Final answer:
The main landing gear on an aircraft are held in the up-and-locked position using mechanical locks, with hydraulic pressure used for movement and aerodynamic forces and gravity providing additional security.
Step-by-step explanation:
The main landing gear are held in the up-and-locked position primarily by mechanical locks. While hydraulic pressure is used to move the landing gear into the correct position, once the gear is in the up-and-locked position, the mechanical locks ensure that they stay retracted during flight. These locks are designed to withstand the vibrations and stresses of flight without failing. Aerodynamic forces and gravity also play roles in ensuring that the gear stays retracted, but they are secondary to the role of the mechanical locks.
It is important to note that while hydraulic systems are critical for the operation of the landing gear, the lock mechanism is a safety feature that helps maintain the gear in the required position regardless of hydraulic pressure. This design philosophy enhances the reliability of the aircraft's systems.