As part of our annual Conditional Inspection, the Sea Harrier is placed on jacks and hydraulic power is applied. The gas motor running is actually our “log splitter” hydraulic generator. It provides the power to actuate the flight controls. All the hydraulic flight controls, etc., are exercised, including retracting and lowering the landing gear. This ensures all the doors open and close, in sequence, and there are no gaps between the doors and the fuselage. A good, tight fit is required. A landing door open or partially open at 600 knots could not only cause handling problems, but significant structural damage to the door or airframe. This video shows some of the many landing gear cycles and the emergency extension at the end.
After the gear has been extended by the emergency system, all the actuators must be bled of air. This is a LOT of work, but must be done to ensure we have a save and operable hydraulic system.