Complex sentence: For the second test-flight, four days later, RATO units were fitted to the aircraft but, because their alignment had been miscalculated, the pilot mistakenly believed that they had not fired and thus shut off the main engines to abort take-off.
Split into simpler sentences: For the second test-flight, four days later, rocket assisted take off (RATO) units were fitted to the aircraft. However, because their alignment had been miscalculated, the pilot mistakenly believed that they had not fired and thus shut off the main engines to abort take-off.