Roshan Bhojwani

1 Angle of Attack

PMDG 777-200 GroundWork - Controls Primary Flight Control System Lesson Introduction Hello and welcome to the PMDG 777-200LR GroundWork Primary Flight Controls system lesson, from Angle of Attack. In this lesson, well have a look at what components make up the 777s primary flight controls system, and how does the flight crew operate such components. Full understanding of the primary flight controls system is crucial, as it is arguably the aircraft system that involves the highest amount of pilot interaction. Not just for the 777, but for all aircraft. Naturally, there are many different types of flight control systems, so lets start by looking at an overview of what they do. Before that, heres the list of lesson topics we are going to cover today.

- Flight controls overview and types, - 777 primary flight controls system overview, - Primary flight controls system modes, - Pitch control through elevators and pitch trim, - Pitch envelope protection, - Roll control through ailerons, flaperons, spoilers and aileron trim, - Roll envelope protection, - Spoilers to aid roll control, - Yaw control through rudder and rudder trim, - Yaw flight stability protection features, - Along the lesson well look at system controls and indicators.

Flight Controls Overview An aircrafts attitude can be changed around three (3) perpendicular axes that intersect at the aircrafts Center of Gravity (CG):

Lateral axis, Longitudinal axis, Vertical axis.

In straight and level flight, external forces like for example, wind, may alter the desired flight path thus creating the need for the aircraft to be maneuvered back to the correct attitude through three (3) types of movements: Pitch (around the Lateral axis), Roll (around the Longitudinal axis), Yaw (around the Vertical axis). To achieve these movements around the aircrafts axes, flight controls are employed. There are two (2) groups of flight controls: Primary and Secondary. Secondary flight controls are covered in the High Lift Control System Groundwork lesson. Normally, the primary flight control surfaces and their direct effects are: Elevators (change in Pitch),

Ailerons (change in Roll), Rudder (change in Yaw). An aircraft is free to rotate within its three axes and it will always turn about its CG, or center of gravity. The tendency to do this is known as a turning moment. A moment is equal to the

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product of the force applied and the distance from which the force is being applied. This is known as arm and it is measured with reference to a defined datum. Because the relationship between force and arm is inversely proportional, the longer the distance from the datum means the force has to be smaller to maintain positive balance, and vice-versa. The flight controls are designed to deflect airflow and produce these forces that make the aircraft turn around its axes. This is done by changing the angle of attack of the control surface thus allowing for a change in lift.

EXAMPLE: When the control column is pulled back the elevators are deflected upwards and due to the lower angle of attack on the control surface, there is a lower lift component in the horizontal stabilizer causing it to go down and thus bringing the aircraft nose up.

Now, normally, small aircraft that fly at relatively low airspeeds have purely mechanical flight controls. This means that pilot input into the control columns or rudder pedals is transmitted directly into the related control surface through cables, pulleys and rods. This implies that as and when the airspeed increases, the amount of force required to deflect a flight control surface would translate to an enormous amount of force required over the control columns or pedals, making it extremely hard for the pilots to comfortably maneuver the aircraft, thus increasing the risk of a pilot incapacitation. Remember, NO emergency is worse than a pilot not being able to safely control his aircraft. To counter this high-speed flight controls problem, hydro-mechanical flight controls were put into service. In this case, relatively small pilot inputs are relayed through cables and pulleys to hydraulic actuators, which make use of hydraulic power to create large amounts of force in the control surfaces. The B737, for example, has hydro-mechanical flight controls. A third type of flight controls system is available, known as fly-by-wire. Initially, only military aircraft would boast this technology, until Airbus implemented it in commercial airliners. The 777 is the very first Boeing commercial aircraft to have fly-by-wire controls. In FBW, pilot input is translated to electronic signals that are sent to flight control computers. These are fed with further aircraft information which then calculate the required flight control surface deflection to meet the pilots input, thus producing more electronic pulses that are then sent to hydraulic actuators on each control surface. Benefits of FBW are:

- A more efficient structure design, - Lesser amount of components, therefore lesser aircraft weight, - Which translates to a better fuel economy, - and the possibility to include computerized flight-envelope protection features.

With this in mind, lets look at the general structure of the 777 primary flight controls system. 777 Primary Flight Controls System Overview As we already mentioned, the 777 uses a fly-by-wire primary flight controls system, or PFCS. This means that commands from the flight crew and the autopilot are translated to calculated electronic pulses towards the respective control surfaces. The operation and components of the system, are however, fairly standard. The control surfaces operated by the PFCS are:

- One aileron on each wing, - One flaperon on each wing,

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- Seven spoilers on each wing, - One horizontal stabilizer, - One elevator on each side of the horizontal stabilizer, - One rudder.

The system operation logic is specifically the following: Pilots input their commands into the control wheels, control columns and rudder pedals. These commands are translated to analog electronic signals that are sent to four Actuator Control Electronics, or ACEs. The ACEs process the signals, change them to a digital format, and send them to three Primary Flight Computers, or PFCs. Whenever the autopilot is active, it feeds information directly to the PFCs, which then send signals to the ACEs and subsequently displace the control surfaces. Metaphorically speaking, the ACEs can be seen as the systems heart, and the PFCs are the systems brain. PFCs take the digital electronic signals and calculate the required control surface deflection. To do these calculations, the PFCs also receive the following information:

- Aircraft airspeed, - Inertial data, - Angle of attack data, - TE Flap position.

The calculated signals are sent back to the ACEs, which then convert them back to digital signals and relay them to the respective control surface Power Control Units, or PCUs. The PCUs are in charge of providing the muscle to operate the flight controls, by making use of hydraulic power. Essentially, PCUs drive the primary flight control system. Its not entirely necessary for you to know how these components and computers exactly work, but knowing their operation and the different logics that are associated to them are primordial. For this reason, this lesson will focus on flight controls from an operational point of view. Moving on, because computers process the pilot input before sending them to the control surface actuators, their displacement may be restricted or limited to protect the aircrafts flight envelope from entering certain potentially dangerous zones. The 777 has three main types of flight envelope protection features, for both manual and auto-flight:

- Stall, - Overspeed, - Bank angle.

Worth mentioning is that the pilot maintains ultimate control authority over the aircraft, regardless of any protection feature being activated. Now, flight controls are normally checked on ground after engine start. If there are no EICAS messages, the system should be working normally. The controls must be free from obstructions, and also must be correct. Meaning for example, when the control wheel is turned to the left, ensuring that the left aileron goes up and right aileron goes down. To perform this check, flight controls surface positions may be seen in the Flight Controls Synoptic Display by pushing the FCTL switch in the display select panel. Pointers indicate surface displacement from their center position and their movement. Horizontal stabilizer trim and rudder trim, in trim units, are also indicated. Certain abnormal conditions are also shown in the flight controls synoptic display. Unknown aileron positions are symbolized by a loss of the respective aileron pointer. A fully black spoiler deflection bar shows unknown spoiler conditions. If a control surface fails, a cross illustrates it. In the lower portion of the synoptics display, there are hydraulic system and ACEs computer indications. If they are green the systems are normal, if they are amber, the systems are failed. Primary Flight Controls System Modes

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Now, the primary flight controls system of the 777 always operates under one of three main laws, or modes, of operation.

- NORMAL, - SECONDARY, - DIRECT.

One of these modes is active at all given times. During normal mode autopilots are operative, autospeedbrakes are operative, all flight envelope protections are available, three PFCs are operative and four ACEs are operative. If the PFCS detects a general failure in the system, or specifically if airspeed and/or inertial reference data is lost, the system automatically switches from NORMAL to SECONDARY mode. In secondary mode, the PFCs still calculate simple surface inputs and relay them to the PCUs via the ACEs, however, autopilots and autospeedbrakes are not available, meaning the aircraft must be manually controlled. Caution must be exercised because even though the aircraft is fully manually controllable, no forms of flight envelope protections are available. Yaw dampening is also degraded, and in some cases may be unavailable, meaning gust lock and dutch roll protection is also degraded or unavailable. When the system reverts to secondary mode, the EICAS caution message displays FLIGHT CONTROL MODE. If any further failure occurs, the PFCs completely disconnect from the ACEs and the system enters DIRECT mode. In direct mode, pilot input is directly relayed to the flight controls, without any input calculations whatsoever. Aircraft controllability is similar to SECONDARY mode, and crew always has full control of the aircraft. When in direct mode, the EICAS displays the caution message PRI FLIGHT COMPUTERS. Direct mode can also be manually selected, however this is not normal practice and must only be done if imperative for safety. To do so, lets look at the PRIMARY FLIGHT COMPUTERS disconnect switch in the overhead panel, which has two positions:

- AUTO: the PFCS operates in normal mode, and if any of the faults we explained earlier occurs, the system automatically reverts to secondary or direct mode. AUTO can also be selected to attempt to restore the system to normal mode if it has changed to secondary or direct mode.

- DISC: the PFCs are disconnected from the ACEs and the PFCS is put into direct mode. Whenever this happens, whether manually or automatically triggered, a DISC light illuminates in amber next to the switch.

In the unlikely event of all electrical power on the aircraft being lost, the horizontal stabilizer and selected spoilers are still manually controllable from the flight deck through a series of cables. This allows for a nominal amount of aircraft controllability, sufficient for maintaining straight and level flight.

Now that weve overviewed the flight controls system, lets jump into the three main types of movements that flight controls induce in the aircraft, starting by pitch. Pitch Control Pitch control is achieved through the use of two elevators and an elevator trim system. Lets first start by talking about the elevators.

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These are movable control surfaces located in the aft portion of the horizontal stabilizer that allow the aircraft to move around its lateral axis, in an up or downward direction. The cockpit control columns control the movement of both elevators simultaneously.

- The left elevator is powered by hydraulic systems L & C, and the L1 & C ACEs. - The right elevator is powered by hydraulic systems L & R, and the L2 & R ACEs.

As we mentioned before, when the control column is pushed forward, signals are sent to the respective ACEs to command the PFCs to generate the respective pitch maneuver. The elevator PCUs receive these signals and provide the required downward displacement, increasing the horizontal stabilizers angle of attack. This produces an upward-pushing force in the tail, thus bringing the aircraft nose down. The PFCs constantly monitor aircraft configuration changes in order to automatically re-position the elevators and thus improve aircraft handling, and passenger comfort. In fact, it makes banking the aircraft a comfortable maneuver for the flight crew, as for all turns below 30 of bank angle, the pilots do not need to add additional column back-pressure to maintain the altitude. For all turns of 30 or more of bank, a little back-pressure must be added. Even though the system is far more modern than conventional flight-control systems, elevator feel forces are provided to the control columns to simulate an increase in the force necessary to displace them, as and when airspeed increases. In normal mode, elevator feel forces are mostly directly proportional to increases in speed. In secondary and direct modes, however, only two forms of feel-forces are provided:

- When TE Flaps are extended which is most generally when airspeeds are relatively low, column forces are low. When TE Flaps are retracted, column forces are high.

The main purpose of feel-forces are to provide protection against overcontrolling the aircraft at high airspeeds. Now, assuming the aircraft has to be manually controlled, a great resource that pilots have is pitch trim. Pitch trim in the 777 is achieved with a movable horizontal stabilizer that is powered by the C & R hydraulic systems, as well as all four ACEs. There are two modes of pitch trim operation: Primary and Alternate. Primary pitch trim is controlled with dual trim switches on each control wheel. There are also two sub-modes of operation. During normal mode, and on ground, the horizontal stabilizer is directly positioned when the trim switches are moved. During flight, trim switches and the autopilot trim instead command inputs to the PFCs to change what is known as the trim reference speed. When in secondary or direct mode, primary trim switches directly command the stabilizer, whether on ground or in flight.

Trim reference speed is the speed that the aircraft would stabilize if there were no control inputs.

The PFCs then send signals to the ACEs. Finally, the stabilizer moves to the commanded position to meet the new trim reference speed, after which the elevators are also displaced so they streamline with the stabilizer. To override the trim switches, opposite column force must be applied. Therefore,

Manual trimming, meaning when autopilots are disconnected, is there to change aircraft airspeed, not aircraft configuration. Once the autopilots are engaged, trim switches are inhibited.

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Two stabilizer position indicators, one being on each side of the control stand have white diamonds to show current relative trim position, in trim units. The FMC calculates and displays a green band that shows the allowable trim range for take-off. This calculation is based upon: Aircraft CG, Gross Weight, Takeoff thrust. The FMC also suggests an ideal trim value, with the previous parameters in mind. This is shown in the takeoff reference page in the CDU. Another way of trimming the aircraft is through alternate pitch trimming. This is achieved by a set of alternate trim levers on the control pedestal, which must be moved together to achieve any pitch trim changes. The alternate pitch trim levers change both the trim reference speed, and the stabilizer position directly, through a series of cables that are linked between the switches and two stabilizer trim control modules, or STCMs. If there is a single STCM failure, the stabilizer will still move but at a slower speed. There are two main rules to operate the alternate trim system:

1. Alternate pitch trim commands have priority over the trim switches on the control wheels. 2. Moving the alternate pitch trim levers during autoflight does cause a displacement in the stabilizer, however, alternate trim must not be used with the autopilot engaged. In fact, it mustnt even be used with any envelope protection features active.

Speaking of flight envelope, there are two features associated to pitch protection: Stall and overspeed protections. Pitch Envelope Protections Lets start with the stall protection. Stalling is obviously a condition that we want to avoid. Fly-by-wire allows for easier and better response to inadvertently entering a stall condition from exceeding the stalling angle of attack. Enhancing crew awareness of a stalling condition is the main objective of the 777 stall protection feature, which operates in the following manner: Stall protection limits the speed at which the aircraft can be trimmed by reducing the likelihood of stick-shaker activation. We must know that the stall warning system is a stick shaker, with no aural warning besides the uncomfortable sound that the shaker produces. *Play stick-shaker sound* Aircraft indicated airspeed can be trimmed to a speed as low as the top of the amber band in the PFD speed tape, which represents minimum maneuvering speed. The pilot must apply continuous back-pressure on the control column to maintain an airspeed below the minimum maneuvering speed. As and when airspeed reaches near stalling speed, the control column forces increase to high levels. In this case, an EICAS caution message will display reading AIRSPEED LOW. If airspeed still decreases below minimum maneuvering speed, and autothrottles are armed but not activated, they automatically activate and advance thrust to minimum maneuvering speed or the MCP selected speed, whichever is higher. This feature is inhibited below 100ft RA. Always remember that in spite of all protections, the aircraft may still enter a stall, therefore caution and safe operation must be exercised to avoid this unwanted condition.

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Moving on from stall protection, another form of pitch envelope protection is against aircraft overspeed. Overspeed protection essentially does the opposite to stall protection, meaning: It limits the airspeed at which the aircraft can be trimmed to maintain maximum operating speed (Vmo/Mmo). Above this speed, constant forward pressure is required in the control columns to keep the aircraft in the overspeed condition and once the columns are released, the aircraft brings down its airspeed to or below Vmo/Mmo. The required forward pushing force increases as bank angle increases. In an overspeed condition, the EICAS illuminates a warning message OVERSPEED and there are also aural alerts. *Play overspeed aural alert* Thats pretty much it for pitch control. Were going to summarize it towards the end of the lesson. Next stop? Roll control. Roll Control In the normal mode, Roll control around the aircrafts longitudinal axis is achieved with one aileron on the outboard portion of each wing; one flaperon located slightly more inboard, and selected spoilers. Flaperons are most generally a concept that not many pilots have heard of. For explanation purposes, flaperons are standard inboard ailerons that also extend and operate as flaps. Both ailerons and flaperons droop a few degrees when TE Flaps are extended, however, they are still fully operational for roll control. During secondary and normal operation modes, the flaperons droop a fixed 20. In this case, ailerons do not droop. Normally, deflection of ailerons, flaperons and spoilers is a function of control wheel displacement. When the control wheels are turned, for example left, the left set of ailerons and flaperons move upward and the opposite wings set move downward. Spoilers only displace upward and begin to extend when additional rolling moment is needed, when the control wheel has been rotated a significant amount. Its important to know that there is a lockout mechanism that prevents ailerons from moving at high speeds. During ground and in flight at slow speeds, ailerons and flaperons control roll, but ailerons lockout and fair to the wings when cruise speed is attained. This is to avoid generating excessive rolling moments at high speed, in order to protect the aircrafts structural integrity. In terms of the FBW aspect of roll control, the structure is very similar to that of pitch control. Input from the pilot controls are sent to four ACEs, that change the signals to digital inputs and relay them to the PFCs. These calculate the required control surface deflection, based on other aircraft information, and send these calculations back to the ACEs, which then command the PCUs to provide the muscle power in the actuators of each control surface. Whenever autopilot is in operation, the autopilot signals are sent directly to the PFCs and then the process is the same from there on. Note that the PFCs backdrives the control wheels, making them displace left or right whenever the autopilot issues roll commands. The pilot may override the autopilot input through sufficient application of force in the control wheel to overcome the backdriven forces. Roll control in secondary and direct modes is mostly the same as in normal mode. Now, also like with the case of elevators, the ailerons also have an aileron trim function. To do so, dual switches on the aisle stand must be pushed to either side. When aileron trim in one direction is used, the system also backdrives the control wheel so that it also tilts slightly in that direction. Both switches move ailerons, flaperons and spoilers in the desired direction. They are spring loaded to neutral. Aileron trim is NOT available, and is inhibited during autopilot operation. Aileron trim is measured in trim units, and is indicated on a scale on top of each control column, near the center.

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Roll Envelope Protections The 777 also has a flight envelope protection feature that is linked to roll control, and that is bank-angle protection, or BAP. As the name implies it, bank-angle protection is there to reduce the possibility of exceeding the normal bank-angles due to weather disturbances or failures of any sort. During IFR flight, a normal bank angle is one that allows a standard rate turn, or 30 of bank, whichever is lower. And so, if the aircraft exceeds a boundary of approximately 35 of bank, the protection feature comes into action and provides control wheel inputs to bring the aircraft back to 30 of bank. Bank angle protection is not available in secondary and direct modes. An indication of an excessive bank angle is made apparent in the PFD. When 35 are exceeded, the indicator turns amber to alert the flight crew. Worth mentioning is that, given the Boeing philosophy, pilots always have ultimate control of the aircraft, and thus may override the bank angle protection commands by continuing to roll the aircraft in spite of having exceeded the bank angle boundary. This must only be done when imperative for safety reasons. Lets now move on to the final segment of this lesson: Yaw control. Yaw Control Yaw control, around the aircrafts vertical axis is achieved through a movable rudder located on the aft portion of the vertical stabilizer. There are three PCUs that move the one rudder and they receive power from all three hydraulic systems, as well as the R, C & L1 ACEs. There are five main aspects related to yaw control in the 777:

- Rudder Pedals, - Rudder Ratio Changer, - Rudder Trim, - Yaw Damping, - Yaw envelope protections.

There are two sets of pedals in the flight deck, and their movement causes a proportional displacement in the rudder. Pedals have a variable force feel, which is not based on airspeed changes like the case of the control column forces we talked about earlier, instead rudder pedal force is based on pedal displacement. The more it is displaced from its zero position, the higher the pedal force. However, airspeed still plays a part in the operation of the rudder. There is a system known as the rudder ratio changer that, given a constant pedal input, reduces the allowable rudder deflection as airspeed increases. This is done to protect the vertical stabilizer and the rudders structural integrity at high speeds. During normal mode, and at low airspeed, the rudder is allowed to deflect fully. As airspeed increases, the PFCs reduce rudder deflection. During secondary and direct modes, rudder ratio is not calculated based on airspeed, but on flap position. To put it simply, when the system senses that TE flaps are down, the system assumes low speed operation and thus allows the rudder to deflect fully, contrary to a case with TE flaps in the up position. In all modes, the rudder deflection ratio always allows sufficient control to counter-act the yawing moment of an engine failure, or to counteract the effect of crosswind crabbing.

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Now, rudder system logic is similar to elevators and ailerons, in the sense that rudder pedal inputs are sent to ACEs, which then communicate with PFCs and then send enhanced signals to the rudder PCUs, to provide muscle to move the rudder. The rudder is also trimmable, with a rudder trim controller that commands rudder trim in all three flight control modes. When the controller is rotated to the left, the rudder deflects left and so does the aircraft nose. Rudder trim also backdrives a movement in the rudder pedals. There are two rudder trim speeds: high and low. A detent in the trim controller limits low and high trim speeds. If the controller is rotated at or below the detent, the rudder displaces at low speed whereas if it is rotated beyond the detent, the rudder displaces at high speed. A MAN TRIM CANCEL switch cancels rudder trim and returns the rudder to the zero position, at a high speed rate. To do this, the button must be pushed however, it is only available in the normal and secondary control modes. Yaw Protection Features Moving on, the 777 yaw control system has several flight envelope protection mechanisms, many more than pitch and roll. These are:

- Yaw Damping, - Gust Supression, - A wheel to rudder cross-tie, - Turn compensation, - Thrust asymmetry compensation.

Yaw Damping Lets start with Yaw Damping. During normal mode, Yaw damping is available to reduce the oscillations caused by dutch roll, as well as aiding in turn-coordination. Contrary to other commercial aircraft, such as the B737, the 777 does not count on an independent yaw damper to achieve the dampening. In fact, the rudder system logic is such that the PFCs themselves issue commands to provide yaw dampening by optimizing the inputs that are sent to the rudder PCUs. In secondary mode yaw damping is available only if inertial data is available to the PFCs. When the system reverts to direct mode, no yaw damping is available. Gust Suppresion Gust suppression is a feature to improve passenger comfort along the flight. The system reduces the impact that gusting wind has on the aircraft, by issuing yaw and roll commands. These commands do not backdrive the control wheel and pedals. Wheel to rudder cross-tie Moving on, the wheel to rudder cross-tie is a feature available to counter the initial yawing moment of an engine failure by using the control wheel. Control wheel inputs allow for around 8 of rudder deflection, however the feature is only available in the normal control mode, and at airspeeds below 210 kts. Turn Compensation The next yaw protection feature is for turns. Turn compensation comes into place only in the normal mode and when the aircraft is being rolled so that in any turn below 30 of bank, the column back pressure required to maintain straight and level flight is eliminated. When turns exceed 30 of bank, but below 60, partial compensation is provided.

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Thrust Asymmetry Compensation Finally, were going to talk about thrust asymmetry compensation, or TAC. TAC is available to assist in controlling the aircraft after an engine failure or when thrust levers are unequal, by providing a high rate rudder command.

For example if the left engine were to fail, a strong left yawing moment would be generated, thus requiring an enormous amount of right-rudder to stay on track. TAC would assist in providing this rapid right rudder response.

The TAC system constantly monitors engine data and when it detects a difference of 10% or more in the thrust output of each engine, it activates a rudder input in the way we described before. The higher the thrust difference, the more rudder deflection is commanded. This rudder deflection is backdriven to the rudder pedals, and is displayed in the rudder trim indicator. The system only corrects the thrust imbalance partially, so that there is still a difference enough that pilots may recognize the thrust asymmetry condition. In spite of TAC coming into place, the pilots may override the commands by making manual rudder pedal inputs. TAC is always available, except:

- When the control system reverts to secondary or direct mode, - When engine thrust data is lost, - When airspeed falls below 70kts on the ground, - When the right thrust reverser has been deployed, - Or when the system has been manually disengaged.

To manually disengage TAC, there is a pushbutton in the overhead panel named THRUST ASYM COMP. When pushed, TAC gets disconnected and an OFF light illuminates in amber. Lesson Summary This wraps up our lesson on the primary flight control system. As weve seen, there are an impressive amount of operation logics, subsystems and modes. More than anything else, you should understand fully the implications of the system reverting to secondary or direct modes. What elements and protection features become inoperative? Perhaps its a good idea that you make a list of all the controls and protections that are unavailable in each mode. To help you out, heres a quick system summary: In the normal mode, all the flight deck inputs for pitch, roll and yaw are fed into actuator control electronic computers, which take the signals and relay the signals to the primary flight computers, which then calculate the required control surface deflection and return the signal to the ACEs, which then command the power control units to hydraulically displace each control surface. In secondary mode, the PFCs calculate simpler commands based on the lack of inertial or airspeed data. When there are more failures, the system reverts to direct mode where the PFCs disconnect from the ACEs and these relay the pilot input directly to the PCUs. All three control axes have trimmable surfaces with cockpit switches. All trim indications are in trim units. Pitch trim is the most prominent in flight and thus has a green normal range band that is displayed to position the trim for takeoff. The 777 FBW function allows for flight-envelope protection features that are based upon protecting the structural integrity of the aircraft and powerplant. At a quick glance, these are Stall, Overspeed and Bank angle protection, as well as a few others we talked about.

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Remember that in a full electrical or hydraulic failure, the system is designed so that the aircraft is still nominally controllable. Only one more lesson to wrap up the controls section. Stay tuned for the following lesson on the high-lift control system. Until then, thanks for watching and

ThrottleOn!