John R81
Makes prefect sense after all the Police only have part of the information regarding the matter, they need to check the aircraft itself and its records which they don't have access to. They also need as previously stated inspections carried out by competent persons into this and they do not have the resources / expertise within the police force. So therefore use experts in the field to do this.
After all if a car crashes and results in the death of persons, the vehicle is examined for defects and also the driver is questioned (as in the case of the Scottish bin lorry) plus witness statements are all provided to the CPS.
You can't expect the police to put a case to the CPS without the missing information and the only persons that have access to this is the AAIB.
I would imagine that the AAIB may not be able to issue their report until the CPS have decided if they will be taking any further action and then possibly after any resulting court case.