Who has said that they want a single type in service? The NZDF air transport study hasn't been released yet and AFAIK no leaks, so we actually don't know what they are thinking. Personally I would be thinking that they could be looking at a two tier transport system because the non replacement of the Andovers will have been hurting so to speak. It is true that the Andovers were becoming economically unsustainable, however the utilisation of the Hercs to undertake Andover type taskings will have been expensive, or the taskings were not undertaken. Until the study is released and / or it is covered in the DWP we will just have to wait.