<sod >The Air Accident Investigation Bureau said it was investigating a "mid-air collision involving an aircraft and a helicopter near Aylesbury".< eod>< sod > The Air Accidents Investigation Bureau said it was investigating a "mid-air collision involving an aircraft and a helicopter near Aylesbury" while police said preservation of life was their top priority. The aircraft involved was "small", The IndependentÃÂÃÂ understands.< eod>