What holds the landing gear up in the aircraft?

The correct choice is the nitrogen accumulator, which is essential in aircraft landing gear systems. In many aircraft designs, including those that operate under Part 135 regulations, the landing gear is retracted and held up by hydraulic pressure. This pressure is often maintained using a nitrogen accumulator.

The nitrogen accumulator acts as a reservoir of compressed nitrogen gas that provides the necessary pressure for the hydraulic system. When the landing gear is retracted, the hydraulics push the gear into the wells, and the accumulator helps maintain that pressure, ensuring that the landing gear remains in the up position during flight.

This system effectively combines the advantages of hydraulic mechanisms with the reliability of compressed gas, allowing for smooth operation while minimizing weight and complexity. It enhances the overall performance of the landing gear system, adhering to safety and efficiency standards crucial for aircraft operation.