<P> The 737's main landing gear under the wings at mid-cabin rotates into wells in the aircraft's belly . The legs are covered by partial doors, and "brush - like" seals aerodynamically smooth (or "fair") the wheels in the wells . The sides of the tires are exposed to the air in flight . "Hub caps" complete the aerodynamic profile of the wheels . It is forbidden to operate without the caps, because they are linked to the ground speed sensor that interfaces with the anti-skid brake system . The dark circles of the tires are clearly visible when a 737 takes off, or is at low altitude . </P> <P> 737s are not equipped with fuel dump systems . The original aircraft were too small to require them, and adding a fuel dump system to the later, larger variants would have incurred a large weight penalty . Boeing instead demonstrated an "equivalent level of safety". Depending upon the nature of the emergency, 737s either circle to burn off fuel or land overweight . If the latter is the case, the aircraft is inspected by maintenance personnel for damage and then returned to service if none is found . </P> <P> Engines on the 737 Classic series (− 300, − 400, − 500) and Next - Generation series (− 600, − 700, − 800, − 900) do not have circular inlets like most aircraft . The 737 Classic series featured CFM56 turbofan engines, which yielded significant gains in fuel economy and a reduction in noise over the JT8D engines used on the − 100 and − 200, but also posed an engineering challenge given the low ground clearance of the 737 . Boeing and engine supplier CFMI solved the problem by placing the engine ahead of (rather than below) the wing, and by moving engine accessories to the sides (rather than the bottom) of the engine pod, giving the 737 a distinctive non-circular air intake . </P> <P> The wing also incorporated changes for improved aerodynamics . The engines' accessory gearbox was moved from the 6 o'clock position under the engine to the 4 o'clock position (from a front / forward looking aft perspective). This side - mounted gearbox gives the engine a somewhat triangular rounded shape . Because the engine is close to the ground, 737 - 300s and later models are more prone to engine foreign object damage (FOD). The improved CFM56 - 7 turbofan engine on the 737 Next Generation is 7% more fuel - efficient than the previous CFM56 - 3 in the 737 classics . The newest 737 variants, the 737 MAX family, are to feature CFM International LEAP - 1B engines with a 1.73 m fan diameter . These engines are expected to be 10 - 12% more efficient than the CFM56 - 7B engines on the 737 Next Generation family . </P>

Who makes the jet engines for the 737