I would like to help clear up a few things up about responsibility specially because there is bound to be the press amongst us.
Firstly. I am quite sure YAKs can only be registered in the LIMITED category Certificate of Airworthiness.
Presuming it is registered in the LIMITED category then it can be used to operate adventure flights by a commercially licenced pilot without the need of an Air Operator Certificate and the regulation that entails.
I am not sure but there may be some requirement for the pilot/plane to be registered with the Warbirds organisation.
Anyway, presuming the aircraft was registered in LIMITED, the chief pilot of Hempel Aviation under my interpretation of CASA rules, would not be responsible for checking the pilots qualifications or medical status simply because it is not operating under the Hempels Aviation AOC to which the chief pilot is responsible.
So I dont think there can be any direct blame on the Hempels Aviation chief pilot as he was not required to monitor status of pilots of another operation. However, how can you tell one operation from another when they co-exist and owned by the same person. Difficult one.
OOW