Tanker Modifications
By the middle of June and the Argentine surrender in the Falklands, six probed Hercules had been delivered to the RAF, all but the first of these also having Omega (XV200 returned to Cambridge later, for the navaid to be fitted). The programme to fit probes to 16 aircraft was completed on 25 October 1982.

Meanwhile, also on 30th April 1982, the company had received word from the Ministry of Defence that it should prepare a TI (Trial Installation) for a Hercules tanker, using the standard Flight Refuelling Ltd hose drum unit (HDU) Mk 17B.

For this purpose, a Hercules C Mk 1 (XV296) with the four long range tanks in the fuselage arrived at Cambridge on 1st May, and this aircraft, converted to tanker configuration, flew for the first time on 8 June; the standard probe was also fitted, to allow the tanker itself to refuel in flight, and Omega was installed.

The tanker modification was achieved by locating the HDU on the cargo ramp, with the drogue deployment box and auxiliary serving carriage on the cargo door. This allowed the Hercules to remain pressurised when the drogue was not deployed; to achieve a system that would permit pressure to be retained during drogue deployment would have been more difficult and time-consuming.

Fuel supply was taken from the main aircraft tanks (not the auxiliary tanks in the fuselage) by tapping the standard dump pumps, and pressure to the fuel supply through the HDU to the receiver was provided by a bleed-air turbine-driven fuel pump. To cool the bleed air and components in the HDU, two ram air intakes and two exhaust ports were incorporated through the pressure hull. Standard external lighting for tanker aircraft was provided, and the control panel was located above the navigator's station, adjacent to the in-flight refuelling panel.

The drogue was successfully deployed on the first two flights from Cambridge on 8th and 10th June and XV296 was then delivered to the A&AEE on 11th June. Dry couplings were made with a Harrier, but some problems were encountered, including a slight buffeting around the rear fuselage that was caused by the HDU pack projecting from the cargo ramp, and overheating of the HDU oil cooler.

The aircraft was returned to Cambridge and based on wool-tufting of the aft fuselage to stud, small strakes were fitted on the cargo ramp; with these and a third ram air intake for cooling fitted, the Hercules tanker flew again on 20th June and next day made a successful wet transfer of 5,900 lb (2676 kg) of fuel to a Buccaneer, at 1,000 lb min (454 kg/min), before returning to the A&AEE on 22nd June.

There followed a further period of development flying, including prods by Sea Harrier, Phantom, Nimrod and Hercules at varying aircraft weights and altitudes. The first tanker was delivered to RAF Lyneham on 5th July, but a problem with the heat exchangers persisted and it was eventually decided to introduce alternative heat exchangers, first flown on the second tanker conversion (XV201) on 12 July.

These were then also fitted to XV296, which was finally delivered on 19 July, while the third and fourth aircraft (XV204 and XV192) were delivered on 21 and 26 July. Thus, Marshall of Cambridge completed the entire design, manufacture, installation, ground and flight testing on all four Hercules C Mk1 (K) tankers within 87 days of the initiation of the first requirement - an achievement of which the company is rightly proud and which was recognised by the award of an OBE to the executive director, engineering, Roy O’Gates.