the observer's rear cockpit. The F.2B often carried a second Lewis gun on the rear cockpit mounting, although observers found the weight of the twin Lewis gun mounting difficult to handle in the high altitudes at which combat increasingly took place in the last year of the war, many preferring one gun. Attempts were made to add a forward-firing Lewis gun on a Foster mounting or similar on the upper wing either instead of (or with) the Vickers gun. Among other problems this caused interference with the pilot's compass, which was mounted on the trailing edge of the upper wing: to minimise this effect the Lewis gun was offset to starboard. Alternative engines Rolls-Royce aero engines of all types were in short supply, which frustrated plans to increase production to enable the F.2B to become the standard British two-seater, replacing the R.E.8 and F.K.8. Efforts were made to find an available alternative powerplant that was both reliable and sufficiently powerful. The Type 15 was fitted with a 200 hp (150 kW) Sunbeam Arab engine. In expectation of a reduction in performance with the less powerful engine, it was planned to supply the Arab-powered Fighters to the "corps" reconnaissance squadrons, reserving Falcon powered examples for fighter–reconnaissance operations. The Arab engine was to be fitted to Fighters produced by sub-contractors under licence, while Bristol-built Fighters would continue to use the Falcon. In the event the Arab engine was not a success; it was found to suffer from serious weaknesses in cylinder and crank-chamber design that led, among other faults, to chronic and severe vibration, while the cooling system also required repeated modification. The "Arab Bristol" was never to become a viable combination, in spite of prolonged development. A few Arab-engined Bristols were at the front late in the war but the British reconnaissance squadrons had to soldier on with the R.E.8 and F.K.8 until the end of hostilities. The Type 16 was fitted with a 200 hp (150 kW) Hispano-Suiza engine. This worked better than the Arab but there was already a severe shortage of Hispano-Suizas for other types, such as the S.E.5a and the Sopwith Dolphin. The 300 hp (220 kW) version of the Hispano-Suiza, suggested for the Type 17, was not available in quantity before the end of the war. Other engines tried or suggested for the F.2B were the 200 hp (150 kW) RAF 4d, the 180 hp (130