On all 737 aircraft produced since 1969, Boeing uses hydraulic thrust reversers to change the direction of the exhaust. This creates a force against forward motion to slow the aircraft. The hydraulic system replaces the earlier pneumatic system used by the manufacturer.
The deflector doors are set at a 35 degree angle to vertical, allowing the thrust reversers to direct the plane's exhaust over the wings inboard and under the wings outboard. This angle prevents the thrust diverters from blowing debris into the wheel well and prevents lifting the plane back off the runway.
The hydraulic design provides several significant advantages for the plane. It employs a longer exhaust pipe than the pneumatic design, which improves the cruise performance of the 737 and reduces drag. Boeing data shows this type of thrust reverser to be twice as effective as older style clamshell thrust reversers, according to Power Plant.
On propeller-driven aircraft, thrust reversal is possible using variable-pitch propellers. The pilot adjusts the pitch so the propeller pushes against the air instead of pulling it. However, the jet aircraft uses the force of the exhaust gas to provide forward motion. Changing the exhaust direction is the only requirement to provide the desired velocity change.