 porterbrook monitors cumulative mileage on passenger vehicles to ensure mileage based maintenance events are undertaken within allowed limits planned extensions are managed to support service operation or maintenance lifecycle optimisation there is currently a total of 96 vehicles with mileages above a maintenance event mileage threshold all are within a controlled management regime there is 1 vehicle stood down awaiting heavy maintenancethere are 16 vehicles with temporary mileage extensions to allow ongoing service operation at the customer s request appropriate technical and operational controls are in place to manage these temporary extensionsthere are 79 vehicles under controlled mileage extensions as part of maintenance lifecycle optimisation work appendix a contains a summary of the overdue maintenance trends by category 