I have four days to learn all the systems and SOPs for the Screaming Whippet and its larger cousin, I guess that must be the Screaming Greyhound. Then they'll schedule flight training and tests, and on passing we will have our job offers finalized and be paid for the time we have spent in school and training. So, no pressure or anything, right? More like pressure down over the top and around to every corner. And pressure that can cause action by going around corners is what the hydraulic system is about.
The hydraulic system provides motive force for the flaps, nosewheel steering, normal landing gear extension, and emergency landing gear extension. It contains one approximately 2.5 L reservoir, serviced with Brayco-882 hydraulic fluid and pressurized to 31 psi with regulated P3 bleed air. It has a relief valve on top which will release pressure over 45 psi. The reservoir supplies fluid through emergency shutoff valves (which are only to be used in emergencies, because otherwise they cut off the fluid supply to two engine driven hydraulic pumps which will be damaged if they are working away trying to pressurize the system to 2000 psi but there is no fluid for them to pump. The high pressure fluid from each hydraulic pump continues through a line containing a low pressure sensor. The sensor illuminates a low hydraulic pressure annunciator at or below about 1250 psi. Both hydraulic pumps must be working to depart, but it's okay if one of the low pressure switches is broken. If a low pressure light is on after both engines are started, you have to figure out if it represents a bad pump or bad sensor.
You can't do this by looking at the hydraulic pressure gauge, because it is located downstream of checkvalves, after the two lines from the two pumps have been merged together, and either pump alone is enough to provide 2000 psi to the system. And you can't do it by momentarily selecting the hydraulic pump off on the good side, because I was serious when I said the hydraulic pumps were to be shut off only in an emergency. So you have to do it by shutting off the engine on the good side. (You do this only on the ground, because if it happens in the air you don't need to worry about being good to depart: you've already departed). With one engine running, if that side's low hydraulic pressure warning light is still on and the system hydraulic pressure is still at 2000 psi, then obviously there is nothing wrong with the pump and it must be the pressure switch or something in the low hydraulic light circuit that is unserviceable, and you are allowed to depart. You have to remember to restart the other engine, though.
The system also includes an accumulator (a buffer for sudden high demand) with a 750 psi nitrogen precharge. There is also a high pressure relief valve if the pressure manages to exceed about 2300 psi, and return lines from the pressure relief valve and the various services, back through a filter to the main resevoir. The reservoir, accumulator, filter and the flap and landing gear selector valves are all located outboard in the left engine nacelle. The engine-driven pumps are in the engines, with the shut-off valves nearby. The pressure gauge and warning lights are in the cockpit, where I can see them.
Each leg of the landing gear has two dual (i.e. up and down) action hydraulic actuators. Both are required to make the gear come up, but only one is required to put the gear down, thanks to assistance from gravity and airflow for the latter task. (It's forward-retracting gear, a first for me). Thus for normal landing gear extension, the landing gear selector valve sends pressure to only one actuator on each gear leg. The extension side of the other three actuators are connected to another parallel section of the hydraulic system that I haven't mentioned yet. In addition to the lines leading to the engine-driven hydraulic pumps, there is a standpipe that holds about one litre of fluid and which connects to an emergency hand pump, and thence to the extension side of those second landing gear actuators. If the pressure in the normal side of the hydraulic system is too low (below about 250 psi) to extend the gear, the pilot turns on the emergency landing gear selector valve, turns the emergency gear release lever, and ensures the gear is fully extended by pumping a handle in the cockpit. A shuttle valve causes the hydraulic pressure gauge to always display whichever is higher, the emergency or normal system pressure.
There is one dual-action actuator for each flap, and the flaps are interconnected such that either actuator can move both flaps through their full 36 degrees of travel, making asymmetrical flap deployment an impossibility unless the airplane is first disassembled. There is no provision for emergency flap extension or retraction. If the normal hydraulic system fails, the flaps are locked where they are by the flap selector valves and the flap lock valves. If electrical power (to operate the valve) or hydraulic power (to operate the flap actuators) should fail during flap movement, the valve fails to the closed position, locking the flap where it was.
Nosewheel steering I will address in another post.