Flight Controls
Revised 28-Feb-2001
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The 727 wing has 28 Deg sweepback and many devices that affect its aerodynamics.  All flight control surfaces are normally powered hydraulically except for the stabilizer, which is trimmed electrically. Switches on the overhead panel control system "A" and system "B" hydraulic pressure to the ailerons, elevators, rudders, and flight spoilers.  Normally they are guarded ON.  Moving these switches OFF shuts off pressure to the associated control units.  The system "A" rudder switch is ganged with the ON-OFF switch for the standby hydraulic system pump motor.  Should hydraulic pressure to the primary flight controls drop below an acceptable level the appropriate amber light on the annunciator on the First Officer's forward instrument panel will come on.
There are two sets of ailerons on the 727, they are designated inboard and outboard.  A hydraulic power unit located in the left main wheel well operates the ailerons.  This unit is supplied hydraulic pressure from both systems "A" and "B".  Either system will provide sufficient power to displace the ailerons through their full travel.  When the control wheel is turned, the hydraulic power unit operates the ailerons through a cable system. When the trailing edge flaps are up, the outboard ailerons are held in a faired position by a lockout mechanism.  When the flaps are extended, the lockout mechanism allows outboard aileron movement.  The amount of outboard aileron movement, in relation to inboard aileron movement, is dependant on the degree of outboard trailing edge flap extension.  Full outboard aileron response is available before the flaps reach normal landing configuration.
Aerodynamic balance tabs and balance panels assist in the operation of the ailerons.  In powered operation, the tabs on all ailerons move opposite to the direction of aileron movement to assist in positioning the aileron aerodynamically.  If all hydraulic power to the aileron is lost the system will automatically shift to manually operate aerodynamic tab control.  That is the inboard aileron tabs operate as control tabs.
Now when the control wheel is turned, the aileron cable moves the inboard aileron tabs and the inboard ailerons are repositioned through aerodynamic action.  Movement of the inboard ailerons is transmitted to the outboard ailerons through the normal cable system. If the flaps are retracted, the outboard ailerons are locked out as in powered operation.  During manual operation the outboard aileron tabs do not shift to a control function, but move in response to the movement of the aileron.
Aerodynamic balance panels are located in the wing structure attached to the leading edge of the aileron and the aft wing spar.  Differential air pressure acting on these panels assists in aileron operation.  This is particularly significant during manual flight control operation.
Artificial feel for aileron movement is provided by a spring-loaded roller and cam mechanism.  This mechanism also centers the control wheel when control pressure is released.  The aileron trim wheel is located on the aft end of the control stand.  Movement of the trim wheel repositions the roller and cam mechanism, changing the control wheel forces.  When trim is changed, the control wheel will have a new neutral feel position.  The ailerons will not move in response to a trim input if there is no hydraulic pressure available, however, the control wheel will reposition.  When the ailerons are operated in flight without hydraulic pressure.  Aerodynamic forces provide feel, and the trim wheel is ineffective.
There are seven spoiler panels on each wing.  The two inboard panels are ground spoilers and can be extended only on the ground through the action of the speed brake lever.  They are operated by hydraulic system "A" pressure.  With the left main gear strut compressed, this linkage opens a hydraulic valve allowing pressure to go to the ground spoiler actuators when the speed brake lever is moved aft.  Since these panels are used only on the ground to spoil lift after landing, they go to the full up position when actuated.  There are no intermediate positions for the ground spoilers.
The remaining five panels on each wing are flight spoilers.  The three inboard flight spoilers are powered by hydraulic system "B".  The two outboard flight spoilers are powered by hydraulic system "A".  Individual actuators operate the spoiler panels.
The flight spoilers, which operate in conjunction with the ailerons, provide the major portion of roll control by spoiling lift on the low wing.  After a small movement of the control wheel, spoiler action is programmed in proportion to the further movement of the control wheel.  A full roll control input would cause the flight spoilers of the low wing to rise to a maximum of approximately 25 degrees.  The spoiler panels of the high wing would remain down.
The flight spoiler panels are also used as in-flight speed brakes when extended by the speed brake lever.  They extend increasingly as the speed brake lever is moved aft.  Maximum extension of the flight spoilers when used as speed brakes is 45 degrees; however, the actuators will allow spoiler blow down at high speed.  While in flight a warning horn will sound intermittently if speed brakes are extended with the wing flaps also extended.
When the speed brakes are extended, spoiler input is still received from control wheel movement; however, this input is influenced by the amount of speed brake extension.  Although there is no operational restriction, care should be exercised in operating ailerons with partial speed brakes due to spoiler mixer inputs.  Extreme roll rates can be experienced in this configuration. Two switches on the upper right of the flight control hydraulic power panel are OFF-ON switches for hydraulic power to the flight spoilers.

AUTO SPOILERS 200 series variants
An auto spoiler system is installed and extends the flight and ground spoiler's automatically after landing.  To arm the auto spoiler system, the speed brake is moved to the arm position.  The spoiler will extend when armed with the main wheel rotation above 60 kts, and strut compression has taken place. The speed brake armed light will illuminate to show that the auto speed brake circuitry is complete.  If armed and a fault exists in the auto spoilers, the DO NOT ARM light will illuminate and the speed brake lever must be moved out of the armed position.  The speed brakes may then be armed manually.  The DO NOT ARM light will illuminate after landing with the speed below 60 kts, until the speed brake lever is restowed. When either No. 1 or No. 2 reverser is actuated during an aborted take off above 60 kts, the spoilers will automatically deploy.  Auto spoilers will also deploy automatically if a landing is made without speed brake in arm when either No.1 or No.2 reverse throttle is actuated, with speed above 60 kts. If a go-around is initiated after landing, moving no.1 or no. 3 thrust lever forward automatically moves the speed brake handle forward to the down position.
To test the system, place the speed brake lever in arm and note the ARMED light illuminated, press each of the test buttons.  When pressed the DO NOT ARM light will be illuminated.
Pitch control of the 727 is accomplished through two independent elevators and a stabilizer.  There are balance panels to assist in elevator movement.  The elevators are powered jointly by systems "A" and "B" and will operate normally with either system alone.  With loss of all hydraulic power, the elevator tabs act as control tabs, the same as the inboard aileron tabs.  Full elevator movement by means of the control tabs is only about 50% of the movement available in powered operation. Actual position of the elevator with respect to the stabilizer, can be observed by reference to the indices on the left and right sides of the rudder and elevator position indicator.  Under normal flight conditions the elevator pointers should be centered.
To provide artificial feel, an elevator feel computer is installed.  With inputs of system "A" and "B" hydraulic pressure, pitot-static pressure and stabilizer position the feel computer furnishes proportional feel to the elevator control system.  The elevators feel computer does not supply boost to the elevator.  When substantial differences in the computer pressure outputs occur, the elevator feel differential pressure light on the SID's panel will come on.  It indicates a possible erroneous control feel.  When the light is on the pilot should avoid abrupt elevator inputs.  Spring tension and aerodynamic forces give feel when the elevators are operated with no hydraulic power.
Pitch trim is accomplished by repositioning the stabilizer.  The stabilizer can be controlled by either of two electric motors or a manual system.  Both of the electric motors and the manual system operate the same jackscrew, which drives the stabilizer.  The high-speed electric trim motor is controlled by these main electric trim switches on the control wheels.  Each main electric control consists of two thumb switches; one is for motor power, the other clutch power.  Both have to be activated to move the stabilizer.  This is a safety feature to prevent one faulty switch from causing a runaway stabilizer. The other trim motor is slow speed, and is controlled by the cruise trim switch an the control stand.  The autopilot uses the slow speed motor for pitch trim. Manual trimming of the stabilizer is accomplished by using the cranks stored in the trim wheels located on either side of the control stand.  The manual trim will override either electrical trim motor.
When the stabilizer is being trimmed electrically by either the main electric trim switch or cruise trim switches the stabilizer trim light on the control stand will come on.  This indicates that one of the trim motor circuits is energized.  When the autopilot is engaged, however, the cruise trim motor runs continuously.  To avoid continuous illumination of the lights it is deactivated during autopilot operation.  The two lever switches to the right of the light are cut-out switches to remove electrical power from the motors.
Trim indices, located on both sides of the control stand, show the position of the stabilizer in units.  The green band denotes limits of stabilizer trim in % MAC.  If the stabilizer trim is not in the green band for takeoff, an intermittent horn will sound when the throttles are advanced toward takeoff setting.  To prevent running the stabilizer to the stops there are electrical trim limits, both nose up and nose down.  When the main trim reaches one half-degree nose up during nose down travel, control automatically switches to the cruise trim motor.  Once the electrical limits are reached, the stabilizer can be trimmed slightly further manually.  As the stabilizer is trimmed into the range near the nose up limit, the elevator neutral will gradually move a few degrees up from faired.  This provides more effective stabilizer trim and more nose down elevator capability when the airplane is trimmed nose up.  Operation of this mechanical linkage can be noted in the cockpit by aft movement of the control column when the stabilizer is trimmed into the range where the system if effective. To stop a runaway stabilizer a stabilizer brake is installed.  A control force opposite to the direction of the runaway will engage the brake.  Once engaged, the stabilizer brake should automatically disengage when opposing pressure is relieved.  If it does not, pulling the stabilizer brake release knob will mechanically release the brake.
Yaw control is accomplished through dual rudders and yaw dampers.  The rudders are hydraulically powered, the upper rudder by system "B" pressure, the lower rudder by system "A" pressure.  As a backup, the lower rudder can also be operated by the standby hydraulic system, which powers a separate rudder actuator.  If all hydraulic power is lost there is no rudder control. Full system "A" pressure is provided to the lower rudder when the flaps are extended.  When the flaps are up pressure to the lower rudder is decreased through action of the rudder load limiter.  Illumination of the rudder load limiter light on the FE's panel indicates that the pressure to the lower rudder is not proper for the inboard flap position.  The upper rudder always operates at reduced system "B" pressure.  Therefore, the pressure changeover monitored by this light is applicable only to the lower rudder. Both rudders have anti-balance tabs, which move in the same direction as the rudder.  Artificial feel for rudder inputs is provided through a spring-loaded roller and cam assembly.
The rudders are trimmed by positioning the rudder trim control on the control stand.  The rudder pedals will reposition during trim input.  As with the ailerons, rudder trim is available only when hydraulic power is available.
The tendency to "Dutch Roll", at high altitude and high airspeed, is counteracted in the 727 by a yaw damper system for each rudder.  The lower rudder yaw damper does not function when that rudder is powered by the standby system.  The yaw dampers receive electrical signals from the rate gyros.  As the nose moves left or right, the rate gyro senses a yaw.  This signal is sent to the yaw dampers , which direct the rudders opposite to the direction of the yaw.  Rudder movement caused by the yaw dampers is not transmitted to the rudder pedals and does not interfere with pilot input to the rudders. A yaw damper test switch on the Captain's forward instrument panel allows testing of the yaw dampers before taxiing.  The yaw dampers are designed to be used continuously.  Therefore, the guarded yaw damper engage switches on the center forward instrument panel are normally ON at all times. On the Captain's forward panel is a rudder-elevator indicator.  The yellow YID flag will appear if the respective yaw damper system loses electrical power or is turned off. If one or both yaw dampers fail, airspeed and altitude restrictions are imposed.  These restrictions are listed in the Limitation section of the Flight Manual.
Find out more about yaw dampers by clicking here

To prevent airflow separation over the rudders at low speeds, vortex generators are installed at the vertical stabilizer. A maximum of two may be missing (not adjacent).

Combining both leading and trailing edge devices provides high lift capability in the 727.  The trailing edge flaps are triple slotted Fowler flaps and are divided into two sets, inboards and outboards.  Each set is bussed separately by torque tubes.
The flap handle on the control stand normally controls flaps.  Flap positions are: UP 2, 5, 15, 20, 25, 30, and 40 with gates at the 2 and 25 positions. Though operational reasons sometimes prevents these all being used (certain stage three requirements).   The position 2 gate prevents inadvertent retraction of the flaps prior to reaching air speed requirements and also acts as a reminder to check the leading edge annunciator before selecting another position.  The position 25 gate establishes the lower end of the takeoff range.  This gate also facilitates locating the go-around position without visual reference to the flap handle.  Both gates require that the lever be lowered into the detent and passed under the gate before it can be moved further.
The flaps are normally operated by hydraulic motors each turning a torque tube for a set of flaps.  One of theme motors is located in each main wheel well.  An alternate method of operating the flaps is by electric motors. One located in each wheel well.  The same torque tubes and jack screw assemblies are used by either the hydraulic or electric motors.
Switches on the overhead panel control the inboard and outboard flap electric motors.  With the Alternate Flaps Master switch ON, the inboard and outboard switches are armed.  The switches must be held down for flap extension but will stay up for flap retraction.  The Alternate Flap Master switch should not be turned ON unless airspeed is 245 knots or below.  The inboard and outboard switches should be positioned off for a few seconds before reversing the direction of flap movement.  Indicators on the center instrument panel show actual flap position.  Each flap indicator has dual pointers, one for each associated left and right flaps.
To prevent a roll control problem due to a differential in trailing edge flap extension, there are asymmetrical protection systems for the inboard pair and outboard pair of flaps.  The indicators have an electric asymmetrical protection system based on separation of the left and right pointers on the flap indicators by about one quarter inch.  This protection system stops flap travel by stopping flow of fluid to the hydraulic motor.  There is no asymmetrical protection when using the alternate flap system other than monitoring the flap position indicators.
Leading edge devices, consisting of four slats and three flaps on each wing, are automatically sequenced to operate with the trailing edge flaps.  The movement of the outboard trailing edge flaps positions the control valve for the leading edge device actuators.  At the position 2, the two middle slats on each wing will extend.  When the trailing edge flaps are extended to position 5, all of the leading edge devices extend.  Once extended the actuators are mechanically locked in the extended position and must be unlocked hydraulically to be retracted.  The three inboard devices, or flaps, are held in the retracted position with the system "A" pressure.  The remaining devices, the slats, are mechanically locked in the retracted position and must be unlocked hydraulically to be extended.  Each leading edge device has its own actuator. The leading edge annunciator on the forward instrument panel will be amber while the leading edge devices are in transit or not properly positioned for trailing edge flap configuration.  When the correct leading edge devices are extended, in agreement with trailing edge flaps, the green light comes on.  Both lights will be off when the leading edge devices are up.
The leading edge device annunciator on the FE's auxiliary panel has a 3 position selector, spring loaded to OFF.  Holding the selector to the right will test all of the lights.  To the left, the actual position of each individual leading edge device is indicated, that is, with no light the device is retracted; amber, the device is partially extended; or green, the device is fully extended.  This annunciator is used when the one on the forward instrument panel remains amber after flap operation, indicating improper leading edge device positioning.
An alternate means of extending the leading edge devices is provided.  With the Alternate Flaps Master switch ON, momentarily moving either of the alternate flaps inboard or outboard switches to DOWN will extend all leading edge devices.
The standby hydraulic system powers a hydraulic motor-pump assembly, which pressurizes the leading edge device actuators to the down position with fluid from a reserve section of the system "B" reservoir.  The devices cannot be retracted by the alternate system.
Under certain conditions a warning horn will sound if high lift devices, speed brakes, or the stabilizer are improperly positioned.  On the ground, the warning horn will sound intermittently when the no. 1 or 3 throttle is advanced to takeoff thrust if the stabilizer trim is not in the green band, the flaps are not in the takeoff range, the speed brake lever is not in the forward detent, or the leading edge devices are not extended.

The trailing edge flaps control various other components an the airplane.
Outboard Trailing Edge Flaps      Inboard Trailing Edge Flaps
Leading edge devices                Stall warning systems
Outboard ailerons                   Air conditioning pack fan
Takeoff warning horn                Auto pack trip system.
Landing gear-flaps warning horn     Lower rudder load limiter
Ground proximity warning system.     Speed brake - flaps warning

Most commercial swept-wing airplanes have a device to warn of an approaching stall.  As an advanced warning if an approach to stall occurs, the computer activates a stick shaker at 1.15 times stall speed.  The stall-warning portion of the overhead panel contains the fail light and test/heater off switch for the Stall Warning unit in the 727.  There is one unit installed on the airplane, which consists of a computer, an angle of attack vane and a stick shaker. The angle of attack vane, located on the right side of the airplane, sends angle of attack information to the stall-warning computer.  The computer also receives information of actual inboard trailing edge flap position.  The computer is powered and the vane is heated any time an airplane generator is powering the electrical bus, provided external power is disconnected.  When the vane heater and computer are both powered, the power off lights will be off. To avoid stall-warning signals while taxiing, the computer, although powered, is not activated until the airplane is airborne.


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