I am trying to understand this schematic of the landing gear hydraulic of the Piper Turbo Arrow III of which the emergency gear extender has been disabled. Although an engineer by training, I cannot make sense of the pump schematic: where is the actual pump, what are the high pressure lines, what does the gear lever do (purely electric), … It just doesn’t make sense in my head.
Am I getting old? Can someone help me explaining it (particularly the pump)?
the Piper Turbo Arrow III of which the emergency gear extender has been disabled
You’re referring to the braindead Piper automatic gear extender, not the emergency release? Without the latter, the aircraft would clearly not be airworthy.
Yes, of course Achima. I mean the disabled Back-up Gear Extension. Thanks :)
I can’t work it out either but the pump is this bit
I went through a pump search exercise here and these gear pumps seem to all work in a similar way. They are gear pumps as in this
and they contain one-way (spring-loaded) valves which make sure that once the pump has pumped the fluid in a particular direction, it doesn’t leak back out again, and this works in both directions (also called “two way blocking”). The pumps also have a crude overcurrent cut-out, implemented with a poorly assembled bimetallic switch which gives more trouble than the pump itself!
What your diagram does not show are the electrics. You should have some relays and some switches. This is the TB20 version
The pump motor, which is a reversible DC brush motor, is at top left.
This system drives the electric pump in both directions and the gear selector chooses the direction. In other airplanes (e.g. Cessna), the pump only operates in one direction and the gear lever is actually a mechanically actuated valve.
The pump is controlled by two pressure switches — low pressure and high pressure. It makes sure there is always operating pressure in the system. The wheels are held in the up position by hydraulic pressure and in the down position by hooks (which are connected to the gear lights in the cockpit).
The emergency system works by dumping the hydraulic pressure and letting the gear free fall. For the main gears, this is enough, for the nose gear there are springs to push it into the locked position. In other designs (e.g. Cessna), hydraulic pressure is required to lower the gear and a manual pump is provided. This makes the Piper system rather robust as the hydraulic system is not required at all to lower the gear.
When down, the shuttle valve goes to the left and the gear up check valve is opened by the pressure activated piston
I guess I typed slowly, will leave it anyway
The pump is the two circles in the diagram, just like on Peter’s pump example. They are a gear type pump. The pump can rotate in two directions, enabling up and down.
It currently shows with gear down.
When you select gear up:
1. Current flows trought the squatch switch (in flight) trought pressure switch (low pressure) trought the gear selector (up) to the gear up relay. The gear up relay powers the UP winding of the DC motor.
2. Motor rotates in UP direction. Pump creates suction and draws in fluid from the reservoir, though the filter, and the checkfilter below the filter. The pump (round circles in diagram) pumps fluid to the right section. There fluid under pressure forces the gear up check valve open, allowing oil to flow trought the orifice to the right manifold. Oil from the manifold is forced in the lower port of the three gear actuators. This is high pressure.
3. The gear actuators will retract, forcing fluid at the low pressure side of the actuator (upper connection) to the left manifold. The shuttvalve is hold in the right position under spring pressure, allowing the fluid to flow back into the reservoir.
4. When the gear reaches the up position, the pressure increases, the pressure switch on the right manifold will stop the pump (and restart the pump if it looses pressure).
When you select down again:
1. The gear switch will enable the down relay, enabling the motor the run in reversed direction.
2. The pump will generate pressure, which make the shuttle valve switch to the left, enableing the pressure to go the upper part of the cilinders, to force them down. In the meantime the plunger in the body is pressed to the right, opening the gear up check valve, to enable the low pressure from the lower part of the cilinders to flow back into the pump.
The emergency mode, and the gear extender, make the pressure in both manifolds equal. Enabling the gear to drop out on it’s own weight, or stay down even when gear up is selected and the speed is to low.
Jesse, LesVing, Achima, Peter: thanks. Your answers are fast and complete: wow.
I think I get it now.
However: how does the pump know to stop in the DOWN position? Is it through the microswitches that also drive the gear lights?
And: I understand this pump to only draw filtered oil from the reservoir when the gear goes up, right?
The system is designed to always keep the hydraulic system within a pressure range. The low pressure switch turns the pump on, the high pressure switch turns it off.
how does the pump know to stop in the DOWN position?
The switches get operated and (via relays) cut the current.
Is it through the microswitches that also drive the gear lights?
You may have separate switches for the gear lights. The TB20 has some low-end Crouzet microswitches for that.
I understand this pump to only draw filtered oil from the reservoir when the gear goes up, right?
The pump I opened up, a Parker one, has filters on the pump inlets.