Chapter: Flight Controls

Overview

This chapter covers the inspection, maintenance, rigging, and troubleshooting of aircraft flight control systems. It addresses the critical safety requirements for primary and secondary flight controls, including ailerons, elevators, rudders, and trim systems. The material emphasizes the regulatory framework governing flight control maintenance, the identification of defects, proper repair procedures, and the systematic approach to diagnosing control system discrepancies.

Key Concepts

1. Control Cable Inspection and Replacement Criteria

Control cables are the backbone of mechanical flight control systems. Their integrity is critical to safe operation, and specific inspection criteria determine airworthiness.

Broken Wire Criteria

Per AC 43.13-1B, control cables must be replaced when any of the following conditions exist:

Any broken wires within the vicinity of a terminal, fitting, or swage — regardless of the number, this condition is immediately unairworthy

More than 3 broken wires in any 12-inch length of cable run

Frayed strands anywhere along the cable length

Corrosion with pitting — corrosion reduces the cable's cross-sectional area and creates stress risers that can lead to catastrophic failure

Critical Considerations

Splicing is never an approved repair for primary flight control cables

Soldering is prohibited because it creates stress risers and can mask internal damage

Cleaning and applying corrosion inhibitor is not acceptable for cables with pitting — the cable must be replaced

Broken wires near pulleys or fairleads indicate possible routing problems, chafing, or pulley damage that must also be investigated

Inspection Focus Areas

Pay particular attention to:

Points where cables pass over pulleys

Areas near turnbuckles and terminals

Sections passing through fairleads

Any location where the cable changes direction

2. Cable Tension and Rigging

Proper cable tension is essential for correct flight control operation. Both excessive and insufficient tension create distinct problems.

Low Cable Tension Effects

Spongy or mushy control feel — the pilot perceives a soft, non-positive response

Excessive free play in the control wheel or pedals before surface movement

Control surface flutter — the most dangerous consequence; flutter is a violent, destructive oscillation that can cause structural failure

Inconsistent control response during maneuvering

High Cable Tension Effects

Stiff or heavy control forces — the pilot must exert excessive effort to move the controls

Restricted control surface travel — the surface may not reach its full deflection limits

Premature wear on pulleys, fairleads, and cable attachments

Binding in the control system

Tension Adjustment Procedure

40.Position all flight controls in the neutral position

41.Reference the manufacturer's maintenance manual for the specified tension range

42.Adjust tension using turnbuckles

43.Account for ambient temperature — cable tension specifications are temperature-compensated

44.Safety all turnbuckles after adjustment

45.Verify full range of motion of all control surfaces

46.Confirm cockpit controls are correctly positioned

Turnbuckle Safety

Turnbuckles must be properly safetied after any tension adjustment

Loose turnbuckles are a common cause of spongy controls and excessive free play

Safety wire must be installed correctly to prevent turnbuckle rotation

3. Control System Free Play

Free play is the movement of a cockpit control before the corresponding control surface begins to move. Excessive free play is a serious safety issue.

Causes of Excessive Free Play

Incorrect cable tension — either too loose or improperly adjusted

Worn cable system components — pulleys, fairleads, and attachment points

Worn bellcrank bearings or bushings

Elongated bolt holes in control system components

Worn hinge points on control surfaces

Loose turnbuckles

Troubleshooting Approach

When excessive free play is detected:

64.Check cable tension first — this isolates cable stretch/slack from worn components

65.Inspect the entire system — cables, pulleys, bellcranks, and attachment points

66.Identify the specific source of play before making adjustments

67.Correct the root cause — do not mask the problem with temporary fixes

Consequences of Uncorrected Free Play

Control surface flutter

Reduced control authority

Autopilot hunting or oscillation (the autopilot servos command movement, but slack in the system causes delayed feedback and overshooting)

Pilot perception of sloppy or imprecise aircraft handling

4. Control System Rigging

Rigging is the process of adjusting the control system so that cockpit controls and control surfaces are properly synchronized.

Neutral Position Alignment

With cockpit controls in neutral, control surfaces must be in their neutral position

A rudder deflected with pedals neutral indicates improper cable tension or adjustment

An aileron that moves before the control wheel moves indicates free play

Travel Limits

Control stops must be adjusted to ensure symmetrical travel

Example: If the rudder travels +25° right but only -22° left, the control stops are incorrectly adjusted

Asymmetric control wheel travel in one direction versus the other is typically caused by improperly adjusted control stops

Control stops prevent overtravel and must be set precisely per the maintenance manual

Direction of Movement

Control surfaces must move in the correct direction relative to cockpit control input

Reversed controls occur when cables are crossed at a bellcrank or quadrant

A reversed control is a critical flight safety hazard

A functional test is required after any rigging work to verify correct direction of movement

Post-Rigging Functional Test

After any rigging procedure:

92.Move cockpit controls through their full range

93.Verify control surfaces move in the correct direction

94.Confirm full travel in both directions

95.Check that neutral positions align

96.Verify control forces are within acceptable limits

5. Control Surface Inspection and Damage Assessment

Structural Damage Assessment

When damage is found on a control surface:

101.Determine if the damage is within the manufacturer's allowable damage limits — this is always the first step

102.If within limits, the aircraft may be returned to service, but the finding should be recorded in the logbook for traceability

103.If beyond limits, repair or replacement is required

104.If no repair procedure exists in the maintenance manual, obtain an approved repair method from the manufacturer

Damage Categories

Cracks — any crack in a flight control surface is a structural defect requiring repair before return to service

Dents — may be cosmetic or structural; assess against allowable damage limits

Corrosion — reduces structural integrity and must be evaluated

Fabric damage — even if the fabric is intact, a dent may affect structural integrity or control surface balance

Repair Documentation

Damage within allowable limits must be recorded in the logbook

Repairs must use approved methods (manufacturer's structural repair manual or AC 43.13-1B Chapter 2)

Ad-hoc repairs are never acceptable

Replacement is only necessary if the manufacturer determines damage is beyond repair

6. Component Wear and Defects

Bellcranks

A cracked bellcrank in a flight control system is a major defect

Without an approved repair procedure, the only safe and legal action is replacement

Welding a cracked bellcrank without an approved procedure is prohibited

Hinges and Bushings

Worn hinge bushings cause excessive play at the control surface

Replacement of worn bushings is the appropriate repair to restore designed tolerance

Tightening bolts or adding washers is not an approved repair for hinge wear

Excessive play at a hinge must be corrected before return to service

Balance Weights

A loose balance weight must be tightened to the manufacturer's torque specification

Removal of balance weights renders the aircraft unairworthy

Proper securing methods include cotter pins or other approved locking devices

Trim Tab Actuators

Excessive wear in a trim tab actuator that fails to hold position requires replacement

Installing used or repaired parts without proper documentation is not acceptable

Safety wiring a defective tab in neutral is not an approved temporary repair

Cotter Pins and Castle Nuts

A missing cotter pin in a flight control system is a serious safety issue

Replace the castle nut with a new one, torque to specification, and install a new cotter pin

Never reuse cotter pins

Thread-locking compound is not a substitute for mechanical locking in flight control systems

7. Troubleshooting Control System Problems

Symptom-Based Diagnosis

Symptom	Likely Cause	Investigation Priority
Spongy controls	Low cable tension or air in hydraulic boost system	Check cable tension; check hydraulic system for air
Stiff/heavy controls	Excessive cable tension or binding	Check cable tension; perform free play check
Excessive free play	Low cable tension or worn components	Check cable tension first, then inspect components
Control surface not responding	Broken or disconnected cable	Inspect control path continuity
Reversed control movement	Crossed cables at bellcrank/quadrant	Verify cable routing
Asymmetric travel	Incorrectly adjusted control stops	Adjust control stops
Trim tab buzzing	Worn hinge	Replace hinge
Autopilot hunting	Excessive free play in control system	Check rigging and cable tension

Systematic Troubleshooting Approach

142.Gather information — pilot reports, inspection findings, maintenance history

143.Perform initial checks — cable tension, free play, visual inspection

144.Isolate the system — determine if the problem is in the cockpit controls, cable system, or control surface

145.Identify root cause — do not treat symptoms without finding the underlying issue

146.Correct the defect — use approved methods and parts

147.Verify the repair — functional test the system

8. Regulatory Requirements

14 CFR 43.13(a) — Performance Standards

Maintenance must be performed using methods that ensure the aircraft is in a condition for safe operation

All repairs must use methods and parts acceptable to the FAA

14 CFR 43.15 — 100-Hour Inspection Requirements

All discrepancies found during a 100-hour inspection must be corrected before the aircraft is returned to service

This includes flight control system defects

14 CFR 43.9 — Maintenance Record Entries

Required elements of a maintenance record entry:

Description of the work performed

Date of completion

Signature and certificate number of the person approving the aircraft for return to service

14 CFR 91.7 — Civil Aircraft Airworthiness

No person may operate a civil aircraft unless it is in an airworthy condition

The pilot-in-command is responsible for determining airworthiness

AC 43.13-1B — Acceptable Methods, Techniques, and Practices

Provides acceptable methods for aircraft inspection and repair

Chapter 2 covers aircraft structures

Chapter 5 covers flight controls

Chapter 7 covers cable operations and systems

Important Procedures

Control Cable Replacement Procedure

171.Remove the old cable, noting the routing path

172.Inspect all pulleys, fairleads, and attachment points for wear or damage

173.Install the new cable following the exact routing path

174.Rig the system to the manufacturer's specifications

175.Adjust cable tension to the specified range for ambient temperature

176.Safety all turnbuckles

177.Verify full range of motion of control surfaces

178.Confirm cockpit controls are correctly positioned

179.Perform a functional test to verify correct direction of movement

180.Document the work in the maintenance record

Control System Free Play Check

182.Position the cockpit control in neutral

183.Apply light pressure in one direction

184.Measure the movement of the cockpit control before the control surface begins to move

185.Repeat in the opposite direction

186.Compare measurements to manufacturer's specifications

187.If excessive, check cable tension first, then inspect system components

Damage Assessment Procedure

189.Document the damage (location, size, type)

190.Consult the manufacturer's maintenance manual for allowable damage limits

191.If within limits — record the finding and return to service

192.If beyond limits — determine if a repair procedure exists

193.If a repair procedure exists — perform the repair using approved methods

194.If no repair procedure exists — obtain manufacturer approval for a repair method or replace the component

Common Relationships Between Concepts

Cable Tension and Control Feel

Low tension → spongy feel, free play, potential flutter

High tension → stiff feel, restricted travel, premature wear

Correct tension → positive, responsive control feel

Free Play and System Wear

Free play is often the first indicator of system wear

Cable tension should be verified before condemning components

Worn components (bearings, bushings, bellcranks) contribute to free play even with correct cable tension

Rigging and Control Travel

Correct rigging ensures symmetrical travel in both directions

Control stops determine maximum travel limits

Cable tension affects both control feel and travel

Hydraulic Systems and Control Feel

Air in hydraulic boost systems causes spongy feel

Hydraulic leaks require repair and functional testing

Hydraulic restrictions cause sluggish response or hardover conditions

Autopilot Interaction with Mechanical Systems

Autopilot hunting often indicates mechanical free play

The autopilot system itself may be functioning correctly

Always verify mechanical control system rigging before condemning autopilot components

Documentation and Airworthiness

All maintenance must be properly documented

Damage within limits must be recorded for traceability

Return to service requires compliance with 14 CFR 43.9

Proper documentation ensures the chain of airworthiness is maintained

Summary

Flight control maintenance requires a systematic approach that combines thorough inspection, accurate diagnosis, and proper repair procedures. The key principles are:

225.Control cables must be replaced when any broken wires, fraying, or corrosion pitting is found

226.Cable tension must be within manufacturer's specifications — both excessive and insufficient tension create safety hazards

227.Free play must be investigated systematically — check cable tension first, then inspect components

228.Rigging must ensure correct direction, neutral alignment, and symmetrical travel

229.Structural damage must be assessed against allowable limits before any repair decision

230.All discrepancies must be corrected before return to service

231.Proper documentation is mandatory for all maintenance actions

Adherence to these principles ensures flight control systems operate safely and reliably, meeting both regulatory requirements and the highest standards of aviation safety.

Flight Controls