Hydraulic System - General
The reservoir, located in the left hand wing at the inboard wing root, is pressurised with regulated engine bleed air to provide a positive supply of hydraulic fluid to the engine driven hydraulic pumps.
The fluid that is used in the system is MIL-H-5606 hydraulic fluid which does not contain fire retarding agents and therefore cannot be used in the Metro hydraulic systems.
A reservoir filler cap on the upper surface of the left hand wing features a dipstick and a sight guage. The fluid level for the hydraulic system is checked by observing the colour of the sight guage on the filler cap. If the guage is dark in colour, sufficient fluid exists in the system for normal operation.
Hydraulic system reservoir located at the left hand wing root
If the reservoir requires servicing, MIL-H-5606 hydraulic fluid can be added by removing the cap and adding fluid until ‘one inch' of fluid shows up on the dipstick.
Ensure that hydraulic system pressure has been removed, before removing the cap. This is achieved by pulling a relief valve located on the lower side of the left hand wing directly underneath the reservoir.
Hydraulic system pressure relief valve located under the left hand wing.
The 'open centre' hydraulic system operates through a solenoid operated system bypass valve which is spring loaded to the open position and routes the pump output to the reservoir.
This bypass valve is energised closed by manual selection of the flap or landing gear levers and blocks the return of the hydraulic fluid, producing hydraulic pressure and actuating whichever system was selected.
If electrical power is lost the valve fails open allowing fluid to circulate throughout the system but deems the sub-systems unusuable.
A pressure relief valve is also enclosed within the bypass valve to limit the amount of system pressure.
Hydraulic system bypassing
If either pump is not producing sufficient flow for normal operation, or, if a pump becomes inoperative, the applicable L/R HYD FLOW LOW annunciator light will illuminate. Should this light illuminate normal system operation should be available, however, refer to the Abnormal section of the Pilot Operating Handbook for correct in flight procedures.
When either hydraulic sub-system is operated, the solenoid operated bypass valve closes and flow is diverted to the landing gear and flap control valves.
The hydraulic pressure produced will activate the HYD PRESSURE ON light located on the left hand annunciator panel. If the HYD PRESS ON light does not illuminate the bypass valve did not close indicating that the hydraulic system is inoperative.
Hydraulic system pressurised
If the HYD PRESS ON light comes on and stays on, the bypass valve failed to close, which will require the flap or landing gear circuit breaker. Please refer to the correct procedures laid down in the Abnormal section of the Pilot Operating Handbook.
When the selected operation has completed, the bypass valves re-opens and the system reverts to open centre operation.