Heavy Metal Systems

david0tey

david0tey

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Long story short, I've been spending a lot of time around the hangar with local mechanics to satisfy my curiosity about how things work. I always leave with more questions than answers. Anyway, this stuff gets me thinking about how different the systems are between the mighty skyhawks of the world and the heavier metal (lets say CRJ size and bigger for the sake of discussion).

I'm particularly interested to know if someone could explain how the hydraulic flight control system works in the bigger aircraft. Obviously a system of cables and pulleys will not suffice. Where does the hydraulic system get its power from? While the aircraft is at the gate, if you yanked the yoke around, could you move the flight controls without engine power? Does the APU give you hydraulics? What about the Sully scenario with no engine power at all?

Please learn me.
 
Engine driven hydraulic pumps and some even have an APU driven pump. Some aircraft like the Cessna 750 even have some electric driven standby hydraulic pumps. The lower rudder for example has its own electric powered hydraulic emergency standby system. The upper rudder on it is completely controlled by the flight guidance system and somewhat follows the lower rudder controlled by the crew. On a citation 750 there is two separate main hydraulic systems with a power transfer unit between the two. The PTU can be driven by the remaining system to pressurize the other if one engine pump fails.

Another note about the 750 is that it is all pull-pull cables between the cockpit and the hydraulic units near the flight control surface. These have several latching and disconnect systems allowing separation of pilot & copilot controls incase an aileron or elevator is jammed. Meaning the elevators can be operated indepent of each other in an emergency.


Most Cessna products also typically use a separate electrically powerd hydraulic system for powere wheel brakes.
 
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My understanding is some of the large iron have a RAT to power hydraulics and give emergency power
 
sferguson524 said:
My understanding is some of the large iron have a RAT to power hydraulics and give emergency power
Click to expand...

Ram air turbines are a thing that pops out the side and basically is a wind powered machine that can either either generate electricity and/or hydraulic pressure depending on how the airplane is built.

Many can use the APU in flight as well.
 
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My airplane has three independent hydraulic systems, and each is capable of controlling the aircraft if the two others are failed. Two of the hydraulic systems are primarily powered by engine driven pumps, which are backed up by electric pumps. One system has two electric pumps only.

As you mentioned, at the gate typically none of the hydraulic systems are powered, so moving the yoke does nothing. I can manually turn on the electric pumps to pressurize the hydraulics, and that will allow me to move the control surfaces.

My APU does not give me hydraulics, and neither does my RAT. Those provide electric power only (the APU provides bleed air as well).

I don't know the Airbus, but I'd imagine it's similar to mine - the system logic will fire up the electric pumps if the engine driven pumps fail. In my plane, the electric pumps will also run anytime the slats/flaps are extended. So for Sully, I'm sure he still had electric pumps powering the hydraulics, even though the engine driven pumps were not working.
 
Hydraulics in general -

A pump moves hydraulic oil. That pressurized oil is directed by a control valve to one side or another of a two sided piston - pump oil into one side and the piston moves one way, pump it into another side and it moves the other way. The "non pumped to" side of the piston is open (also by the control valve) to drain unpressurized oil back to a reserve system where it will be picked up by the pump again in the future.

Between the pump and control valve you usually have some sort of accumulator or reservoir. The purpose is to smooth out any pulses from the pump and to provide a supply of pressure while the pump isn't running. The reservoir is a chamber which is oil on the bottom and sealed air on the top. The pump pushes oil into the reservoir and fills it up with oil. Because the air is sealed on the top, the oil compresses the air and is under pressure. When the control valve asks for more oil, it takes it from the reservoir. When the reservoir gets low, the pump fills it up again.

That's the basic idea of hydraulics....clear as mud? Would pictures help?
 
david0tey said:
Long story short, I've been spending a lot of time around the hangar with local mechanics to satisfy my curiosity about how things work. I always leave with more questions than answers. Anyway, this stuff gets me thinking about how different the systems are between the mighty skyhawks of the world and the heavier metal (lets say CRJ size and bigger for the sake of discussion).

I'm particularly interested to know if someone could explain how the hydraulic flight control system works in the bigger aircraft. Obviously a system of cables and pulleys will not suffice. Where does the hydraulic system get its power from? While the aircraft is at the gate, if you yanked the yoke around, could you move the flight controls without engine power? Does the APU give you hydraulics? What about the Sully scenario with no engine power at all?

Please learn me.
Click to expand...

It depends on the hydraulic system. The power for hydraulics can com from the arms of the pilot, same as braking hydraulics comes from your feet in a Skyhawk. You can also boost hydraulics with an engine, or electric driven pump. You can even hydraulically boost cable and pulley systems. Then there is fly by wire actuation which can be direct electromechanical servos, or electro-hydraulic servos, locally powered or loop powered.

A given plane may actually have more than one system available for redundancy.
 
david0tey said:
Long story short, I've been spending a lot of time around the hangar with local mechanics to satisfy my curiosity about how things work. I always leave with more questions than answers. Anyway, this stuff gets me thinking about how different the systems are between the mighty skyhawks of the world and the heavier metal (lets say CRJ size and bigger for the sake of discussion).

I'm particularly interested to know if someone could explain how the hydraulic flight control system works in the bigger aircraft. Obviously a system of cables and pulleys will not suffice. Where does the hydraulic system get its power from? While the aircraft is at the gate, if you yanked the yoke around, could you move the flight controls without engine power? Does the APU give you hydraulics? What about the Sully scenario with no engine power at all?

Please learn me.
Click to expand...

Depends on the airplane. On the DC-9 family, you sure can, no hydraulics required to move the flight controls (well, you move the tabs, not the actual flight control). As was pointed out already, the CRJ certainly needs hydraulic power to move things.

The DC-9 has an engine powered hydraulic pump on each engine, as well as an AC powered electric hydraulic pump, and an ALT transfer pump, which allows the electric pump to power both sides of the system.
 
I'll try to keep it simple.

Sources of hydraulic pressure can be:
Engine driven pump
Electric driven pump
Pneumatic driven pump (air motor using engine bleed air)
RAT - ram air turbine mounted to hydraulic pump
PTU - power transfer unit which uses hyd pressure from one system to drive a motor to drive a pump for a second system.

There are switches in the cockpit to operate the various pumps. Typically the engine pumps will pressurize the system automatically once the engine starts to turn unless the pump is selected off. Most other pumps have to be selected on by the pilot. In the case of the RAT, the pump will operate as soon as the RAT is deployed into the air stream.

The hydraulic pressure is then sent to the flight control actuators which often have various names such as Power Control Unit (PCU) or Power Control Actuator (PCA) and others. These actuators are what will extend and retract depending on which side of the actuator piston the fluid is ported to thereby moving the flight control up and down or left and right.

Whether the actuator is commanded down, up, left, right or neutral is dependent on pilot or autopilot input. This input is transmitted either via mechanical means (cables or push rods) or via wiring (i.e. fly-by-wire). There is normally a valve on the PCA/PCU that is repositioned either mechanically or electrically (solenoid). This is what will port the hydraulic pressure to the proper side of the actuator.

Also in the case of flaps, hydraulic pressure can be used to drive a motor which via torque tubes will turn gearboxes that drive jackscrews which will move the flaps up and down.

The hydraulic fluid is stored in reservoirs and the pumps draw fluid from them and the fluid is returned to the reservoir after passing through whatever devices it is sent to. The reservoir is frequently pressurized by bleed air or other source to provide a head pressure to help prevent pump cavitation. You might also have an accumulator which helps to maintain pressure on the system.

There will be various filters throughout the system to keep the hydraulic fluid clean. These will typically be in both the pressure and return sides of the system. There are also pressure switches and sensors and temp sensors and switches that are used for monitoring the systems.

Again, this is a simplified overview and I have left a fair amount out but hopefully I have given enough info to provide a little enlightenment.

So yeah, slightly more complicated than a 172. :yes:
 
Yep, and the next model plane may bring in yet some new element of technology to the picture.
 
Also, you asked about a Flight 1549 scenario -

1) They restarted the APU. Until then, the only hydraulic power came from the engines windmilling. There was also the RAT windmill thing that deployed for power.

2) APU provides power to the hydraulic.

3) If the pump dies there's a secondary.

4) If both pumps die, you have a limited amount of hydraulic power to what's left in the reservoir. That's going to depend on the airplane. After that...you're ballistic.

Incidentally - did you know that the co pilot for 1549, Jeff Skiles, had just completed his IOE? 1549 was the last leg of his first 4 day trip as a qualified pilot.
 
Just to clarify, the APU has a generator which supplies electrical power. With that they can operate the electric hydraulic pumps. The APU itself does not have hydraulic pumps.
 
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