During cruise the right engine began to surge and vibrate. Normal actions to restore engine performance were unsuccessful and the engine was shut down. About four minutes later the left engine lost power in a similar manner to the right. The pilot carried out a forced landing but the nosewheel was torn off when it struck a ditch. A subsequent examination revealed no evidence of pre-impact defects or malfunctions of either engine which might have contributed to the reported power losses. Atmospheric conditions at the time were conducive to the formation of carburettor icing. The aircraft was not equipped with a carburettor air temperature gauge, and the pilot had not applied carburettor heat until the initial loss of power from the right engine. At the same time, he had moved the mixture levers to the full rich position, which would have reduced the exhaust gas temperature. This in turn would reduce the amount of heat available to disperse any ice accumulation. It was considered probable that if an appreciable amount of ice had formed, there was insufficient time available for it to be dispersed by the heat applied, before the pilot was committed to a forced landing.