Complex sentence: To aim bombs, the bombardier had to open a hatch in the nose of the airplane which extended the bomb sight out of the fuselage, but to everyone's comfort, the Mk IV replaced this hatch with a slightly extended transparency.
Split into simpler sentences: To aim bombs, the bombardier had to open a hatch in the nose of the airplane which extended the bomb sight out of the fuselage. To everyone's comfort, Mk IV replaced this hatch with a slightly extended transparency.