M — Airframe (Cellule)Chapter 3 · 24 practice questions

Chapter 3: Aircraft Assembly & Rigging

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Chapter: Aircraft Assembly & Rigging

Overview

This chapter covers the fundamental principles and procedures for assembling aircraft structures and rigging flight control systems. Aircraft assembly involves the correct installation of structural components, fasteners, and control system elements, while rigging refers to the adjustment and alignment of flight controls to achieve specified travel, neutral positions, and system harmony. Proper assembly and rigging are critical for aircraft safety, handling characteristics, and structural integrity. The material addresses fastener selection and installation, control cable systems, control surface adjustment, and the troubleshooting of common rigging discrepancies.

Diagram — Aircraft Assembly & Rigging Aircraft Assembly & Rigging — Control Cable System Schematic LEGEND Cable (tension) Turnbuckle Pulley Cable terminal Control Cable System — Elevator Rigging (Typical) FUSELAGE STRUCTURE Control Column P1 P2 P3 P4 TB Turnbuckle ELEVATOR Hinge UP DOWN CABLE TENSION 50-70 lbf (222-311 N) CONTROL TRAVEL 25° UP / 15° DOWN Neutral Forward cable Aft cable Rigging Procedure Notes 1. Set control column to neutral position (rigging pin installed) 2. Adjust turnbuckle to achieve specified cable tension 3. Verify elevator travel angles with protractor 4. Check for system harmony and freedom of movement PULL PULL

Key Concepts Explained in Detail

Control Cable Systems

Control Cable Rigging Control Cable Rigging COCKPIT Stick / Wheel Output Drum LINKAGE & INSPECTION TENSIOMETER Measure at mid-point of longest free span INSPECTION POINTS: • Pulleys (wear/seizing) • Pins & Turnbuckles CABLE CRITERIA: • 0 broken wires allowed • Wear < 10% diameter CONTROL SURFACE FREE PLAY (Backlash) Check at hinges RIGGING PRINCIPLES & SAFETY TENSION & HARDWARE • Measure precisely away from pulleys (friction). • Turnbuckle: Max 3 threads visible after adj. • Cotter pin: One leg flat, other bent up over bolt head. • Self-locking nut: Min. 1 thread exposed. Max 3 threads exposed TRAVEL & GEOMETRY • Pedal free play ≠ Surface free play. (Linkage vs Hinge Bearings) • Differential aileron travel: Set by clevis/lever geometry. • Excessive sag = Loss of authority. Up (e.g. 20°) Down (e.g. 15°) CRITICAL LIMITS MANDATORY REPLACEMENT IF: • Any broken wire (regardless of count). • Wear > 10% of nominal diameter. • Nicopress sleeve deformed. • Swaged terminal cracked. • Cable/structure clearance < 1/8".

Control cables transmit pilot inputs from the cockpit to flight control surfaces. They consist of wire rope, terminals (swaged or Nicopress sleeves), and hardware such as turnbuckles, pulleys, fairleads, and swivel joints.

Cable Tension Measurement:

Cable tension measurement location Cable Tension Measurement Point PULLEY Fixed guide PULLEY Fixed guide Tension Tension MEASUREMENT POINT OPTIMAL Middle of the span unsupported TENSIOMETER MEASUREMENT TO AVOID Pulley friction → inaccurate reading MEASUREMENT TO AVOID Pulley friction → inaccurate reading Unsupported span (free of any contact) Optimal measurement point (middle of span) Zones to avoid (pulley friction) Control cable The most accurate tensiometer reading is obtained at the middle of the longest unsupported span, free from friction effects.

The most accurate tension reading is obtained at the midpoint of the longest unsupported span. At this location, the cable is free from friction effects caused by pulleys, fairleads, or other contact points. Measurements taken near pulleys or fittings can be affected by friction, leading to inaccurate readings.

Cable Identification:

Control cables must be identified with a metal tag attached to the cable, as per manufacturer's instructions. Markers can wear off over time, etching can damage the cable, and length alone is not sufficient for identification.

Minimum Clearance:

Per AC 43.13-1B, a minimum clearance of 1/8 inch must be maintained between a control cable and any structure, fairlead, or other component. This prevents chafing, which can lead to cable wear and eventual failure.

Cable Wear and Damage:

  • Any broken wires in a control cable require immediate replacement. AC 43.13-1B states that a cable must be replaced if it has any broken wires. Splicing is not permitted on control cables.
  • Any measurable wear exceeding 10% of the cable diameter requires replacement. 25% wear is significant and could lead to failure. Replacing the fairlead alone does not address the worn cable, and lubrication is not a repair.

Swaged Terminals and Nicopress Sleeves:

  • A deformed Nicopress sleeve indicates improper swaging and must be replaced. Pull testing may not reveal all defects, and tape is not an acceptable repair.
  • A cracked swaged terminal is a critical defect. The cable assembly must be replaced because the swage is a one-time process and cannot be repaired. Sealant, re-swaging, or stop-drilling are not acceptable.

Turnbuckle Adjustment:

Turnbuckle thread engagement check Turnbuckle Adjustment: Visible Threads Enlarged view of the turnbuckle — correct assembly 3 threads max visible RULE: A maximum of 3 threads must be visible between the barrel and the terminal after final adjustment — ensures sufficient thread engagement. Comparative view — thread engagement CORRECT Threads engaged along the full length of the barrel — 3 visible threads max Full engagement guaranteed INCORRECT Too many visible threads — insufficient thread engagement in the barrel Risk of deformation under load too many M-AIRFRAME ch3 — Aircraft Assembly and Rigging — Transport Canada

After final adjustment, no more than three threads should be visible between the barrel and the terminal. This ensures sufficient thread engagement. Fewer than three threads indicates possible over-tightening or insufficient engagement.

Swivel Joints:

Swivel joints (e.g., rod ends) allow for angular misalignment between the control rod and the bellcrank or horn, preventing binding. Rod length adjustment is done via threaded rod ends. Friction reduction is a secondary benefit, and fail-safe is not a primary purpose.

Fasteners and Structural Assembly

Bolt Selection and Installation:

Bolt grip length and thread engagement Bolt Reach and Thread Engagement BOLT TOO LONG — WITH WASHERS INCORRECT Method Shimming washers Head Correct reach Nut Excessive length (interference risk) ⚠ Shimming washers modify the reach and can cause inadequate tightening or stress concentration. CORRECT ASSEMBLY — SELF-LOCKING NUT CORRECT Method Head Correct reach Self- locking nut nylon insert At least ONE full thread Full thread engagement TORQUE — AN3 BOLT 1/4 inch + self-locking nut 50 to 70 in-lb Use a calibrated torque wrench ✗ Never cut a bolt to fit — risk of damaging threads and creating stress concentrators. BOLT ORIENTATION • Head down on wing spar: prevents loosening from vibration • Nut down: avoids accumulation of oil or water
  • Bolts must be the correct grip length. Using washers to compensate for a long bolt changes the grip length and can lead to improper clamping. Cutting bolts is not recommended as it can damage threads and cause stress risers. Thinner nuts are not standard.
  • For self-locking nuts, the bolt must extend at least one full thread beyond the nut to ensure full thread engagement. Thread-locking compound is not needed, and backing off is incorrect.
  • Bolts are typically installed with the head on the side that sees the most vibration to prevent the nut from backing off. On a wing spar, heads are often placed on the bottom to prevent the nut from loosening due to downward vibration during flight.

Torque Values:

  • Per AC 43.13-1B, the recommended torque for a 1/4-inch AN3 bolt with a self-locking nut is 50-70 in-lb. Lower torques may not provide adequate preload, while higher torques could overstress the bolt or nut.
  • When using a crowfoot adapter at 90 degrees to the torque wrench handle, the effective length of the wrench is not changed, so the torque reading is accurate. If the adapter is in line with the handle, the reading must be corrected.

Cotter Pins:

Cotter pin installation pattern Correct installation of the cotter pin Castle nut — Mechanical locking method Overview — Locked assembly T Bolt Castle nut Cotter pin Cotter pin (yellow) Castle nut Bolt IMPORTANT: The cotter pin adds NO clamping force. Bending procedure — Side view 1 Insert the cotter pin Pass the cotter pin through the slots of the nut and the hole in the bolt. 2 Bend the lower branch Bend the lower branch over the flat of the nut, flat against the surface. 3 Bend the upper branch Bend the upper branch upward, against the end of the bolt. The cotter pin prevents nut rotation caused by vibrations — without changing the tightening torque. Vibrations
  • The cotter pin mechanically locks the nut in place, preventing rotation due to vibration. It does not add clamping force (that is from torque).
  • One prong of the cotter pin should be bent over the flat of the nut, and the other prong bent up over the end of the bolt. This prevents the pin from rotating and ensures security.

Flight Control Rigging

Control Surface Travel and Neutral Position:

  • Excessive aileron droop reduces the effective travel range, decreasing roll authority. This must be corrected.
  • If the elevator neutral position is 2 degrees trailing edge up from the stabilizer (when specification calls for 0 ± 0.5 degrees), the trailing edge up elevator creates a downward aerodynamic force on the tail, causing a nose-up pitch.

Differential Travel:

Aileron differential travel via bellcrank geometry Differential aileron deflection Healthy geometry — asymmetric deflection neutral +20° -15° 20° 15° pivot Red rod = upward · Blue rod = downward Misadjusted rod — symmetry destroyed ✗ neutral +12° -23° 12° 23° pivot Rod too short — reduced up travel, excessive down travel Why differential deflection? Aerodynamic effect The upward-deflected aileron creates less drag than the downward-deflected one. Asymmetric deflection (more up than down) reduces undesirable adverse yaw. Yoke/lever geometry The rod attachment point is offset from the lever axis. This offset naturally creates unequal up/down travel ranges. Incorrect adjustment A rod that is too long or too short shifts the neutral point and alters the stops. Result: reduced roll authority on one side, risk of adverse yaw. Typical standard: 20° up, 15° down — check with a protractor on the control surface, rods disconnected

Differential travel is often built into the control system via the geometry of the bellcrank or control horn. If the travel is not symmetric (e.g., 20 degrees up and 15 degrees down when specification calls for 20 degrees up and 20 degrees down), the rigging of the control rod (pushrod) is likely incorrect, affecting the neutral position and travel. Cable tension affects both directions equally, and worn hinges cause free play, not differential travel.

Control System Free Play:

Free play at the pedals that is not transmitted to the rudder indicates slack in the control system, likely at a connection (e.g., bolt hole wear, loose clevis pin). Worn hinge bearings would cause free play in the rudder itself. Excessive cable tension would reduce free play, and worn cable would cause slack but also affect tension.

Control System Binding:

Binding in moving parts (pulleys, bellcranks) directly increases control forces. Incorrect cable tension (too high) can also increase force, but binding is more likely to cause a significant increase. Misaligned rods cause binding, while worn hinges reduce friction, not increase it.

Trim Tabs:

Trim tab deflection and control column force Effect of trim tab position on control forces Trim tab deflected UPWARD T.T. Airflow Downward force PUSH Stick AERODYNAMIC RESULT • The tab deflected upward creates a downward force on the elevator. • The pilot must push on the stick to counteract. Trim tab deflected DOWNWARD T.T. Airflow Upward force PULL Stick AERODYNAMIC RESULT • The tab deflected downward creates an upward force on the elevator. • The pilot must pull on the stick to counteract.

A balance tab moves opposite to the aileron, creating an aerodynamic force that assists the pilot in moving the aileron, thereby reducing control forces. It does not directly increase roll rate, improve balance (though it helps), or provide trim.

If the trim tab is deflected upward, it creates a downward force on the elevator, requiring forward pressure (push) to counteract. The opposite would be true for a downward tab.

Important Formulas, Regulations, and Procedures

Regulatory References:

  • AC 43.13-1B: Acceptable Methods, Techniques, and Practices for Aircraft Inspection and Repair. This document provides guidance on cable tension, clearance, torque values, and turnbuckle adjustment.

Key Procedures:

  • Cable Tension Check: Measure at the midpoint of the longest unsupported span.
  • Turnbuckle Adjustment: Ensure no more than three threads are visible between the barrel and the terminal.
  • Cotter Pin Installation: Bend one prong over the flat of the nut and the other prong up over the end of the bolt.
  • Bolt Installation: Ensure correct grip length and at least one full thread beyond self-locking nuts.

Critical Torque Values:

  • 1/4-inch AN3 bolt with self-locking nut: 50-70 in-lb.

Common Relationships Between Concepts

  • Cable Tension and Control Forces: Incorrect cable tension (too high) can increase control forces, but binding in moving parts is a more likely cause of significant increases.
  • Cable Wear and Clearance: Insufficient clearance (less than 1/8 inch) leads to chafing and cable wear, which requires replacement if wear exceeds 10% of diameter.
  • Swaged Terminal Defects: Deformation or cracking indicates improper swaging and requires replacement, as the swage is a one-time process.
  • Trim Tab Position and Control Column Force: Upward deflection of the trim tab requires forward pressure on the control column, while downward deflection requires aft pressure.
  • Differential Travel and Pushrod Rigging: Asymmetric travel is often caused by incorrect pushrod length, affecting the neutral position and travel range.
  • Free Play and System Slack: Free play at the cockpit controls without corresponding movement at the control surface indicates slack in connections, not at the surface hinges.

Diagram

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