– flying for long hours in intensely cold weather at – revealed several problems with these engines. They had a poor manifold fuel-air distribution and poor temperature regulation of the turbo-supercharger air, which resulted in frequent engine failures (detonation occurred as the result of persistent uneven fuel-air mixture across the cylinders caused by the poor manifold design). Specially formulated fuels were a necessity for the P-38 as were specific spark plugs needed for specific cylinders. The turbo-supercharger had additional problems with getting stuck in the freezing air in either high or low boost mode; the high boost mode could cause detonation in the engine, while the low boost mode would be manifested as power loss in one engine, resulting in sudden fishtailing in flight. These problems were aggravated by sub-optimal engine management techniques taught to many pilots during the first part of WWII, including a cruise setting that ran the engine at high RPM and low manifold pressure with a rich mixture. These settings can contribute to over-cooling of the engine, fuel condensation problems, accelerated mechanical wear, and the likelihood of components binding or "freezing up." Details of the failure patterns were described in a report by General Doolittle to General Spatz in January 1944. In March 1944, the first Allison engines appearing over Berlin belonged to a group of P-38Hs of 55th Fighter Group, engine troubles contributing to a reduction of the force to half strength over the target. It was too late to correct these problems in the production lines of Allison or GE, and as the numbers of Merlin-engined P-51 Mustangs based in England mounted up through the end of 1943 and into 1944, the P-38s were steadily withdrawn from Europe until October 1944 when they were no longer used for bomber escort duty with the Eighth Air Force. A few P-38s would remain in the European theater as the F-5 for photo reconnaissance. The P-38 had fewer engine failures in the Pacific Theater, where operating techniques were better developed (such as those recommended by Charles Lindbergh during his development work in the theater),) and the Japanese did not operate at such high altitudes. Using the same P-38Gs which were proving difficult to maintain in England, Pacific-based pilots were able to use the aircraft to good advantage including, in April 1943, Operation Vengeance, the interception and downing of the Japanese bomber carrying Admiral Isoroku Yamamoto. New P-38