off  or Avoid Pairings bid that has an Else Start Next  attached to it, the N-PBS Scheduler attempts the Else  Start Next before awarding pairings from the stack, unless the  pairing was awarded for  language credit coverage.

If multiple pairing stacks have been triggered, the N-PBS Scheduler assigns a pairing from each triggered  stack, starting with the stack closest to the beginning of the bid period and continuing to the end of the bid period, unless a priority stack date  has been triggered.

A priority stack date is a critical period that takes precedence  over all other pairings that need to be awarded first. The priority stack  date helps to ensure that seniority for a specific date isn’t violated  by Coverage Awards.