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ATR 42/72-600 Systems Guide — Chapter J: Flight Controls

2026-06-23

Chapter J. FLIGHT CONTROLS (FCOM DSC 27)

1. PRIMARY MECHANICAL FLIGHT CONTROL TOPOLOGY (Page 109)

The primary flight control network architecture utilizes mechanical cables, pulleys, push-pull rods, and quadrant assemblies to transmit pilot inputs directly to the aerodynamic control surfaces.

  FLIGHT CONTROL SURFACE ACTUATION MAP
  ┌────────────────────────────────────────────────────────┐
  │                 PILOT INPUT CONTROL COLUMNS            │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [Tensioned Cable Runs]
  ┌───────────────────────────┴────────────────────────────┐
  │               MECHANICAL SEPARATION VALVES             │
  │        (Allows isolation if mechanical jams occur)     │
  └─────┬─────────────────────┬─────────────────────┬──────┘
        │                     │                     │
        ▼                     ▼                     ▼
  ┌───────────────┐     ┌───────────────┐     ┌───────────────┐
  │   ELEVATORS   │     │   AILERONS    │     │    RUDDER     │
  │(Pitch Tracking│     │(Roll Tracking)│     │ (Yaw Tracking)│
  │  Rigid Cable) │     │(Cable/Hyd Tabs)     │(Rigid Linkage)│
  └───────────────┘     └───────────────┘     └───────────────┘
  • Power Assistance Profiles: Pitch (Elevators) and Yaw (Rudder) are entirely mechanical and manually driven, relying on aerodynamic tabs to reduce control forces. Roll control utilizes manually driven mechanical ailerons augmented by hydraulically assisted spoiler panels.

2. ROLL CONTROL ARCHITECTURE & SPOILER INTEGRATION (Page 110)

Roll guidance is achieved through coordinated inputs from the mechanical ailerons and dual hydraulically driven spoiler panels.

2.1. Mechanical Layout & Roll Disconnect Options

  • Control Separation: The Captain's control wheel is cabled directly to the left aileron assembly, while the First Officer's control wheel drives the right aileron network. Under normal operations, a roll-coupling torque tube links these systems together.
  • Roll Unjamming Mechanics: If a mechanical component or pulley jams in flight, applying high breakout force across the control wheels splits the torque tube connection. This isolates the jammed side and allows the surviving pilot wheel to maintain control of the aircraft's roll axes using either a single remaining functional aileron or the spoiler circuit.
  • Hydraulic Flight Spoilers: Two spoiler panels are installed on the upper trailing edge of each wing. They are powered by the main hydraulic distribution networks (Left Spoilers link to the Blue System, Right Spoilers link to the Green System) and deploy automatically to assist roll authority whenever control wheel input exceeds 10 degrees of bank deflection.

Chapter J. FLIGHT CONTROLS (CONTINUED)

2.3. YAW CONTROL SYSTEM & MECHANICAL ARCHITECTURE (Page 111)

Directional control along the yaw axis is achieved via the all-moving vertical rudder surface, driven mechanically by individual pilot rudder pedal loops.

  YAW MECHANICAL CONTROL PATHWAY
  ┌────────────────────────────────────────────────────────┐
  │                 PILOT & CO-PILOT PEDALS                │
  └───────────────────────────┬────────────────────────────┘
                              │
                              ▼ [Rigid Rods & Cable Tension]
  ┌───────────────────────────┴────────────────────────────┐
  │           RUDDER TRAVEL LIMITER UNIT (RTLU)            │
  │     (Modulates mechanical arc tolerances via airspeed) │
  └───────────────────────────┬────────────────────────────┘
                              │
               ┌──────────────┴──────────────┐
               ▼                             ▼
  ┌────────────────────────┐    ┌────────────────────────┐
  │    SPRING TAB UNITS    │    │   MECHANICAL TRIM TAB  │
  │ (Aerodynamic Relief)   │    │ (Yaw Dimming/Offsetting)│
  └────────────────────────┘    └────────────────────────┘

2.4. Rudder Travel Limiter Unit (RTLU) Operational Law

To prevent catastrophic structural loads or torsion twisting on the vertical stabilizer tail fin assembly during high-speed configurations, the Rudder Travel Limiter Unit (RTLU) automatically restricts the maximum angular travel arc of the rudder based on current airspeed metrics.

  • Low Speed Regime: At airspeeds below $185\text{ kt}$, the RTLU expands its physical stops, permitting maximum rudder authority up to a full $27^\circ$ deflection boundary either side of the centerline. This ensures full aerodynamic directional control during engine-out configurations at takeoff or landing approach.
  • High Speed Regime: As airspeed accelerates up toward cruise limitations, the RTLU narrows its internal stop gates progressively, clamping down the physical limit until the rudder is restricted to a maximum $3.5^\circ$ deflection boundary.
  • System Alerts: If the RTLU internal electric actuator fails to transition its profile stops to match live Air Data Computer (ADC) inputs, an FLT CTL RTLU FAULT caution displays on the EWD. Pilots must then follow specific airspeed limits specified in the QRH to protect the vertical stab cell.

2.5. Mechanical Flight Control Gust Lock Mechanics

A mechanical cockpit gust lock assembly prevents external surface wind gusts from hammering and damaging the control rods, hinge pins, and balanced cables while the aircraft is parked on the ground.

  • Cockpit Control Lever: Located on the right-hand lateral side of the center pedestal console, accessible from the First Officer operating station.
  • Interlock Security Protocol: Moving the lock lever forward to the LOCKED position physically traps the mechanical input runs for the elevator and aileron quadrant bases. To prevent taking off with locked controls, the structural framework features a safety mechanical block: locking the control surfaces mechanically limits the movement of the primary Power Levers, preventing them from being advanced past the Flight Idle gate.

3. PRIMARY FLIGHT CONTROLS POSITION INDICATION & ASYMMETRY SYSTEMS (Page 112)

Flight control position monitoring is achieved via integrated synchro transmitters and rotary variable differential transformers (RVDTs) that feed surface tracking metrics back to the Integrated Modular Avionics (IMA) processors.

3.1. MFD Flight Control Display Page

Selecting the SYS / FLT CTL option key on the Electronic Flight Control Panel (EFCP) calls up the visual tracking schematic page layout on the Multi-Function Display (MFD).

  • Aileron and Spoiler Metrics: Displays digital angle markers and graphic bar paths confirming left and right spoiler deployment arcs.
  • Pitch and Yaw Scales: Shows real-time alignment marks for both the left and right elevators, alongside current rudder deflection angles.
  • Trim Tracking Ranges: Replicates mechanical trim tab positions, with explicitly highlighted green bands mapping out acceptable Takeoff (T.O) safety launch tolerance sectors.

3.2. Pitch Trim Asymmetry & Safety Runaway Protection

Because the horizontal stabilizer utilizes a split elevator design, the system must actively guard against structural asymmetry scenarios where the left and right control paths drift out of alignment.

  • Asymmetry Trigger Limits: If a mechanical component failure causes a deviation greater than a calibrated safety margin between the left and right elevator profiles, internal cross-check monitors detect the delta.
  • Automated Defensive Isolation: The Multi-Function Computers (MFC) immediately disconnect electrical power to the main pitch trim motor. This isolates the runaway drive lines to freeze the trim tabs in place, locks out automatic pitch commands from the autopilot, and generates a red flashing FLT CTL PITCH ASYM warning on the EWD screen.

4. TRIM SURFACES CONTROL MODES AND PEDESTAL INTERFACES (Page 113)

The aircraft trim network provides stabilization adjustments across three independent axes: Pitch, Roll, and Yaw.

  COCKPIT TRIM INPUT MODULES
  ┌────────────────────────────────────────────────────────┐
  │ YOKE DUAL PITCH SWITCHES ──► Main Electric Pitch Trim   │
  │                                                        │
  │ PEDESTAL TOGGLE SWITCH   ──► Standby Electric Backup   │
  │                                                        │
  │ PEDESTAL ROTARY WHEELS   ──► Roll and Yaw Axis Trim    │
  └────────────────────────────────────────────────────────┘

4.1. Axis Control Priority Routing

  1. Pitch Trim Control Channels:
  2. Normal Mode (Main Electric Trim): Driven via the split thumb-switches mounted on the outboard grip of each pilot control wheel yoke. Both sides of the split switch must be pushed simultaneously to complete the circuit. This safety design prevents an accidental continuous electrical short circuit from triggering a single-line trim runaway event.
  3. Standby Backup Mode: Controlled via a guarded toggle switch on the center pedestal console or overhead panel space. Actuating this switch routes electrical power directly to a secondary electric motor winding, bypassing normal automated control loops.

  4. Auto Trim Mode: Operated directly by the Flight Guidance Computers (FGC) when the Autopilot is active to counter aerodynamic changes without pilot intervention.

  5. Roll and Yaw Trim Configurations:

  6. Managed via independent manual rotary dials located on the lower pedestal face behind the power management quadrant. Turning the dials inputs direct mechanical biases into the aileron and rudder tab linkages.

5. FLAPS MECHANICAL CONTROL & ACTUATION HYDRAULICS (Page 114)

The trailing edge flap system utilizes hydraulically driven mechanical components to vary wing lift profiles during low-speed operational flight profiles.

5.1. System Hardware Layout

  • Central Hydraulic Actuator Motor: A dual-winding hydraulic motor component mounted inside the center wing fuselage crossover root. Powered directly by fluid pressure from the primary BLUE Hydraulic System.
  • Mechanical Torque Tubes: Rigid mechanical drive shafts extend symmetrically out from the central motor into both the left and right wing structures. These turn individual mechanical jackscrew assemblies to slide the flap panels smoothly along their structural roller tracks.
  • Monitoring Synchros: Symmetrical electronic feedback sensors are mounted at the far outer ends of both flap tracks to continuously transmit surface displacement data back to the processing units.

6. FLAPS SELECTION COCKPIT INTERFACE & MONITORING LOGIC (Page 115)

The flap system is managed via a notched control lever on the center pedestal.

  FLAPS SELECTOR DETENT PATHWAY
  ┌────────────────────────────────────────────────────────┐
  │        [ 0° ]  ──► Fully Retracted / Cruise Profile    │
  │          │                                             │
  │        [ 15° ] ──► Standard Takeoff / Approach Setting │
  │          │                                             │
  │        [ 30° ] ──► Maximum Drag / Full Landing Angle   │
  └────────────────────────────────────────────────────────┘

6.1. Flaps Asymmetry Protection and Structural Over-speed Guard

Because an unequal flap configuration causes severe rolling moments, the control loops apply strict monitoring rules via the Multi-Function Computers (MFC):

  • Asymmetry Lockdown Circuit: If the feedback synchros track a positional delta greater than $2^\circ$ of structural misalignment between the left and right flap wings, the system triggers protection logic:
  • An emergency hydraulic block valve closes immediately, isolating fluid pressure to freeze the drive assembly in its current position.
  • The system blocks any further input command attempts from the cockpit lever.

  • Generates an explicit amber flashing FLT CTL FLAPS ASYM caution line on the EWD screen.

  • Structural Flap Over-speed Protection: If the aircraft's airspeed accelerates past the structural limit velocity ($V_{FE}$) for the currently selected flap angle, an electronic warning alarm alerts the pilot to retract the lever immediately. This protects the flap attachment tracks from structural damage or bending.