 In the early 1970s, the RAF issued an Air Staff Requirement for the development of a new interceptor intended to replace both the Phantom and the Lightning. One initial proposal was a plan introduced by McDonnell Douglas for a variable geometry wing Phantom. This was rejected by the RAF owing to the fact that there was little apparent improvement in performance over the existing Phantom, and that it might affect the development of the "Multi-Role Combat Aircraft" (MRCA). Another idea that the MRCA concept, which eventually evolved into the Panavia Tornado, could be used in the interceptor role, was ruled out, as both its avionics and engines were optimised for the low level interdictor mission. The UK's partners in the MRCA project displayed no enthusiasm for the idea of developing an air defence version of the Tornado, so the UK alone began the process, with the authorisation for what came to be known as the Tornado ADV issued in March 1976. With the aircraft in development, the initial plan was for the Tornado to replace the remaining two squadrons of Lightnings, as well as all seven squadrons of Phantoms. However, while the Tornado was in development, the RAF looked at interim measures to replace the Phantom, which had been in service for over a decade by 1980, and was beginning to suffer from fatigue issues; one proposal looked at was the possibility of leasing or purchasing a quantity of F-15 Eagles to re-equip 19 and 92 Squadrons, the units stationed in Germany. Further suggestions were that up to 80 F-15s be procured, to replace the Phantom and Lightning squadrons then in service, or even cancel the Tornado entirely and purchase the F-15 with UK adaptations (specifically to be fitted with the AI.24 Foxhunter radar developed for the Tornado, and capable of carrying the Skyflash air-to-air missile). Ultimately, the F-15 option was not seriously considered, as it was felt there would not be time or cost savings over this plan against the Tornado. Subsequent to this came a decision that the Tornado, once it had entered service, would only re-equip three of the Phantom squadrons, with two retained in the UK and two in Germany. Eventually, the Tornado accounted for the two FG.1 squadrons at RAF Leuchars (43 and 111 Squadrons), plus two FGR.2 units (23 Squadron and 29 Squadron), with 56 and 74 Squadrons remaining with the Phantom.
