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PMDG 777-200 GroundWork - Controls Hydraulic System Lesson Introduction Hello and welcome to the PMDG 777 Hydraulic System GroundWork lesson, from Angle of Attack. Today, we will be comprehensively analyzing the complex 777 hydraulic system architecture in order to allow you to understand it properly, as it is a crucial system upon which many important components of the aircraft rely on. Well have a look at the following topics in this lesson:

- Well start by talking about what is understood by hydraulics, and what components make up a simple hydraulic system.

- General overview of the hydraulic system on the 777. - Left and Right Hydraulic systems, - Center Hydraulic system, - Ram Air Turbine. - Well also be talking about abnormal hydraulic pressure, temperature and quantity

conditions, after which well look at - Controls and Indicators associated to the system, - and well finish off with a lesson summary.

Hydraulics and Aircraft Hydraulic Systems Aircraft hydraulic systems are used to move and displace surfaces, or other equipment such as the landing gear, which are too big or heavy to be moved by muscle power alone. Many aircraft use hydraulics, ranging from light piston-powered Pipers, all the way to the 777. Even though the components vary hugely in complexity, they all revolve around one basic principle: Pascals Law. Pascals Law states that when a force is applied to an uncompressible and enclosed fluid, that force is transmitted equally in all directions and the magnitude of the applied force is maintained constant throughout the fluid. In simple terms, if there is an increase in the pressure at any point in the fluid, there will be an equal increase in every other point, within the same enclosed container. The most basic form of a hydraulic system involves a container with a fluid, and two pistons of the same size. Lets look at what happens if pressure is exerted on the first piston. Because of Pascals Law, the pressure will be transmitted to the second piston too, causing it to displace upward. This case assumes that both pistons are of same size, however, what happens when they arent? The equation to keep in mind is: Pressure equals Force divided by Area. (P=F/a) Because the relationship between force and area is directly proportional, at a constant pressure if the area increases, so will the force, in the same proportion. Lets put this into context: If we give the first piston a 1m

2 area, and the second piston a 5m

2 area, as we apply a 5Pa

pressure upon the first piston, that pressure will remain constant toward the base of the second piston. Considering the second piston has a five-time greater area, we end up getting a 25N force from it. Thus, the second piston will have exerted a five-time greater force than the one we initially applied to the first piston. This idea explains the reason behind the implementation of hydraulic systems. They allow us to multiply forces and move very large surfaces that would otherwise be very heavy to move simply mechanically.

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Lets apply what weve learned to a very simplified and hypothetical diagram of the elevators on the 777. Assuming the elevators are actuated up and down by a single piston that moves either front or back, whenever the control column in the flight deck is pulled back, hydraulic fluid is pumped under pressure to the actuators, forcing the piston backward and thus lifting the elevator. Similarly, when the control column is pushed forward, the pumped hydraulic fluid is directed toward the other side of the piston, forcing it forward and bringing the elevator down. The piston has a large surface area, and that is what exerts a massive force on the elevator mechanism, enabling it to move even when the aircraft is traveling fast and aerodynamic forces acting on it are really large. With this in mind, lets dig deeper into the 777 and have a look at what components and properties make up its hydraulic system. 777 Hydraulic System Overview The 777 has three independent hydraulic systems that supply pressurized hydraulic fluid to operate aircraft systems, including:

- Primary Flight Controls, - Landing gear actuation, - Thrust reversers, - Main landing gear braking, - Main and Nose landing gear steering, - Leading edge slats, - Trailing edge flaps.

The three main hydraulic systems are the Left, Right and Center system, based on the location of most of their components. There is also a backup form of hydraulic power, achieved through the Ram Air Turbine, or RAT, which provides both hydraulic and electric power to the aircraft in an emergency situation. The electrical capabilities of the RAT are discussed in the 777 Electric System lesson, and the hydraulic capabilities will be covered later on during this lesson. Each main hydraulic system supplies hydraulic fluid that is stored within three hydraulic reservoirs and is pressurized by bleed air from the pneumatic system. It is important to monitor the output pressure and temperature, as well as the fluid quantity within the reservoirs. All these parameters are indicated in the flight deck, and any abnormal condition is shown on the EICAS. Two categories of pumps provide the adequate hydraulic fluid pressure in each hydraulic system. There are one or more primary and demand pumps. Primary pumps normally operate continuously. Demand pumps, as their name implies it, operate only when there is demand for additional hydraulic power, such as during flaps or landing gear extension. Each hydraulic system powers a specific set of aircraft components and there is considerable redundancy built-in into the system, so that in the event of one or more hydraulic system failures, the aircraft may still remain controllable. It is important to emphasize that any one hydraulic system can provide adequate, if not normal, aircraft controllability. Lets now move on to specifics, starting with the left hydraulic system. Left Hydraulic System Architecture The left hydraulic system, which is color-coded red, powers the following aircraft components:

- Left Outboard Elevator and Right Outboard Elevator, - Left Outboard Aileron and Right Outboard Aileron,

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- Rudder Middle Power Control Unit, - Spoilers 2,4,11 and 13, - Left Outboard Flaperons, - Left engine thrust reverser.

In short, it powers components in the PFCS, or primary flight control system, and the left thrust reverser.

The left hydraulic system draws hydraulic fluid from the left hydraulic reservoir, located in the left engine aft strut fairing. The total volume of the left reservoir is 12.6 GAL, however, it normally contains only 7.4 GAL. Engine bleed air pressurizes the reservoir, therefore the engines must be on and running, and the respective engine bleed switch must be ON.

In the case of a bleeds-off takeoff, there would be no bleed air to pressurize the hydraulic reservoirs, however, it is not necessary as at low altitudes, the reservoirs can deliver an adequate amount of fluid. It is recommended however that the pneumatic source be switched back to engine bleed air as soon as practicable in order to avoid fluid foaming and other conditions within the hydraulic reservoir.

Hydraulic fluid is supplied under pressure to the left systems primary, and demand pumps. The primary pump for the left hydraulic system is an engine-driven pump (EDP), located in the left side of the left engine. The engine driven pump is turned by the engine main gearbox and is rated at 48 gpm at 2850psi and 3900rpm speed. The pressure downstream of the engine driven pump is maintained at approximately 3000psi. A cockpit switch located in the overhead hydraulic panel controls the operation of the engine driven pump. The switch, called PRIMARY L ENG, is a pushbutton with two positions: ON The left EDP pressurizes the hydraulic system when the engine is running. OFF The left EDP is turned off and depressurized.

A primary pump FAULT light illuminates in amber when there is low primary pump pressure, excessive primary pump fluid temperature, or the pump has been manually selected off.

When the pumps are turned off, a depressurization solenoid gets energized and blocks the flow of hydraulic fluid from the pump. The engine driven pump has a supply shutoff valve that closes and prevents the flow of hydraulic fluid between the EDP and the associated hydraulic reservoir. The supply shutoff valve is actuated when the related engine fire switch is pulled UP, which in turn also depressurizes the EDP.

Hydraulic fluid, while flammable, in most cases will not directly light up when put into contact with fire, but it will burn lightly and propagate an enormous amount of heat.

Like we mentioned earlier, the hydraulic systems are further pressurized when system logic senses an increase in demand, with the so-called demand pumps. The left hydraulic system demand pump is an alternating-current motor-driven pump (ACMP), or simply an electric pump. The left system demand pump is located in the left engine strut, aft of the left reservoir. There must be an operative source of AC electric power for the demand pumps to function and minimize the hydraulic requirement from the engine driven pumps. The electric pumps also deliver hydraulic fluid that is pressurized to approximately 3000psi, however, the output fluid volume is only 6gpm at 2850psi, which is considerably lower than that of the engine driven pumps we talked about earlier.

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The left system demand pumps are controlled by the DEMAND L ELEC selector, which has the following positions: ON The pump runs continuously,

AUTO The pump operates when either the related system, or engine driven pumps pressure are low, or when the system logic anticipates an increase in the hydraulic demand. OFF The pump is off. A demand FAULT light illuminates in amber below the selector when there is low demand pump pressure, high demand pump fluid temperature, or the demand pump has been selected off, similarly to the primary pump FAULT light logic.

Right Hydraulic System Architecture Moving on, before we talk about some potentially unwanted conditions, such as hydraulic fluid leaks and their effects, lets jump ahead into describing the other two hydraulic systems, starting with the right system. For the most part, the right hydraulic system is identical in components and operation to the left hydraulic system. It only differs in the aircraft systems that it powers, and so, the right system which is color coded green, is in charge of supplying power to the following aircraft components:

- Right Inboard elevator, - Rudder Lower Power Control Unit, - Spoilers 3,6,9 and 12, - Left and Right Inboard Flaperons, - Right Stabilizer Trim Control Module, - Right Engine Thrust Reverser, - Normal main gear braking.

In short, the right hydraulic system powers components in the primary flight control system (PFCS), the right thrust reverser, and normal braking. While discussing the right system, we dont want to repeat all of the explanations of the almost-identical left system, so lets quickly summarize them. The right hydraulic system is fed with bleed-air pressurized hydraulic fluid drawn from the right hydraulic reservoir. The reservoir, located in the right engine aft strut fairing, has a capacity of 12.6 GAL, yet normally contains 7.4 GAL. The right system has a primary engine-driven pump, and a demand electric-driven pump. Both pumps deliver fluid at approximately 3000psi, however, the engine driven pump delivers 8 times the volume of the electric-driven pump, measured at 2850psi. Both pumps have the same controls as the left system in the overhead panel, namely, the PRIMARY R ENG pushbutton and the DEMAND R ELEC selector, both having exactly the same positions and operation logics as in th left system. Remember that FAULT lights illuminate when the related pump pressure is low, output fluid temperature is excessive, or the pump has been manually turned off. Finally, the right system supply shutoff valve shuts off the hydraulic fluid flow between the engine-driven pump and the right reservoir whenever the related engine fire switch has been pulled UP. Lets carry on by discussing the center hydraulic system. Center Hydraulic System Architecture And so, the center hydraulic system, color-coded blue, is in charge of supplying hydraulic power to the following aircraft components:

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- Left Inboard Elevator, - Left and Right Inboard Ailerons, - Rudder Upper Power Control Unit, - Spoilers 1,5,7,8,10 and 14, - Right Outboard Flaperons, - Center Stabilizer Trim Control Module, - Alternate and Reserve Main gear braking, - Normal and Reserve Nose gear steering, - Nose and Main landing gear actuation, - Normal main gear steering, - LE Slats and TE Flaps Primary extension system.

Right away we notice that from all three systems, the center system powers considerably more items than the other two hydraulic systems making it larger, more complex and demanding the need for a more elaborate structure. The center system draws hydraulic fluid from the center system reservoir, located in the aft section of the right wheel well. The reservoir is easily visible in grey color when inspecting the wheel well and it has a total capacity of 25.6 GAL, however normally only contains 11 GAL. The center hydraulic system supplies pressurized fluid to four pumps, as opposed to the other systems having only two. There are two primary pumps and two demand pumps. Remember that demand pumps only operate during periods of high system demand and to provide a backup hydraulic power source. Primary pumps in the center system are alternating current motor-driven pumps (ACMPs), or simply electric-driven pumps. The pumps are the same as the left and right system demand pumps, meaning they deliver 6gpm of hydraulic fluid at 2850psi. The PRIMARY ELEC C1 and C2 cockpit switches in the hydraulic panel control the operation of the center primary electric pumps. The switches are pushbuttons with the following positions:

ON The C1/C2 electric-driven pump operates and the center hydraulic system gets pressure. OFF The C1/C2 electric-driven pump is turned off. Similarly to the previous systems, there is a FAULT light which illuminates in amber when there is low pump pressure, high pump fluid temperature or when the pump has been selected off.

There are two operation logics for the center system primary pumps: on the ground, and in flight. On the ground, and with a single power source such as an external ground cart, or the APU, the C2 pump will only run provided the C1 pump is not selected on. Like this, the C1 pumps load wont be shed if there are either of the following conditions:

- One engine driven generator is operative, - There is primary and secondary external power, - Or there is primary external power, plus the APU generator is on.

During flight, primary center pumps logic is the following: The C2 and C1 pumps may be on simultaneously, but the C2 pump will only be load shed when:

- All other electric pumps are running, - Available electric power is reduced to only one source, - Engine-driven generator capacity is exceeded due to high system demand.

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The generators and electric power, amongst others, are members of the electric system, and are carefully explained in the 777 Electric System Groundwork lesson.

Now, the center hydraulic system also has two demand pumps, which are air-driven. The air-driven pumps are powered by pressurized bleed-air from the aircrafts pneumatic system, and they are located in the left wing-fuselage fairing behind the left wheel well. The ADPs are rated at 53gpm at 4315rpm, however, they may operate in a reserve mode supplying 63gpm at 5160rpm. Center system air-driven pumps are controlled by the DEMAND AIR C1 and C2 selectors in the hydraulic panel, which have the following positions: ON The demand pumps run continuously,

AUTO The pumps only operate when the related system or primary pump pressure is low, or to reduce a large system demand. OFF The pumps are turned off. A FAULT light with the same logic as in previous cases is available and it illuminates in amber when pump output pressure is low, output temperature is high, or the pump has been switched off.

The center system demand pumps, however, have a restriction. If both air-driven pumps are manually selected ON, only the C1 demand pump will operate. If both air-driven pumps are selected to AUTO, no two air-driven pumps will operate simultaneously. Now, considering that the center system powers an incredible amount of components, it has a few protection mechanisms for some non-normal conditions. Lets talk about what happens if hydraulic quantity in the center reservoir is sensed to be low. In this situation, the center hydraulic isolation system (CHIS) comes to life when the center system reservoir quantity is less than 40% of the normal service level. The objective of the CHIS is to protect the aircraft by dividing the center hydraulic system into different parts:

Center system pressure is isolated from nose landing gear actuation and steering, as well as from the leading edge slat hydraulic lines. Nose gear hydraulic pressure is reconnected when the aircraft airspeed decreases below 60 KCAS and the leading edge slats still remain electrically operable in the secondary and alternate modes, which will be explained in the 777 Heavy Lift Controls lesson.

This means that the output from all the center system primary and demand pumps goes to: Trailing edge flaps, Main landing gear actuation and steering, Primary flight control system. The only exception is the center primary pump C1, which is an ACMP, where the pump output goes exclusively to the reserve braking system. The CHIS stops isolating the system when the center reservoir quantity becomes more than 70% and the airspeed is lesser than 60 KCAS for five seconds, or when both engines and both EDPs operate normally for 30s.

Now we must think, what could cause hydraulic fluid quantity in the reservoirs to decrease to such a point that the system components must be isolated? The answer is simple, yet involves a complex analysis. Hydraulic fluid leaks. Reservoir Hydraulic Fluid Leaks and Heat Exchange Process

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Each system reservoir has a standpipe. The standpipe is used to give priority to a nominal amount of fluid to remain in the reservoir for a reserve or non-normal condition. In the case of the left and right hydraulic system reservoirs, which are identical, out of the normal 7.4 GAL of fluid quantity, there are 2 GAL below the standpipe. For the center system reservoir, out of the normal 11 GAL of fluid, there are 1.2 GAL below the standpipe. The standpipes top is located about 1/5 high from the bottom of the reservoir. For the left, right and center system reservoirs, the fluid may flow either out of the port at the bottom, or from the standpipe. The crucial difference lies in the following: The left and right system provides fluid to the electric motor-driven demand pump from the bottom of the reservoir, and to the engine-driven primary pump from the top of the standpipe.

This means that if a leak were to develop in the engine-driven pump hydraulic lines, the reservoir quantity would only drop to about the top of the standpipe. At this point the related system quantity indicators in the Hydraulic status display will indicate about 20%. The remaining fluid in the reservoir will be sufficient for the electric motor-driven pump to operate. However, if a leak were to develop in the electric motor-driven pump hydraulic lines, a complex scenario would arise because the reservoir quantity would be drained to a point where fluid quantity would reach 0%.

The center system reservoirs standpipe supplies fluid to the primary electric motor-driven pump 2, to both demand air driven pumps and to the ram air turbine. The electric motor-driven pump 1 gets fluid from the bottom of the reservoir.

This means that if a leak were to develop in the hydraulic lines of the ACMP 2, either ADP or the RAT, the center reservoir quantity would drop to the level of the top of the standpipe. The system quantity indicators will still indicate around 20%, which is an appropriate quantity for the ACMP 1 to pressurize the center system. If a leak were to develop in the ACMP 1 hydraulic lines, the center system reservoir quantity would drain to 0%.

Before we finish of this section and move on to the famous Ram Air Turbine, lets talk about a considerably important process that hydraulic fluid must go through. Heat exchange. As you probably figured out by now, the hydraulic fluid moves through hundreds of components, valves and systems, before it is filtered and returned to the hydraulic reservoirs, only to start the cycle once again. In all this process, the fluid absorbs an enormous amount of energy, which manifests itself in the form of heat, thus increasing the fluids temperature. In order to extend the hydraulic fluid service life, its temperature must be brought down during operation. The way this is done is by exchanging heat with cold fuel from the fuel tanks. Fuel in the wing tanks for example can be as cold as -50C, however, the warmer it is before entering the combustion stages of the engine, the lesser amount of energy is needed to raise its temperature during combustion. In short, fuel is cold but needs to be warm, and hydraulic fluid is warm but needs to be cold. So both of these fluids are put into indirect contact inside heat exchangers. Aluminum finned tubes are used to exchange heat, because aluminum is a great electricity and heat conductor. Worth mentioning, the heat exchange process only uses fuel from the left and right fuel tanks. Each hydraulic system has one heat exchanger. The left system heat exchanger is in the left main fuel tank. The right system heat exchanger is in the right main fuel tank.

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The center system heat exchanger is in the right main fuel tank.

There have to be 709 GAL and 1080 GAL in the left and right main fuel tanks respectively to cool the hydraulic fluid from the related systems.

Ram Air Turbine Throughout the lesson, weve had a look at the three main 777 hydraulic systems. An incredible amount of redundancy is built into the overall system, and as we stated earlier, the aircraft remains nominally controllable with any one hydraulic system. Remember that the 777-200LR, provided the operator has the respective certification, is approved for ETOPS-180 or higher, meaning the aircraft is authorized to fly with only one engine operative for 180 min or less until a suitable diversion airport is available. This obviously means that the on board aircraft systems must prove to be tremendously reliable and fail-safe. However, there is always the possibility of things going wrong. All hydraulic system output pressures may become low due to very remote causes involving failures and leaks, therefore it is important to have a backup system for hydraulic pressure generation, at least for the primary components that require hydraulic power. To illustrate, the flight controls are a primary component, as opposed to the slightly less important leading edge slats, which are not indispensable. For this reason, the 777 relies on a Ram Air Turbine, or RAT, that supplies an emergency source of hydraulic power to operate the aircrafts flight controls that are connected to the center hydraulic system. The RAT also has electric power generation capabilities, which are covered in the 777 Electrical system GroundWork lesson. The RAT is located aft of the right wheel well and is made up of a two-blade variable pitch turbine. Specifically in the hydraulic context, this turbine turns a shaft at 4510rpm that subsequently turns a pump.

The RAT hydraulic pump is rated at 10gpm at 2850psi. A considerable amount when compared to the ACMPs 6gpm at 2850psi.

The RAT can be deployed either automatically or manually. To manually deploy the RAT, the RAM AIR TURBINE switch in the overhead hydraulic panel must be pushed. The switch then energizes an actuator and deploys the RAT, which is normally stowed by a compressed spring. A lock mechanism allows the blade to turn only when the turbine is fully extended, to avoid damage to the RAT well during extension. The RAT manual extension switch has two lights:

A green PRESS light that illuminates when the RAT is deployed and center hydraulic system pressure recovers to greater than 1500psi. An amber UNLKD light that illuminates when the RAT is not in the stowed position. There is no way to stowing the RAT in flight, as only maintenance personnel can perform that job on ground.

The RAT will also extend automatically when any of the following three conditions are met:

- Both engines have failed and the center hydraulic system pressure is low, or - Both AC Transfer buses are not energized, - All three hydraulic system pressures are low.

This sums up our discussion about the RAT.

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Now, how does the flight crew tell when a hydraulic systems pressure is low? Or the fluid temperature is excessively high, triggering the amber FAULT lights we discussed earlier? Hydraulic System Cockpit Indicators and EICAS Messages In the cockpit, the STAT display switch on the display select panel must be pushed to bring up the hydraulic status on the multifunction display. In here, we find two types of information. On one hand, hydraulic reservoir quantity is displayed as a percentage of the normal service level, where 1.00 would be the normal service level and 0.50 would correspond to half of that. Next to the quantity indications for each system, we may find a LO, OF or RF message. LO is displayed in amber when the reservoir quantity is low.

OF is displayed in white when the reservoir is over-full, however, this message is inhibited during flight. RF is displayed when the reservoir requires refilling. This message is also inhibited during flight, and is displayed when the system anticipates that the fluid will soon come to a low level, not when the fluid quantity is already critically low.

Aside from quantity, we also find each systems pressure information in psi. The information is presented according to the system pump with the highest pressure. Remember that the average hydraulic pressure to be encountered in during flight is approximately 3000psi. All this information is also presented in the hydraulic synoptic display, which is accessed by pushing the HYD synoptic switch in the display select panel. This display combines the hydraulic system information along with a general schematic of the hydraulic architecture to ease the cross-check process. Amber colors display over the pumps and valves for example when they present failures. Under certain non-normal conditions, certain EICAS messages and advisories may be displayed. For example, when any hydraulic system pressure drops below 1200psi, a HYD PRESS SYS EICAS caution message appears, along with an aural beep. The message is followed by the system that is causing the low pressure message, either L, R, C, a combination of two, or all three in the form of L+R+C. You can take a look at all the EICAS messages associated to the 777 hydraulic system in the handouts section of this video. Lesson Summary To summarize all the information in this lesson, the 777-200LR is powered by three main hydraulic systems. An engine-driven pump is the primary pump for the left and right hydraulic systems, whereas an electric motor-driven pump is their demand pump. The left and right hydraulic systems are virtually identical, and only differ in the components they power. Two electric motor-driven pumps are the primary pumps for the center system, and two bleed-air driven pumps are the demand pumps for the same. The ram air turbine may also power the center system flight control components in case an emergency source of hydraulic power is needed. The hydraulic panel in the overhead has controls for each hydraulic system pump, and for the ram air turbine. Cockpit indications in the hydraulic synoptic page display temperature, pressure and fluid quantity indications. Weve seen the enormous amount of components and systems that make up the 777 hydraulic architecture. It is important to understand them properly as they will give you a clear picture of what systems you wont be able to count on in case there is a hydraulic system failure.

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Last but not least, well run through the general components that each system powers, once again.

The left hydraulic system supplies power to the primary flight control system, and the left thrust reverser.

The right hydraulic system supplies power to the primary flight control system, the right thrust reverser, and normal wheel braking.

The center hydraulic system, probably the most important amongst the three, powers the primary flight control system, leading edge slats, trailing edge flaps, landing gear actuation, alternate and reserve braking, normal and reserve nose gear steering and the main gear steering.

If you had to lose a hydraulic system completely during cruise flight, which hydraulic system would you choose to lose and why? Leave your ideas in the comments section below. Stay tuned for the following lesson on the 777 Primary Flight Controls System. Until then, thanks for watching and

ThrottleOn!