Complex sentence: It was not until November 1941 that the Me 262 V1 was flown with BMW engines, which both failed during the test, the prototype having to return to the airfield on the power of the piston engine, which was still fitted.
Split into simpler sentences: It was not until November 1941 that the Me 262 V1 was flown with BMW engines, which both failed during the test. The prototype aircraft had to return to the airfield on the power of the piston engine, which was still fitted.