GENERAL PRINCIPAL When aircraft are flying at high speed, the aerodynamic forces acting on the control surface are such that it is not possible to move the surfaces without some form of assistance. This assistance can be provided in the form of hydraulic or electric power or a combination of both of the systems. This system is important for the aircraft to fly.
. The purpose of the powered flight control unit fitted on the aircraft is to: 1) Aircraft speed and aerodynamic loads imposed on control surface too great for the pilot to overcome using manual force only. 2) Assist the pilot to overcome this load 3) No need for normal form of aerodynamic assistance, example: balance tab and spring balance tab
EXPLAINATION OF POWERED FLIGHT CONTROL UNIT (PFCU)
Figure 1: Powered Flight Control Unit System Drawing
Artificial Feel
With purely mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot, allowing tactile feedback of airspeed. With hydro mechanical flight control systems, however, the load on the surfaces cannot be felt and there is a risk of overstressing the aircraft through excessive control surface movement. To overcome this problem, artificial feel systems can be used.
With total hydraulic or electric power moving the control surface, it becomes difficult for the pilot to gauge the amount of control movement required for any maneuver. This is because the pilot's control does not have any sensation of the aerodynamic loads on the control surface. To avoid overstressing the aircraft it is necessary to re-introduce, artificially, this sense of actually moving the control surface, in other words, to provide Artificial Feel.
Fully powered aircraft normally will have no feel of the aerodynamic load. Pilot can’t instinctively position the control surface in relation to speed, attitude and altitude. Rapid movement will damage, over stress the airframe and may become unmanageable Artificial feel device incorporated in the control system give some indication of the aerodynamic force.
Fly-by-Wire
A fly-by-wire (FBW) system replaces manual flight control of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control computers determine how to move the actuators at each control surface to provide the expected response. Commands from the computers are also input without the pilot's knowledge to stabilize the aircraft and perform other tasks. Electronics for aircraft flight control systems are part of the field known as avionics.
Fly-by-Wire means that the aircraft flying controls is based on electrical and electronic devices. In this system, pilot doesn’t feel any aerodynamic forces either artificial or other forces. Computer will judge the pilot command whether can be made by that control surface by speed of movement and the range. This shows that the computer will decide the best command from the pilot. When pilot over select, this can make aircraft go violent move and damage the airframe or control surface devices. Computer will give maximum movement under those circumstance thus aircraft or control surface will not over stressed.
TYPE OF POWERED FLIGHT CONTROL UNIT (PFCU)
There are 3 basic types PFCU that is: * Valve ram type
Consist valve and ram assembly supplied with hydraulic power from the aircraft hydraulic system. * Self-contained type
Self-contained hydraulic system with an electrical supply from aircraft electrical system to operate the hydraulic pump. * Manual reversion type
Allows the pilot to operate the controls manually if the PFCU fail.
Valve Ram Type PFCU
The main part valve ram type PFCU are: * Jack body * Servo valve * Pilots input * Jack ram
Figure 2: Valve Ram Type PFCU
The valve ram type PFCU comprises of Jack body that is connected to control surface. Equal area jack ram connected to the structure. Integral servo valve connected to the pilot’s control. Servo valve is neutral when delivery port are closed and the fluid is trapped in jack is hydraulically locked. Applied input signal make servo valve slide move to open port to pressure and one to return. Jack body move along the ram under hydraulic pressure move the control surface.
When the input cease, the pilot control (slide) in its new position and continues to move relative servo valve slide to neutral position. These close delivery and return ports and ensure hydraulic lock within the jack. Now control surface in the position as command by the pilot. “Follow up” movement by jack body is the negative feedback feel to the pilot. The selection by pilot will give the input to PFCUs until its stop and jack body will catch the input and the movement is small and takes a second. The same PFCU can be mounted in 2 ways that are: 1) Fully powered operation 2) Power- assisted operation
* Fully Powered Operation
The system is operated by hydraulic power. Input from pilot move the servo valve to allow hydraulic fluid to operate hydraulic jack and control surface. Pilot has no feel therefore artificial feel is provided by Q feel unit. PFCU body move the control surface, jack ram is attached to the structure. “Summing link” between pilot input and PFCU output is use as the negative feedback link.
Figure 3: Fully Powered PFCU
Pilot puts an input into the PFCU the summing link will initially pivot above point A on the ram. This will cause the servo valve to select to move the ram. The movement of the ram will be in such a direction as to try to de-select the servo-valve, so as soon as the pilot stops his/her input the jack ram will de-select the servo valve via the summing link pivoting about point C.
Example of the system operation: * Pilot inputs to the left. * Summing link rotates about A in an anticlockwise direction. * Servo valve selects. * Jack ram move to the right. * Pilot ceases input. * For a fraction of a second jack ram continues to move. * With pilot holding the control stationary the summing link rotates anti-clockwise about point C. * This movement will de-select the servo valve setting it into the neutral position and holding the
Figure 4: PFCU with Summing Link
* Power assisted operation
Input link and output link connected in a way that some loading felt by the control surface is felt by the pilot (feedback). Pilot control will move the PFCU to operate. The loading felt by the pilot input lever through the jack ram by using lost motion bush system (allow small play) at the connection of the jack ram to the pilot input. Linked control column with control surface will give a proportional feedback feel to the pilot, so no need for artificial feel.
Figure 5: Power Assisted PFCU
Self-Contained Type PFCU
This system has no external hydraulic power supplies; internally they have their own complete hydraulic system that includes: * Pumps * Valves * Reservoirs * Pipelines * Jacks
All of these components are built in one case. The only external connections are electrical and the control rods (input and output).
An electric motor continuously drives a bank of hydraulic pumps. Movement of the pilot’s input causes a servo valve to move the main bank of pumps, which causes fluid to be pumped to one side of the jack. The other side of the jack is connected to suction. Movement of the jack moves the control surface and a feedback link mechanism.
Figure 6: Self-Contained Type PFCU
Manual Reversion Type PFCU
This system allows the pilot to operate the controls manually if the PFCU fail. Manual operation will be heavy with reduced control authority but it is a reliable emergency standby measure. When manual reversion occurs the pilot will move the control surface directly via the PFCU. In this case the unit just acts as another link in the system. The PFCU goes into manual mode by: 1) Disconnecting the jack ram from the structure. The jack ram is connected to the structure by a hydraulically operated lock mechanism which disengages automatically to allow the jack ram to slide freely back and forth. 2) Allowing fluid to transfer freely from one side of the jack ram piston to the other by a hydraulically operated transfer valve which normally closed. When normal supply fail, the valve open and allows free movement of hydraulic fluid from one side of the piston to the other – with a warning to the pilot.
In both cases the pilot’s input is via the servo valve input and the PFCU moves in response to the pilot’s force thus moving the control surface. In other words the PFCU acts as a control link between the control system and the control surface, and has no other function.
Figure 7: Manual Reversion Type PFCU
PRIMARY FLIGHT CONTROL UNIT Primary flight control unit consists of aileron, elevator and rudder. All of these flight control surface function to move or maneuver the aircraft. All of this control surface moves the aircraft in three ways. There are yaw, roll and pitch. The explanation of the flight control surface is shown on table below.
Table 1: Airplane controls, movement, axes of rotation, and type of stability
AILERONS
Ailerons control roll about the longitudinal axis. The ailerons are attached to the outboard trailing edge of each wing and move in the opposite direction from each other. Ailerons are connected by cables, bell cranks, pulleys or push-pull tubes to each other and to the control wheel. Moving the control wheel to the right causes the right aileron to deflect upward and the left aileron to deflect downward. The upward deflection of the right aileron decreases the camber resulting in decreased lift on the right wing. The corresponding downward deflection of the left aileron increases the camber resulting in increased lift on the left wing. Thus, the increased lift on the left wing and the decreased lift on the right wing cause the airplane to roll to the right.
ELEVATOR
The elevator controls pitch about the lateral axis. Like the ailerons on small airplanes, the elevator is connected to the control column in the cockpit by a series of mechanical linkages. Aft movement of the control column deflects the trailing edge of the elevator surface up. This is usually referred to as up elevator. The up-elevator position decreases the camber of the elevator and creates a downward aerodynamic force, which is greater than the normal tail-down force that exists in straight-and-level flight. The overall effect causes the tail of the airplane to move down and the nose to pitch up. The pitching moment occurs about the center of gravity (CG). The strength of the pitching moment is determined by the distance between the CG and the horizontal tail surface, as well as by the aerodynamic effectiveness of the horizontal tail surface.
RUDDER
The rudder controls movement of the airplane about its vertical axis. This motion is called yaw. Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface, in this case, to the vertical stabilizer, or fin. Moving the left or right rudder pedal controls the rudder. When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction. By pushing the left pedal, the rudder moves left. This alters the airflow around the vertical stabilizer/rudder, and creates a sideward lift that moves the tail to the right and yaw the nose of the airplane to the left. Rudder effectiveness increases with speed, so large deflections at low speeds and small deflections at high speeds may be required to provide the desired reaction. Powered flight control unit is used on the entire primary flight control unit. This is to assist the pilot in controlling the control surfaces when flying the aircraft. The PFCU acted on the ailerons, elevator/stabilizer and rudder.
Figure 8: Example of Rudder PFCU
Figure 9: Example of Elevator PFCU
BASIC POWERED FLIGHT CONTROL UNIT (PFCU) SYSTEM
The PFCU is located at the control surface and can be either, Moving Ram or Moving Body design. The Moving Body design is a simpler system, but has the disadvantage of requiring valuable space to move into, and any high mass transfer can affect the aircraft balance/trim.
Mechanical input to a powered flying control unit is felt at the servo valve. The servo valve can be of a simple spool valve design, which when held in the neutral position, prevents the ram being moved by a hydraulic lock.
MOVING RAM PFCU
Hydraulic pressure is felt at the servo valve awaiting an input from the pilot. Whilst the pilot does not make an input to the servo valve, the PFCU remains in position by means of a hydraulic lock.
INPUT MADE
As the pilot makes an input by movement of the servo valve, the servo valve moves allowing hydraulic pressure through into the jack body, forcing the piston to move and allowing return fluid to flow back to the reservoir. The jack will stop at the required position by means of a feedback link.
FEEDBACK
To enable the flying control to be moved in relation to the input, the input must be cancelled at the specified point. To achieve this, a feedback link is attached between the jack ram and the servo valve. As the control moves, the feedback link gradually removes the input, until the servo valve is closed when the selected position is reached.
MOVING BODY PFCU
Hydraulic pressure is felt at the servo valve awaiting an input from the pilot. Whilst the pilot does not make an input to the servo valve, the PFCU remains in position by means of a hydraulic lock. Movement of the servo valve by the pilot directs fluid to either side of the piston head and creates movement of the PFCU Body whilst the ram is attached to the aircraft structure.
INPUT MADE
As the pilot makes an input to the servo valve, the servo valve moves allowing hydraulic pressure through the servo into the jack body, forcing the piston to move and the return fluid is permitted to flow back to the reservoir.
SPRING FEEL
Basic spring feel units are normally attached to the control columns for ease of use and adjustment. Basic spring feel systems have the disadvantage that the resistance is constant throughout the speed range.
ARTIFICIAL ‘Q’ FEEL SYSTEMS
"Q" feel is an artificial force felt at the control column, which increases as the aerodynamic pressure at the control surface increases. Thus, a "Q" feel system has to simulate the feel of the actual control surface loading that is lost with the use of powered controls, preventing the pilot damaging the aircraft by pulling excessive "g" loads. Artificial "Q" feel units have to increase the control column resistance, in proportion to the square of the airspeed. In general, "Q" feel systems can be either mechanically or hydraulically operated.
MECHANICAL ‘Q’ FEEL
The control rods are connected at one end of the slotted bell crank lever and the spring cartridge at the other. As a pilot demand is made, this lever pivots about the roller and the spring provides a resistance to the movement.
Relative positions of the fulcrum arm determine the amount of feel felt back at the control column. The fulcrum arm is positioned by means of an electrical linear actuator.
BOEING 737 SYSTEMS
* The primary flight controls, ailerons, elevators and rudders, are hydraulically powered. Hydraulic power is provided from hydraulic systems A and B; either system can operate all primary flight controls. * The ailerons and elevators may be operated manually if required. The rudder may be operated by the standby hydraulic system if system A and/or B pressure is not available
Ailerons
The ailerons provide roll control around the airplane’s longitudinal axis. The ailerons are positioned by the pilots’ control wheels. The A and B FLT CONTROL switches control hydraulic shutoff valves. These valves can be used to isolate each aileron, as well as the elevators and rudder, from related hydraulic system pressure .The Captain’s control wheel is connected by cables to the aileron power control units (PC’Us) through the aileron feel and centering unit. The First Officer’s control wheel is connected by cables to the spoiler PCUs through the spoiler mixer. The two control wheels are connected by a cable drive system which allows actuation of both ailerons and spoilers by either control wheel. With total hydraulic power failure the ailerons can be mechanically positioned by rotating the pilots’ control wheels. Control forces are higher due to friction and aerodynamic loads.
Elevators
The elevators provide pitch control around the airplane’s lateral axis. The elevators are positioned by the pilots’ control columns. The A and B FLT CONTROL switches control hydraulic shutoff valves for the elevators. Cables connect the pilots’ control columns to elevator power control units (PCUs) which are powered by hydraulic system A and B. The elevators are interconnected by a torque tube. With loss of hydraulic system A and B the elevators can be mechanically positioned by forward or aft movement of the pilots’ control columns. Control forces are higher due to friction and aerodynamic loads.
Rudder
The rudder provides yaw control about the airplanes vertical axis. The A and B FLT CONTROL switches control hydraulic shutoff valves for the rudder and the standby rudder. Each set of rudder pedals is mechanically connected by cables to the input levers of the main and standby rudder PCUs. The main rudder PCU is powered by hydraulic system A and B. The standby rudder PCU is powered by the standby hydraulic system .The standby rudder PCU is powered by the standby hydraulic system. The standby hydraulic system is provided as a backup if system A and/or B pressure is lost. With the standby PCU powered the pilot retains adequate rudder control capability. it call be operated manually through the FLT CONTROL switches or automatically
ROLL CONTROL SCHEMATIC
PITCH CONTROL SCHEMATIC
YAW CONTROL SCHEMATIC
FLIGHT CONTROL PANEL
1 - Flight control switches STBY RUD - activates standby hydraulic system pump and opens standby rudder shutoff valve to pressurize standby rudder power control unit.
OFF - closes flight control shutoff valve isolating ailerons. Elevators and rudder from associated hydraulic system pressure
ON (guarded position) - normal operating position.
2 - Flight control low pressure light
Illuminated (amber)
• indicates low hydraulic system (A or B) pressure to ailerons. Elevator and rudder
• deactivated when associated FLIGHT CONTROL switch is positioned to STBY RUD and standby rudder shutoff valve opens.
3 - Flight spoilers switches
ON (guarded position) - normal operating position
OFF - closes the respective flight spoiler shutoff valve.
4 - Yaw damper light
Illuminated (amber) -yaw damper is not engaged
5 - Yaw damper switch
OFF - disengages yaw damper
ON-
* engages main yaw damper to main rudder power control unit if the B FLT CONTROL switch is in the ON position * engages standby yaw damper to standby rudder power control unit if both the A and B FLT CONTROL switches are in the STBY RIJO position
6 - Standby hydraulic light
STANDBY HYDRAULIC LOW QUANTITY Light
Illuminated (amber) - * indicates low quantity in standby hydraulic reservoir * Always armed.
STANDBY HYDRAULIC LOW PRESSURE Light
Illuminated (amber) - * indicates output pressure of standby pump is low * Armed only when standby pump operation has been selected or automatic standby function is activated.
STBY RUD ON Light * illuminated (amber) - indicates the standby rudder system is commanded On to pressurize the standby rudder power control Unit.
7 - Alternate flaps master switch
OFF (guarded position) - normal operating position.
ARM - closes TE flap bypass valve, activates standby pump. and arms the alternate flaps position switch
8 - Alternate flaps position switch
UP-
* electrically retracts TE flaps * LE devices remain extended and cannot be retracted by the alternate flaps system.
OFF - normal operating position
DOWN (spring loaded to OFF) - * (momentary) fully extends LE devices using standby hydraulic pressure * (Hold) electrically extends TE flaps until released.
9 - Feel differential pressure light
Illuminated (amber) - * Indicates excessive differential pressure in the elevator feel computer.
10 - Speed trim failure light
Illuminated (amber) - * indicates failure of the speed trim system * Indicates failure of a single FCC channel when MASTER CAUTION light recall is activated and light extinguishes when Master Caution System is reset.
11 - Mach trims failure light
Illuminated (amber) - * indicates failure of the speed trim system * Indicates failure of a single FCC channel when MASTER CAUTION light recall is activated and light extinguishes when Master Caution System is reset.
12 - Automatic slat failure light
Illuminated (amber) - * indicates failure of the auto slat system * Indicates failure of a single Stall Management Yaw Damper (SMYD) computer when illuminated during MASTER CAUTION recall and extinguishes when master caution system is reset.
Cockpit PFCU control
RUDDER
1. Rudder pedal
2. Rudder trim indicator
3. Rudder trims OFF flag
4. Rudder trim control
5. Yaw damper indicator
AILERON/ELEVATOR/FLIGHT SPOILER
1. Aileron trim indicator
2. Aileron trim switch
3. Control wheel
4. Control column
The Boeing737 use hydro-mechanical system to control the aircraft. Compared to simple mechanical system the:
Advantage * With hydraulic flight control systems, aircraft size and performance are limited by economics rather than a pilot's strength. * The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft. Hydraulically powered control surfaces help to overcome these limitations
Disadvantage * With purely mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot, allowing tactile feedback of airspeed. With hydro-mechanical flight control systems, however, the load on the surfaces cannot be felt and there is a risk of overstressing the aircraft through excessive control surface movement.
REFERENCES 1) Aircraft Flight Control. Note on Powered Flight Control Unit –PFCU 2) http://www.scribd.com/doc/52717363/PFCUs-Pt-1 3) http://www.free-online-private-pilot-ground-school.com/Flight_controls.html 4) http://en.wikipedia.org/wiki/Aircraft_flight_control_system 5) http://www.737ng.co.uk/B_NG-Flight_Controls.pdf
