Development of the quieter, more fuel-efficient CFM56-3 series of engines allowed Boeing to improve its most popular jetliners and produce the advanced-technology 737-300, -400, and -500. The engines are larger and mounted forward of the wing struts instead of tucked directly under the wing as in the -100 and -200 versions. Advancements on the new-generation 737s included a stregthened wing to accomodate greater gross weight, tip extensions to the wings and horizontal stabilizers, and stregthened wheels, tires and brakes. The 737-200, -300, -400, -500 have the same handling characteristics so that pilots can use the same Type rating to fly all of them. Airlines' spares inventory and maintenance are simplified by making a substantial number of replaceable parts on the -300, -400, -500 identical to those used on the -200. The 737-400 is about 10 feet longer than the 737-300. It holds 19 more passengers and has a gross weight of up to 150,000 pounds, while retaining the same fuel efficiency, low noise, and low operating costs as its predecessor.