This article also describes language coverage awards, which is not in all configurations.
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 take 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 are 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.
Note: 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.
As 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. Due to these interactions, on rare occasions, it may appear as if the system has violated seniority by. For example, a junior crewmember holding a day off specifically requested by a more senior crewmember. 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.
Note: 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. In addition to the existing primary stack date, PBS offers a secondary priority stack date. The secondary date can only be enabled if the primary unstacking date is enabled.