In February 1965, Boeing began to produce the 737 twin-engine jet. This jet, a shorter version of the 707/727 with a more radically swept-back wing, filled a market need for a more fuel-efficient, no-frills aircraft. The smaller size of the 737 enables the jet to fly cost-effectively to smaller airports and more remote locations. In 1964, the original specifications called for a plane with a capacity of between 60 and 85 passengers with an operating range of between 100 and 1,000 miles. Most importantly, the plane had to be able to break even economically with a 35 percent load factor. Today's 737s carry 100 passengers with the range and load factors remaining the same. Three decades and a few design and fuel-efficiency modifications later, the 737 continues to be manufactured. Over the years, it has become the bestselling aircraft ever built. At any one point in time, there are about 1,200 737s in the air. That means that a 737 takes off somewhere in the world every 5.3 seconds. The 737 fleet has flown about 124 million hours in service, traveling 49 billion nautical miles or 90 billion kilometers. Of the 3,000 737s created, most are still flying, which means that many have lasted over a quarter of a century. When the 737 was originally conceived, engineers at Boeing predicted a life expectancy of 75,000 flights, or takeoff-and-landing cycles. Based on the behavior of the current planes still flying long after the predicted 75,000 flights, a Boeing 737 may last as long as 195,000 cycles (flights). Besides longevity, the 737 jets have the advantage of cockpit design and systems commonality with other Boeing jets. The Boeing 737 has wing-mounted engines that provide less interference drag, a better center of gravity position, and more usable cabin space at the rear. From a reliability point of view, the wing-mounted engines allow better access to engines for maintenance and require less pipework for fuel. The weight of the engines also provides bending
relief from the lift of the wings. The marked similarity in cockpit design has simplified pilot training. Cockpit similarities allow pilots to transfer easily from cockpit to cockpit, and cockpit familiarity increases flight safety since in emergency situations pilots are not confused by control-surface differences.
Cockpit and systems commonality provides the benefits of interchangeable parts, components, subassemblies, and subsystems. Maintenance is easier since repair people familiar with one plane can transfer that knowledge to another. Parts availability, and therefore aircraft availability increases with interchangeable parts. The designers of the 737 kept reliability and cost in mind during the design phase. Much of the 727 design, particularly the fuselage cross section, is replicated in the737. This gives cost savings in tooling commonality while providing six-abreast seating in the cabin. The 737 and 727 share 60 percent of their parts, including doors, leading edge devices, nacelles, cockpit layout, avionics, components, and other fittings. Redundant and fail-safe systems are also a part of the Boeing 737. The plane has two engines but is able to fly with only one. The two engines support two electrical systems. Two fuel systems exist on the 737. If a loss of pressurization occurs on an aircraft, an emergency backup oxygen system exists. During its life, the 737 rudder has been redesigned to include three hydraulic rudder control devices instead of the original two. The added system provides better rudder control in the event that one of the devices fails.
All aircraft, whether private or commercial, maintain aircraft logs. In these logs, pilots record all flights and information pertaining to the length of flight, weather conditions, system failures, or repairs needed. The logs provide a record of routine maintenance, as well as information about.
loss of pressurization, turbine failure, engine overheat, false alarms with the warning lights, and other such failures. Extensive tests at Boeing and field-tested information from the 3,000 aircrafts' actual logs of flight experiences have been used to make system design changes, improve manufacturing methods, modify quality checks, and select the most reliable components.
In an industry where 10 to 15 percent of the operating costs are devoted to maintenance, these aircraft are maintained in such a manner that they will last indefinitely. Planes are systematically inspected, cleaned, repaired, reinforced, rebolted, and resealed. This type of maintenance exceeds original factory standards and protects the planes from deterioration due to wear and tear and corrosion. However, as the planes age increases, the maintenance cost per flight hour increases significantly. The airline industry rule of thumb says that at the 25-year mark, a plane's maintenance will cost approximately double the maintenance costs associated with a new plane. The mean life of a jet engine is approximately 15,000 hours. Jet engines can be rebuilt, overhauled, or replaced to prolong the life of the entire jet. With the Boeing 737, pilots and mechanics have discovered that engine wear can be reduced by decreasing the amount of engine thrust by 4 to 5 percent, enabling the engine to run 30 to 40 degrees cooler. Communication of this sort has resulted in significantly reduced engine wear and prolonged engine life.