Disruptions to plans and schedules are inevitable and can be expensive. The requirements from airlines and rail operators to manage disruptions are demanding and multi-faceted.
The requirements are fairly easy to express
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Find the 'best' solution.
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Make as few changes as possible.
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Get back on schedule as quickly as possible.
In order to solve these problems, we have developed TPAC Schedule Recovery that:
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Quickly finds a range of good/acceptable solutions.
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Helps the user determine which solution is best on the day and allows them to use their wider knowledge of the situation.
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Makes as few changes as possible and returns to schedule quickly.
TPAC Schedule Recovery solves problems caused by unplanned events. These include:
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Weather
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Closure of facilities
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Congestion
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Equipment problems
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Crew availability
Each one of these - and there may be more than one at a time - prevents the schedule being executed as planned and a problem in one part of the network often quickly affects others.
TPAC Schedule Recovery finds a range of solutions which:
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Reduce the potential negative impact for passengers.
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Limit the cost (if any) of additional resources.
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Reduce the impact on related functions, such as maintenance, support staff, etc.
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Return the operation to schedule as soon as possible.
This is an important part of the system. It generates solutions using a combination of techniques:
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A decision tree where each node is an action on the network.
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Different initial actions to generate a range of reasonable solutions.
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Limited actions (cancel, delay, etc) which can be applied at any part of the network.
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A predictor/corrector algorithm for efficiently comparing solutions and rejecting poor solutions early.
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Recursion whereby any solution can be improved using the same procedure as the original.
The benefits of this approach are:
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The algorithm only makes changes that have real benefits.
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It is fast; less than one minute for a fleet size of more than 200.
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The process can be truncated yet still yield acceptable solutions.
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The operator can view a range of solutions and make his/her own assessment.
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The algorithm modifies the schedule to cope with disruption, rather than completely rebuilding it.
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It can consider all the constraints, e.g., equipment, crew, maintenance, etc, in one process.
The Solution Manager enables the user to look at a wide range of solutions.
Each solution comes with costs which are either hard ($) or soft (e.g., passenger delay minutes), and implications (e.g., equipment out of place for overnight maintenance).
The user looks at all solutions and can reorder based on best or worst in any category and prune away undesirable solutions and/or solutions which have unacceptable implications. Out of the solutions remaining after this exercise the best solution is selected.
Because the algorithm works on the basis of changes to the schedule, it is straightforward to provide the user with information indicating how the schedule was modified. This makes it very easy to understand and rationalize.
Schedule Recovery has been field tested for a customer with a fleet size of 200+ aircraft.
The conclusions are:
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In excess of 25 acceptable solutions can be produced in minutes on a workstation or PC.
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The system produces solutions as good as a very experienced Operations Controller.
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Using the Solution Manager, the user can take account of external factors.
Further Information
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