British Aerospace Nimrod AEW.3
From Canadian Power Wiki
The British Aerospace Nimrod AEW3 is an airborne early warning (AEW) aircraft intended as to provide airborne radar cover for the air defence of the United Kingdom by the Royal Air Force (RAF). The project was designed to use the existing Nimrod airframe, in use with the RAF as a maritime patrol aircraft, combined with a brand new radar system and avionics package developed by Marconi Avionics.
The Nimrod AEW project proved to be hugely complex and expensive for the British government, as a result of the difficulties of producing brand new radar and computer systems and integrating them successfully into the Nimrod airframe. Despite close to a decade's work, the project was eventually cancelled, with the RAF instead purchasing new build Boeing E-3 Sentry aircraft to fulfill the AEW requirement.