Complex sentence: The flight was then 3 kilometers from the aerodome and flying at a speed of 450 km/h when it had still not released the landing gear; in the preparations for landing the navigator forgot to set his altimeter to the correct pressure.
Split into simpler sentences: The flight was then 3 kilometers from the aerodome and flying at a speed of 450 km/h when it had still not released the landing gear. In the preparations for landing the navigator forgot to set his altimeter to the correct pressure.