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the local island gossip being so BlueIslands can save money from Eurocontrol costs
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The only regular routes which go outside airways are Alderney-Bournemouth and Alderney-Shoreham. A quick glance at a chart will show that there is a lack of suitable airways for the whole of these routes, unlike Alderney-Southampton, which has its own airway with a specially low base level to accommodate the BN2 performance issues. Cost is only a very small part of the equation and would never be allowed to compromise safety.
As for the Trislander autopilot issue, I have never been in an Aurigny one, but 700hrs in Trislanders leads me to believe that there is no CAA approved autopilot made for them. That of course may only be the case for fitting a new one. If it's always been there that may be different.