Joined: 11 Dec 2005
|Posted: Sat Aug 04, 2012 12:22 pm Post subject: Flight Controls (Technical Notes)
|PRIMARY FLIGHT COMPUTERS Disconnect Switch
In DISC position:
• disconnects the primary flight computers (PFCs) from the flight control system
• puts the flight control system in the direct mode
• AUTO can be reselected to attempt restoration of secondary or normal mode operation.
In AUTO position:
• the flight control system operates in the normal mode
• system faults automatically cause the system to switch to the secondary or direct modes.
Pitch Trim Switches
• in the normal mode in flight, changes the trim reference airspeed.
• in the normal mode on the ground, moves the stabilizer
• in the secondary and direct modes, moves the stabilizer.
The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes.
Alternate Pitch Trim
The levers move the trim reference airspeed (normal mode) and also move the stabilizer (all modes).
The alternate pitch trim levers are linked to the stabilizer trim control modules (STCM) via control cables, and then mechanically to the stabilizer.
Alternate pitch trim commands have priority over wheel pitch trim commands in all flight control modes.
Moving the alternate pitch trim levers with the autopilot engaged does not disconnect the autopilot, but does move the stabilizer.
Moving the alternate pitch trim levers during stall or overspeed protection does move the stabilizer, but does not remove column forces.
Note: The alternate pitch trim levers should not be used with the autopilot engaged, or during stall or overspeed protection.
The pilot controls command these system electronic components:
• four actuator control electronics (ACEs)
• three primary flight computers (PFCs).
The ACEs receive input signals from all pilot controls. The ACEs send control signals to the primary flight control surfaces. The ACEs can transmit pilot control inputs directly to the control surfaces, or they can send the pilot inputs to the PFCs. When the ACEs are sending pilot inputs to the PFCs, the ACEs receive control commands back from the PFCs and use the commands to position the flight control surfaces.
The PFCs use information from other airplane systems (such as air data, inertial data, flap and slat position, engine thrust, and radio altitude) to compute control surface commands for enhanced handling qualities.
The autopilot also sends commands to the PFCs, which then produce control surface commands.
Flight Control System Normal Mode
In the normal mode during manual flight, the ACEs receive pilot control inputs and send these signals to the three PFCs. The PFCs verify these signals and information from other airplane systems in order to compute control surface commands. These commands are then sent back to the ACES. The four ACES send enhanced signals to the flight control surface actuators.
Flight Control System Secondary Mode
When the PFCs can no longer support the normal mode due to internal faults or lack of required information from other airplane systems, they automatically revert to the secondary mode. The ACEs continue to receive pilot control inputs and send these signals to the three PFCs. However, the PFCs use simplified computations to generate flight control surfaces commands. These simplified commands are sent back to the ACEs, where they are sent to the control surface actuators the same way as in the normal mode.
The simplified PFC control laws used in the secondary mode affect airplane handling qualities and some functions are not available in the secondary mode.
However all flight control surfaces remain operable.
Flight Control System Direct Mode
The ACEs automatically transition to the direct mode when they detect the failure of all three PFCs or lose communication with the PFCs. The direct mode can also be manually selected by moving the PRIMARY FLIGHT COMPUTERS DISCONNECT switch to DISC. In the direct mode, the PFCs no longer generate control surface commands. Pilot inputs received by the ACEs are sent directly to the control surface actuators. The direct mode provides full airplane control for continued safe flight and landing. Airplane handling qualities are approximately the same as in the secondary mode. In the direct mode, some functions are not available.
Secondary and direct modes do not provide automatic pitch compensation for:
• thrust changes
• gear configuration changes
• flap and speedbrake configuration changes
• turns to 30° bank angle
In the unlikely event of a complete electrical system shut–down, cables from the flight deck to the stabilizer and selected spoilers allow the pilot to fly straight and level until the electrical system is restarted.
Thrust Asymmetry Compensation
If the thrust level on one engine differs by 10 percent or more from the other engine, TAC automatically adds rudder to minimize yaw. TAC does not fully compensate for the failed engine so the pilot can recognize engine failure through roll/yaw cues. These roll/yaw cues are greatly reduced when compared to an airplane operating without TAC.
Wheel to Rudder Cross–Tie
A wheel to rudder cross–tie function provides the capability of being able to control the initial effects of an engine failure using control wheel inputs only. Control wheel inputs can deflect the rudder up to 8 degrees. Wheel to rudder cross–tie is operative in flight below 210 knots in the normal mode.
Flap and Slat Modes
Three modes of flap and slat operation are possible:
• Primary (Hydraulic = Center hydraulic system)
• Secondary (Electric - Left and right AC busses)
• Alternate (Electric - Left and right AC busses)
The flaps and slats can operate independently in either the primary or secondary mode. However, independent flap and slat operation in the alternate mode is not possible.
The secondary mode is automatically engaged whenever the primary mode fails to move the flaps or slats to the selected position.
Once engaged, the secondary mode remains engaged until the affected system surfaces are fully retracted or center hydraulic system pressure is restored.
In the secondary mode, the flaps and slats are positioned by electric motors.
Autoslats are not available in the secondary mode. Therefore (below a certain set speed) the slats are fully extended at all flap positions. If actual airspeed exceeds that speed, the slats retract to the midrange position, or will not extend beyond the midrange position.
If the slats are in the midrange position (flaps 1 through 20) when the secondary mode is engaged, they remain in that position until the flaps are retracted to UP, or extended beyond 20.
Slat and flap operation time in the secondary mode is greatly increased.
The alternate mode allows direct manual operation of the flaps and slats through the secondary drive electric motors.
The alternate mode must be manually selected. The alternate flaps ARM switch:
• Disables normal control
• Arms the alternate mode
• Engages the electric motors
• Flap lever no longer controls flaps/slats.
The three–position alternate flaps selector extends and retracts the flaps and slats.
The flaps and slats extend simultaneously, but slat retraction is inhibited until the flaps are up.
Alternate mode flap and slat extension is limited to slats midrange and flaps 20.
Asymmetry protection, uncommanded motion protection, autoslats, and flap/slat load relief are not available in the alternate mode.
Slat and flap operation time in alternate mode is greatly increased.