M — Airframe (Cellule)Chapter 11 · 25 practice questions

Chapter 11: Flight Controls — Primary & Secondary

Includes 8 animated diagrams — view them live in the interactive theory reader.

Flight Controls — Primary & Secondary

Primary Flight Controls — Ailerons, Rudder, Elevator Primary Flight Controls — Ailerons, Rudder, Elevator Longitudinal Axis Vertical Axis AILERONS Axis: LONGITUDINAL Motion: ROLL ELEVATOR Axis: LATERAL Motion: PITCH RUDDER Axis: VERTICAL Motion: YAW SECONDARY CONTROLS • Flaps: Lift/Drag • Trim Tabs: Balancing • Slats: Anti-stall • Spoilers: Roll/Descent Ctrl MECHANICAL LINKAGE Cables & Pulleys or Push-pull Tubes

1. Overview

This chapter covers the theory, construction, operation, inspection, and maintenance of aircraft flight control systems. It encompasses both primary controls (ailerons, elevators, rudder) and secondary controls (trim tabs, flaps, stabilizers). The material focuses on the mechanical and hydraulic systems used to transmit pilot inputs to control surfaces, common defects, troubleshooting procedures, and the regulatory framework governing their airworthiness. A key theme is the critical importance of precise rigging, correct cable tension, and the absolute prohibition of operating with damaged components.

Diagram — Flight Controls — Primary & Secondary Flight Controls — Primary & Secondary Systems Mechanical Linkage Hydraulic Assist Electrical Signal Component Secondary Control PILOT INPUTS Control Column / Pedals / Yoke Cable / Push-Pull PRIMARY CONTROLS Ailerons (Roll) Elevator (Pitch) Rudder (Yaw) HYDRAULIC ACTUATORS Servo Valves Linear Actuators Hydraulic Power Pack SECONDARY CONTROLS Trim Tabs Flaps / Slats ⚠ CRITICAL Cable tension & rigging must be within TC limits REGULATORY CAR 625 / AWM 571 Airworthiness directives

2. Key Concepts Explained in Detail

2.1 Control System Types and Components

Flight control systems are categorized by the method used to transmit pilot commands to the control surfaces.

  • Mechanical Systems: These use a combination of cables, pulleys, push-pull tubes, bellcranks, and linkages. They are common on light aircraft and are characterized by direct pilot feel.
Flight control transmission system types Types of Flight Control Systems Mechanical vs hydraulically assisted vs fly-by-wire Mechanical System Cables and pulleys Stainless steel / synthetic fiber cables Critical tension: too low = slack, too high = fatigue and failure Push-pull tubes Rigid links — no deformation tolerated (stress concentration) Light and medium aircraft Inspection: broken wires, corrosion, wear. Tensiometer mandatory. Feel: "spongy" if tension low, "stiff" if corrosion / pulley misalignment, "heavy" if excessive friction Hydraulically Assisted System Mechanical control + assistance Valve Actuator return Hydraulic pressure = assistance Hydraulic failure Insufficient pressure → controls "heavy" (increased effort) Internal PCU leak → limited travel without visible external leak PCU — Power Control Unit Distribution spool + actuator Diagnosis: wheel at stop, surface not Inspection: check for external leaks, pressure verification, functional tests Fly-by-wire (fully powered) Electrical signal → computer → PCU Signal Computer PCU Hydraulic or electric actuators Redundancy and fault management software Advantages • Weight reduction (no cables) • Flight envelope protected by software • Built-in self-test and diagnostics Failure management Multiple redundancy (triple/quadruple) Progressive degradation in case of failure Maintenance: software test, verification of sensors and actuators evolution evolution Chapter 11 — Flight Controls | AME Training — Transport Canada | Standard 571.02 / 571.03 / 571.06
  • Cables and Pulleys: Cables transmit tension loads around corners via pulleys. Correct cable tension is vital. Over-tensioning causes high stress and premature fatigue failure. Under-tensioning (slack) leads to a "spongy" feel, control lag, and potential for the cable to jump off pulleys.
  • Push-Pull Tubes: These are rigid rods that transmit both tension and compression. They must be free of bends, cracks, and corrosion. Any bend, even if slight, can create stress risers and is only acceptable if explicitly allowed by the manufacturer's maintenance manual.
  • Hydraulically Boosted Systems: In larger aircraft, hydraulic power assists the pilot's mechanical input. The pilot moves a control valve, which directs hydraulic fluid to an actuator. Low hydraulic system pressure is the most common cause of a "heavy" control feel, as the boost is reduced.
  • Fully Powered (Fly-by-Wire) Systems: The pilot's control input is converted into an electrical signal sent to a computer, which then commands a hydraulic Power Control Unit (PCU). The PCU contains a control valve and an actuator. An internal leak across the control valve spool can prevent the actuator from achieving full travel, even with full pilot input, resulting in limited surface deflection.
PCU internal leak and limited surface travel PCU Internal Leak and Limited Travel Power-assisted hydraulic system — Animated PCU cutaway Power Unit (PCU) PISTON rod A B DISTRIBUTION SPOOL INTERNAL LEAK Hydraulic pressure Fluid return Control Surface (flight control) FIXED STRUCTURE CONTROL SURFACE neutral position full travel expected (35°) 15° LIMITED TRAVEL (partial deflection) mechanical linkage Pilot Command — Cockpit STICK Full command Electrical signal: 100% command COMPARATOR Actual vs. commanded position position feedback (LVDT) LVDT Distribution spool Internal leak Control surface (actual position) Expected position Diagnosis: internal leak → limited travel with no external leak

2.2 Control Surface Deflection and Aerodynamic Effects

The relationship between control input and aircraft response is fundamental.

  • Ailerons: For a roll to the right, the control wheel is turned right. This causes the right aileron to move up (reducing lift) and the left aileron to move down (increasing lift). This differential movement is a critical functional check.
Aileron differential movement for roll Differential aileron movement in roll REAR VIEW (schematic) LEFT WING RIGHT WING FUSELAGE LEFT AILERON moving down lift ↑ RIGHT AILERON moving up lift ↓ ROLL TO THE RIGHT roll axis CONTROL — WHEEL TURNED RIGHT rotation Wheel turned to the right (clockwise as seen from cockpit) CRITICAL FUNCTIONAL TEST 1. Turn the wheel fully to the right 2. Verify: right aileron moving UP 3. Verify: left aileron moving DOWN ⚠ REVERSED SENSE = CRITICAL FAULT Crossed cables — immediate flight prohibition link
  • Elevator: Pulling the control column back moves the elevator up, generating a nose-up pitch moment.
  • Rudder: Pushing the left rudder pedal moves the rudder left, yawing the nose to the left.
  • Trim Tabs: These are small, hinged surfaces on the trailing edge of a primary control surface. They are used to aerodynamically balance the control forces. A trim tab deflected up will force the primary control surface down (e.g., elevator tab up trims the aircraft nose down). Incorrect rigging of a trim tab can cause a "heavy" control feel, as the pilot must fight the aerodynamic imbalance.
Trim tab deflection and control force direction Trim Tab Direction and Control Force Tab deflected UP STABILIZER ELEVATOR TAB Deflection upward Aero force Result: ELEVATOR DOWN STABILIZER ELEVATOR TAB Elevator down Lift ↑ Nose up → Nose down Tab deflected DOWN STABILIZER ELEVATOR TAB Deflection downward Aero force Result: ELEVATOR UP STABILIZER ELEVATOR TAB Elevator up Lift ↓ ! IMPROPER ADJUSTMENT — CONSEQUENCES • "Heavy" control force: an improperly adjusted tab increases the force required by the pilot. • Risk of FLUTTER (aerodynamic oscillation): self-sustaining oscillation that can destroy the structure. • Check adjustment per AMM — any deviation requires correction before flight (CARs 571.02).

2.3 Critical Defects and Airworthiness

Safety is paramount. Any defect in a flight control system renders the component unairworthy and must be corrected before flight.

  • Cracked Components: A crack in any flight control component (pulley, bellcrank, bracket, control column) is a safety hazard. The part must be replaced. Grease or any temporary repair is not an approved remedy.
  • Cable Damage:
  • Broken Wires: Control cables must be inspected for broken wires. Manufacturer's limits (e.g., more than 2 broken wires in one strand within an inspection window) are mandatory. Exceeding these limits requires immediate cable replacement. Splicing control cables is not an approved repair.
  • Corrosion: Moisture ingress can cause internal corrosion, leading to stiffness and potential failure. Inspection for corrosion and checking pulley alignment are key troubleshooting steps.
  • Jackscrew Galling: In horizontal stabilizer trim systems, the jackscrew is a critical component. Galling (adhesive wear) on the threads is a serious defect that mandates replacement. No temporary repairs are permitted.
  • Bent Push-Pull Tubes: If the manufacturer's manual prohibits bends, the tube must be replaced. Straightening is not an approved repair.
  • Misaligned Balance Tabs: A misaligned balance tab can induce catastrophic aerodynamic flutter. Precise rigging is essential.

3. Important Procedures and Regulations

3.1 Regulatory Framework (CARs Standard 571)

All maintenance actions must comply with the applicable regulations. The key principles are:

  • Manufacturer's Instructions (571.02): Maintenance must be performed in accordance with the aircraft maintenance manual (AMM) and other manufacturer-approved data. This is the primary authority for all procedures, limits, and rigging specifications.
  • Approved Data for Repairs and Modifications (571.06): Any repair or modification to a flight control system requires approved data (e.g., a Supplemental Type Certificate (STC) or a repair design approval). An AME cannot use generic data like AC 43.13-1B if the manufacturer has provided specific instructions or if the repair is outside the scope of standard practices.
  • Recording Defects (571.03): All defects, discrepancies, and maintenance actions must be properly recorded in the aircraft's technical records (e.g., journey log). A defect must be corrected before the aircraft is released for flight.

3.2 Rigging Procedures

Flight control rigging sequence Control Rigging Order STEP 1 Set cockpit control to neutral Rigging pin P STEP 2 Adjust cables and turnbuckles with tension meter Tension meter STEP 3 Bring control surface to neutral following the cables Surface neutral STEP 4 Check full travel and direction of movement Functional test ⚠ WHY THE ORDER IS IRREVERSIBLE If you adjust the control surface first, then the cockpit control, the surface neutral will not match the control neutral. Rigging must ALWAYS start with the cockpit control. Cable Diagram — Ailerons Wheel TURNBUCKLE Aileron R Aileron L Direction of Movement — Roll to the Right ↑ Goes up ↓ Goes down Roll to the right Wheel turned right → Right aileron up Left aileron down ✓ Check full travel and symmetry

Rigging is a systematic process to ensure correct control travel and feel.

  1. Start from the Cockpit: Always begin rigging from the pilot's control (e.g., control wheel, rudder pedals, trim wheel). Set the control to its neutral position, often using a rig pin.
  2. Adjust the Transmission System: Adjust turnbuckles to achieve the correct cable tension as specified in the AMM. Cable tension is measured with a tensiometer.
  3. Set the Control Surface: With the cockpit control in neutral, adjust the system so the control surface is also in its neutral position.
  4. Verify Travel: After rigging, perform a full functional test to verify correct direction of movement, full travel in both directions, and smooth operation without binding.

3.3 Troubleshooting Logic

Abnormal control feel troubleshooting tree Decision Tree: Abnormal Control Efforts SYMPTOM Heavy control (excessive effort) Spongy control (elastic feel) No response (free wheel, fixed surface) Limited travel (stop reached, partial surface) Control reversal (opposite direction) PROBABLE CAUSES / DIAGNOSIS Mechanical binding (cables, pulleys) Insufficient hydraulic pressure Excessive cable tension Improperly adjusted compensator Cable tension too low Slack cable (stretched) Play in pulleys Loose connection at station Broken cable Faulty cable attachment Mechanical stop reached Internal leak in PCU (piston bypass) Crossed cables (reversal) Incorrect compensator adjustment Reversed connection at surface CORRECTIVE ACTIONS 1. Lubricate and check alignment of pulleys and cables 2. Check hydraulic pressure (pump, accumulator, leaks) 3. Adjust cable tension 1. Measure tension with tensiometer (must be within limits) 2. Adjust turnbuckles 1. Inspect connection at wheel (chain, attachment, pins) 2. Replace broken cable 1. Check stop travel (compare to manual) 2. Test PCU for internal leak 1. Check cable routing (crossing = major criticality) 2. Complete re-rigging

Systematic troubleshooting is essential for efficient maintenance.

  • "Heavy" Control Feel:
  1. Check for mechanical binding (cables, pulleys, hinges).
  2. Check hydraulic system pressure (for boosted systems).
  3. Check cable tension (too high or too low).
  4. Check trim tab rigging (aerodynamic imbalance).
  • "Spongy" Control Feel: Most likely caused by low cable tension or cable stretch.
  • No Control Response with Free Input:
  1. Check the connection at the cockpit control (e.g., chain, cable attachment). This is the most logical first step.
  2. Check for a broken cable or a disconnected linkage.
  • Limited Surface Travel:
  1. Check for mechanical binding or a physical stop.
  2. Check for internal leaks in the PCU (in powered systems).
  3. Check for incorrect cable tension causing slack.
  • Slow or Intermittent Operation (e.g., Flaps):
  1. Check for mechanical binding (e.g., flap tracks, jamming). This is the most likely cause, especially if the motor is drawing high current and running hot.
  2. Check the electrical system (motor, wiring, limit switches).
  • Reversed Control Movement: This is a critical safety issue. The most likely cause is crossed or incorrectly rigged cables.

4. Common Relationships Between Concepts

  • Cable Tension and Control Feel: Correct cable tension is the single most important factor for proper control feel. Over-tension leads to high stress and fatigue. Under-tension leads to a spongy feel, lag, and potential for the cable to jump pulleys.
  • Hydraulic Pressure and Control Force: In boosted systems, low hydraulic pressure directly translates to higher pilot control forces (a "heavy" feel).
  • Trim Tab Rigging and Control Force: An incorrectly rigged trim tab creates an aerodynamic imbalance that the pilot must constantly overcome, resulting in a heavy control feel, even if the mechanical system is free and correctly tensioned.
  • Internal Leaks and Surface Travel: In a fully powered system, an internal leak across the PCU control valve spool can prevent the actuator from reaching its full stroke, limiting surface deflection even with full pilot input.
  • Damage Limits and Airworthiness: The manufacturer's manual defines the boundary between a serviceable and an unairworthy condition. A defect within the allowable limit is serviceable. Any defect exceeding the limit, or any defect in a component where the manual prohibits damage (e.g., a cracked pulley), renders the part unairworthy and requires replacement.
  • Rigging Sequence and Neutral Position: The rigging process must start from the cockpit control. If the control surface is set to neutral first, the cockpit control may not be centered, leading to incorrect travel and feel. The entire system must be re-rigged from the beginning.

Diagram

Practice this chapter

Reinforce Flight Controls — Primary & Secondary with 25 Transport Canada–style practice questions, matched to your weak areas.

Try Free Practice QuestionsView Pricing