Ad-hoc use of police helicopters in the early 1990’s saw Hertfordshire Constabulary call upon air support from the Metropolitan and Thames Valley Police. In 1995 Hertfordshire and Bedfordshire Police conducted a successful trail using an aircraft loaned from Police Aviation Services. In 1996 the Chiltern Air Support Unit was formed, comprising Hertfordshire Constabulary, Thames Valley Police and Bedfordshire Police.
Two Aerospatiale AS355 helicopters, one based at Luton Airport and one at RAF Benson in Oxfordshire, provided over 20hrs of daily response coverage. Police officers from the three forces were seconded to the Air Support Unit in the role of Air Observer, four of which were from Hertfordshire Constabulary.
A second hand AS355 F1 helicopter was purchased and modified for the police role, working from Luton Airport. This aircraft had tail code G-CPOL and police call sign XA99.