The Kestrel trials gave an encouraging early assessment of the practicality of V/STOL combat aircraft. Continuing the progress, the designation P.1127(RAF) was introduced to indicate a new line of development from the Kestrel, though sharing common ancestry. Considering that the P.1127(RAF) was claimed to represent a 90 per cent redesign of the Kestrel, the aircraft were produced remarkably quickly. The first (XV276) flew on 3 August 1966, and the last (XV281) on 14 July 1967, by which time the aircraft had been renamed ‘Harrier’.
The powerplant for these six machines and early production Harrier GR1s was the 19,000lb thrust Pegasus 6 or Mk 101. The wing itself was further developed, not only to accommodate higher weights, but also to cure the Kestrel’s poor longitudinal stability when carrying underwing stores. A more conventional ventral airbrake was also added, and, for the first time, pilots had the benefit of a ‘zero-zero’ (i.e. ground level with no forward speed) Martin-Baker Mk 9 ejection seat.
The Harrier’s primary role in service was seen to be attacking the enemy’s second-echelon armoured units in the face of relatively advanced air defences. It would therefore operate at high subsonic speed and mainly at low level, although its Hunting BL755 cluster weapons and 68mm SNEB rockets required more height in the attack phase. At that time the Harrier was a world leader in terms of its navigation/attack system, as a result of Britain’s development effort on similar equipment for the ill-fated BAC TSR2. A HUD (head-up display) allowed the pilot to fly safely at high speed and low level, since it largely eliminated the need to look down into the cockpit. Its moving-map display proved an attractive feature to the US Marine Corps when they came to evaluate the Harrier in 1968.
The original plan was to have one squadron of Harrier GR1s in Britain and one in West Germany, on which basis an initial order was placed for sixty aircraft. The first production contract for the GR1 was signed in early 1967, and XV738 made its maiden flight on 28 December of that year. Subsequently the RAF decided to increase its Harrier force to four squadrons (three of which were to be based in Germany) and additional orders were approved. The single-seat Harrier GR1/1A/3 production eventually totalled 118 aircraft, excluding the six development machines.
Training RAF pilot-instructors on the Harrier began at Dunsfold in January 1969, and on 1 April the Harrier Conversion Unit (HCU) was formed at RAF Wittering. This is the date normally quoted for the Harrier’s entry into service. On 1 October 1970, the HCU became No. 233 Operational Conversion Unit (OCU) to provide, within No. 1 Group, conversion and operational training for Harrier pilots.