Development of the quieter, more-fuel-efficient CFM56-3 series of engines allowed Boeing to improve its most popular jetliners in the 1980s 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 older versions of the 737. Boeing gave all three versions the same popular interior as the 757, with contoured sidewall panels providing additional head and elbow room and a wider aisle than earlier airplanes. The new-generation 737s included a strengthened wing to accommodate greater gross weight, tip extensions to the wings, horizontal stabilizers, and strengthened tires, wheels and brakes. The 737-200, -300, -400, and -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 and -500 identical to those used on the -200. On June 4, 1986, Piedmont airlines became the kickoff customer for the 737-400. The aircraft 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. It retains the same fuel efficiency, low noise, and low operating costs as its predecessor. In 1993, Boeing began to offer improved versions of the 737 family, the Next-Generation series of 737 jetliners, the 737-600, -700, -800 and the -900. The Next-Generation 737s continue to build on the strengths that made the 737 the world's most successful commercial airliner, while incorporating improvements and value-added technology for the 21st century.