10 Questions
What is the purpose of the gain schedule in the flight control system?
The gain schedule adjusts the control surface deflection gains based on flap handle or landing gear position to optimize control response for different flight conditions.
When is the fixed high gain used in the flight control system?
The fixed high gain is used when the aircraft is in the landing configuration.
What mode does the flight control computer enter if all four command/monitor lanes disagree?
The flight control computers (FCCs) enter direct mode if all four command/monitor lanes disagree.
How does direct mode differ from alternate mode in terms of redundancy management?
Direct mode has reduced redundancy management compared to alternate mode, with less cross-comparison of data inputs to ensure validity.
Can the flight control reset switch upgrade the system from direct mode to normal mode? Why or why not?
No, the flight control reset switch cannot initiate an upgrade from direct mode to normal mode.
What is the function of the yaw (rudder) damper in the flight control system?
The yaw (rudder) damper provides yaw damping when the AOA (angle of attack) limiting and autopilot (AP) functions are not available.
What happens when the flight control reset switch is used to upgrade from alternate mode to normal mode?
When upgrading from alternate to normal mode using the reset switch, it clears the monitors in the flight control computers (FCCs) and actuators.
What is the purpose of having redundant command/monitor lanes in the flight control computers?
The redundant command/monitor lanes provide fault tolerance and enable cross-comparison of data to detect and isolate faults.
How do the control laws differ between alternate mode and direct mode?
The control laws used in alternate mode and direct mode are identical.
What could cause the flight control computers to disagree across all four command/monitor lanes, resulting in entry to direct mode?
A common software fault across all lanes could cause the disagreement that leads to direct mode entry.
Study Notes
FCC Inputs and Outputs
- In BACKUP PITCH trim switch mode, the HSCU moves the stabilizer directly without involving the FCC at a constant predetermined rate.
- The FCC receives ARINC 429 bus data from various aircraft systems, including:
- Inertial reference system
- Attitude heading reference system
- Air data system
- Radar altimeters
- MAU AP cards
- MAU central maintenance system
- FECU
- HSCU
- Brake control unit
- Landing gear control unit
- FADEC
- The FCC transmits data to aircraft systems on three separate A429 buses:
- External 1: primarily provides CAS and FDR data via MAUs
- External 2: primarily provides Central Maintenance Computer (CMC) data via MAUs
- Control: provides actual control data only to the horizontal stabilizer
FCC Discrete Signal Connections
- Discrete signal connections to the FCCs come primarily from switches located on various cockpit control panels.
- Program pins are used to program the FCC location.
FCC Channels and Redundancy
- Each FCC has two independent and redundant channels (A and B), making a total of four control channels.
- Channel A and B modules are not interchangeable due to dissimilar hardware.
- Like modules (channel A COM/MON or channel B COM/MON) in a particular channel are interchangeable.
- Each channel has its own 28 Vdc power input, computing and input/output resources.
- The standby channel is ready to take control at any point should something happen to the active channel.
- Between the two channels of the same FCC, only one channel at a time will transmit to a given REUs bus.
FCC Processing Lanes
- Each channel has two processing lanes: Command and Monitor.
- There are a total of eight lanes of communication (four control channels, each with two processing lanes).
- Like modules (channel A COM/MON or channel B COM/MON) have identical hardware, but different software (C and ADA language compilers).
- Each module contains both sets of software and will function as either command or monitor when configured to do so.
Gain Schedule and Yaw Damper
- The gain schedule is based on either flap handle or landing gear position.
- Fixed high gain is used when the aircraft is in the landing configuration.
- Fixed low gain is used in the high-speed region.
- The yaw (rudder) damper may be available when AOA limiting and AP are not available.
Flight Control Reset Switch
- The flight control reset switch can be used to upgrade from alternate mode to normal mode provided the faults that caused entry to alternate mode have been cleared.
- Reset clears monitors in FCC, actuators.
Simple Mode
- The FCCs enter direct mode if all four FCC command/monitor lanes disagree.
- Direct mode is similar to alternate mode, but with reduced redundancy management.
- The control laws used are identical, but the FCCs operate in a more simple manner.
- The flight control reset switch cannot initiate an upgrade from direct to normal mode.
Test your knowledge on how the aircraft flight control system operates in different modes and handles inputs from various aircraft systems. Learn about the ARINC 429 bus data and its connections to the Flight Control Computers (FCCs).
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