A pitch change apparatus for a propeller assembly having a row of propeller blades which rotate around an axis of the assembly, the apparatus comprising: a hydraulic actuator for angular displacement of propellers of the propeller assembly, the hydraulic actuator having a piston and cylinder arrangement which defines fine and coarse fluid chambers, whereby increasing the volume of the fine chamber at the expense of the coarse chamber results in angular displacement of the propellers to a finer pitch, and increasing the volume of the coarse chamber at the expense of the fine chamber results in angular displacement of the propellers to a coarser pitch, one or more fine hydraulic lines carrying hydraulic fluid to and from the fine chamber, and a coarse hydraulic line (68a") carrying hydraulic fluid to and from the coarse chamber; wherein the apparatus further comprises a pressure-sensing check valve which fluidly isolates the coarse chamber from the coarse hydraulic line if the pressure in the coarse hydraulic line falls below a threshold pressure, wherein the check valve has: a valve body which is movable between an open position in which the coarse hydraulic line can carry hydraulic fluid to and from the coarse chamber, and a closed position in which the coarse chamber is fluidly isolated from the coarse hydraulic line, the valve body being biased towards the closed position; and, a pilot piston and cylinder which define a pilot chamber in fluid communication with the coarse hydraulic line, the pilot piston being movable to counteract the bias on the valve body and thereby move the valve body to the open position when the pressure in the pilot chamber is at or above the threshold pressure; characterized in that the pilot chamber is located at one side of the pilot piston, and the pilot cylinder has a leakage line extending from the opposing side of the pilot piston to provide a pressure difference across the pilot piston when the pilot chamber is at or above the threshold pressure. a hydraulic actuator for angular displacement of propellers of the propeller assembly, the hydraulic actuator having a piston and cylinder arrangement which defines fine and coarse fluid chambers, whereby increasing the volume of the fine chamber at the expense of the coarse chamber results in angular displacement of the propellers to a finer pitch, and increasing the volume of the coarse chamber at the expense of the fine chamber results in angular displacement of the propellers to a coarser pitch, one or more fine hydraulic lines carrying hydraulic fluid to and from the fine chamber, and a coarse hydraulic line (68a") carrying hydraulic fluid to and from the coarse chamber; wherein the apparatus further comprises a pressure-sensing check valve which fluidly isolates the coarse chamber from the coarse hydraulic line if the pressure in the coarse hydraulic line falls below a threshold pressure, wherein the check valve has: a valve body which is movable between an open position in which the coarse hydraulic line can carry hydraulic fluid to and from the coarse chamber, and a closed position in which the coarse chamber is fluidly isolated from the coarse hydraulic line, the valve body being biased towards the closed position; and, a pilot piston and cylinder which define a pilot chamber in fluid communication with the coarse hydraulic line, the pilot piston being movable to counteract the bias on the valve body and thereby move the valve body to the open position when the pressure in the pilot chamber is at or above the threshold pressure; characterized in that the pilot chamber is located at one side of the pilot piston, and the pilot cylinder has a leakage line extending from the opposing side of the pilot piston to provide a pressure difference across the pilot piston when the pilot chamber is at or above the threshold pressure. a valve body which is movable between an open position in which the coarse hydraulic line can carry hydraulic fluid to and from the coarse chamber, and a closed position in which the coarse chamber is fluidly isolated from the coarse hydraulic line, the valve body being biased towards the closed position; and, a pilot piston and cylinder which define a pilot chamber in fluid communication with the coarse hydraulic line, the pilot piston being movable to counteract the bias on the valve body and thereby move the valve body to the open position when the pressure in the pilot chamber is at or above the threshold pressure; characterized in that the pilot chamber is located at one side of the pilot piston, and the pilot cylinder has a leakage line extending from the opposing side of the pilot piston to provide a pressure difference across the pilot piston when the pilot chamber is at or above the threshold pressure.