What component is essential for maintaining airplane controllability in the 737-800?

The essential component for maintaining airplane controllability in the 737-800 is both hydraulic systems A and B. Aircraft like the Boeing 737-800 rely heavily on hydraulic systems to operate critical control surfaces, which include ailerons, elevators, and rudders. These control surfaces are vital for maneuvering and stability during flight.

The hydraulic systems provide the necessary pressure to move these control surfaces and ensure that the pilots have responsive and effective control over the aircraft, particularly during critical phases of flight such as takeoff, landing, and in response to turbulence. If one system were to fail, the second system acts as a backup, allowing for continued controllability, which is essential for safe operation.

While the standby hydraulic system, landing gear system, and cabin pressure control system are all important for various functions within the aircraft, they do not play the primary role in maintaining controllability. The standby hydraulic system provides redundancy but does not singularly ensure flight control. The landing gear system is essential for takeoff and landing but does not affect the aircraft's maneuverability in flight. Similarly, the cabin pressure control system is crucial for passenger safety and comfort but does not influence the control surfaces or airplane stability during flight.