FAA Airframe Written TestChapter 5 · 40 practice questions

Chapter 5: Landing Gear Systems

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Landing Gear Systems

Overview

The landing gear system is one of the most critical airframe systems on any aircraft. It supports the aircraft's weight during ground operations, absorbs landing and taxi loads, and—on retractable gear aircraft—provides a means to reduce aerodynamic drag during flight. This chapter covers the fundamental principles, components, maintenance practices, and troubleshooting procedures associated with landing gear systems, including shock struts, wheels, tires, brakes, retraction mechanisms, and position indicating systems. A thorough understanding of these systems is essential for the aircraft maintenance engineer (AME) to ensure the safe operation of the aircraft on the ground and in the air.

Key Concepts

1. Landing Gear Configurations

Landing Gear Systems: Tricycle vs Conventional Gear Layout LANDING GEAR SYSTEMS — TRICYCLE vs CONVENTIONAL GEAR LAYOUT FAA A&P Exam Prep · AC 43.13-1B · 14 CFR Part 23 TRICYCLE GEAR (NOSE WHEEL) 14 CFR §23.2150 ▲ BRAKING LEVEL ATTITUDE • Nose wheel + 2 main wheels — stable triangle • Forward visibility on ground — unobstructed • Allows aggressive braking without nose-over • Preferred for high-performance & jet aircraft CONVENTIONAL GEAR (TAILWHEEL) 14 CFR §23.2150 PITCHING • 2 main wheels + tailwheel — taildragger • Forward visibility limited on ground • Braking must be careful — nose-over risk • Requires "S-turn" taxi technique GROUND HANDLING COMPARISON STABILITY HIGH STABILITY MODERATE BRAKING AGGRESSIVE BRAKING CAUTION FAA AC 43.13-1B Chapter 7 · Landing Gear Systems · A&P Airframe Oral & Practical

Aircraft landing gear is typically arranged in one of two primary configurations:

  • Tricycle Gear: This configuration features a nose wheel and two main wheels. It is the most common arrangement on modern aircraft due to its inherent stability during ground operations, improved forward visibility for the pilot, and its ability to facilitate more aggressive braking.
  • Conventional Gear (Tailwheel): This configuration features two main wheels and a tailwheel. While less common today, it is still found on some taildragger aircraft, particularly bush planes and vintage aircraft. It requires different ground handling techniques.

2. Shock Struts (Oleo Struts)

Shock Strut (Oleo Strut) Operation - Landing Gear Systems SHOCK STRUT (OLEO STRUT) OPERATION FAA A&P Prep — Landing Gear Systems · AC 43.13-1B N₂ GAS (SPRING) HYDRAULIC FLUID (MIL-PRF-5606) AIRFRAME WHEEL AXLE OUTER CYLINDER INNER PISTON METERING PIN IMPACT REBOUND PHASE 1 — IMPACT (COMPRESSION) • Piston compresses N₂ gas • Gas acts as a spring, storing energy for rebound • Fluid forced through orifice PHASE 2 — DAMPING (ENERGY DISSIPATION) • Metering pin restricts fluid flow • Fluid friction converts kinetic energy to heat • Prevents bouncing/oscillation PHASE 3 — REBOUND (EXTENSION) • Compressed N₂ gas expands • Piston returns to extended position smoothly • Orifice still restricts flow PHASE 4 — SETTLE (STABLE) • Strut at normal extension • Ready for next landing event KEY: N₂ gas = spring · Hydraulic fluid = damper · Metering pin controls damping rate OLEO STRUT CROSS-SECTION LEGEND N₂ Gas Chamber Hydraulic Fluid Piston Rod Metering Pin

The shock strut, or oleo strut, is the primary component responsible for absorbing the energy of landing and taxiing. It combines hydraulic fluid and compressed air or nitrogen to provide a smooth, damped action.

  • Construction: A typical oleo strut consists of an outer cylinder attached to the airframe and an inner piston (sliding tube) attached to the wheel axle. The piston moves inside the cylinder, compressing the gas and forcing hydraulic fluid through a metering pin and orifice.
  • Operation: On impact, the piston compresses the gas, which acts as a spring. Simultaneously, the fluid is forced through the metering pin, which dissipates energy as heat, damping the oscillation. The gas then expands to return the strut to its extended position.
  • Servicing: Struts are serviced with a specific type of hydraulic fluid (e.g., MIL-PRF-5606) and nitrogen gas. The correct fluid level and gas pressure are critical for proper operation. Water contamination in the strut is a serious issue, as it can cause corrosion and freezing. The correct action per AC 43.13-1B is to drain, flush, and recharge the strut with the correct fluid and nitrogen.
  • Leaks: A slow leak around the upper bearing indicates a failing seal. A recurring leak after repeated servicing is not acceptable; the strut must be disassembled, and the seals and bearing area inspected and replaced as necessary. A leaking strut after a hard landing also indicates a seal failure that must be repaired by disassembly and replacement of the O-rings.

3. Wheels, Tires, and Brakes

  • Wheels: Main wheels are typically constructed from aluminum alloy. They consist of two halves (inboard and outboard) bolted together, with the tire mounted between them. The wheel houses the brake assembly and wheel bearings.
  • Wheel Bearings: These are tapered roller bearings that allow the wheel to rotate freely on the axle. Excessive play in a wheel bearing indicates wear or damage and requires replacement. Tightening the axle nut will not correct this and can cause further damage. Grease leakage from a bearing indicates a failed seal or over-greasing, requiring removal, cleaning, and inspection of the bearing and seal.
  • Tires: Aircraft tires are bias-ply or radial and are designed to withstand high loads and speeds. A tire with a cut that exposes the cord is unairworthy and must be replaced. Patching is not an approved repair for structural damage. Uneven tire wear patterns are diagnostic:
  • Center wear: Overinflation.
  • Edge wear (both sides): Underinflation.
  • Inside edge wear: Excessive toe-in or camber (wheel misalignment).
  • Wheel Balancing: After a tire replacement, the wheel/tire assembly must be balanced. An out-of-balance assembly will cause a vibration that increases with speed. This is a classic symptom.
  • Brakes: Aircraft brakes are typically hydraulically actuated disc brakes. They can be single-disc or multi-disc. A bluish discoloration on a brake disc indicates excessive heat from friction, often caused by a sticking caliper. This can cause the aircraft to pull to one side during braking. Fluid on the inside of a wheel rim or tire is a critical finding, indicating a leak from the wheel seal or brake assembly, which must be identified and corrected before return to service.

4. Retraction Systems

Landing Gear Retraction Cycle - Hydraulic System Operation LANDING GEAR RETRACTION CYCLE SELECTOR VALVE HYDRAULIC ACTUATOR GEAR POSITION UP-LOCK / DOWN-LOCK SELECTOR VALVE SPOOL HANDLE UP HYDRAULIC ACTUATOR EXTEND → ← RETRACT 3000 PSI GEAR POSITION GEAR UP LOCK MECHANISMS UP-LOCK ENGAGED DOWN-LOCK PIN SAFETY SWITCH RESERVOIR MIL-PRF-5606 SKYDROL HYD PUMP 3000 PSI RETRACTION CYCLE PHASES: 1. SELECT UP 2. PRESSURE 3. GEAR FOLDS 4. UP-LOCK 5. VALVE CLOSE 6. EXTEND NORMAL CYCLE: Gear handle UP → Pressure flows → Gear retracts → Up-lock engages → Hydraulic pressure relieved EXTENSION CYCLE: Handle DN → Down-lock must engage positively (mechanical spring + overcenter) 14 CFR §23.729 / §25.729: Landing gear retraction system must be designed for safe extension even after hydraulic failure PRESS RELIEF 1 2 3 4 LEGEND: Pressure (UP) Return (DN) CYCLE

Retractable landing gear systems use a combination of mechanical linkages, hydraulic actuators, and electrical controls to raise and lower the gear.

  • Actuators: Hydraulic actuators provide the force to move the gear. They are controlled by a selector valve, which directs pressurized fluid to either the "up" or "down" side of the actuator.
  • Mechanical Linkages: These include drag braces, side braces, and torque links. They maintain gear alignment and transfer loads to the airframe.
  • Torque Links: These links keep the wheel axle aligned with the strut. A loose torque link after re-rigging indicates improperly secured fasteners. Castellated nuts must be torqued to specification and secured with cotter pins.
  • Uplocks and Downlocks: These are mechanical mechanisms that lock the gear in the up or down position.
  • Uplock Hooks: Engage a roller on the gear to hold it in the retracted position. If the actuator is fully stroked but the gear is not locking up, the uplock hook is likely out of adjustment or worn.
  • Downlock Springs: These springs assist in locking the gear in the down position. If they are weak or corroded, the gear may not lock down, especially during emergency extension, which relies on gravity and airflow.
  • Emergency Extension: This system provides a backup method for lowering the gear if the primary hydraulic system fails. It often uses gravity, airflow, and spring force. If the gear extends but does not lock down, the downlock springs are a primary suspect.
  • Gear Doors: These fairings streamline the gear wells. If the gear retracts but the doors do not close, the door actuator may not be stroking fully due to rigging issues.

5. Position Indication and Warning Systems

Landing Gear Position Indication Troubleshooting - FAA A&P Exam Prep Landing Gear Systems - Gear Position Indication Troubleshooting AC 43.13-1B / 14 CFR Part 43 UP-LOCK SW DOWN-LOCK SW GEAR POSITION INDICATOR PANEL UP GEAR UP DN GEAR DOWN ⚠ WARNING Gear handle position: DOWN (gear retracted) SCENARIO 1: Gear retracts, "UP" light is OFF Gear handle UP → gear retracts → red UP light stays dark. Most likely: burned-out indicator bulb (90% of cases). If bulb OK: misadjusted up-lock limit switch. TEST BULB CHECK: INDICATOR BULB SCENARIO 2: Gear up, green "DN" light stays ON Gear handle UP → gear retracts → green DN light remains lit. Cause: stuck-closed down-limit switch. Switch contacts remain closed, simulating gear-down. CHECK: DOWN-LIMIT SWITCH TROUBLESHOOTING FLOW 1. Gear handle UP, observe lights 2. UP light OFF → press TEST BULB OK? NO YES REPLACE BULB CHECK UP-LOCK SWITCH 3. Adjust up-lock switch per AMM 4. Verify: gear cycle test ✓ ALL LIGHTS CORRECT UP HORN FAA A&P Prep • Landing Gear Systems • Gear Position Indication Troubleshooting • AC 43.13-1B UP SWITCH STATE
  • Limit Switches: These electrical switches are mechanically activated by the gear or its linkages to indicate the gear's position.
  • Up-Lock Switch: Closes when the gear is fully retracted and locked, illuminating the "gear up" (red) light.
  • Down-Lock Switch: Closes when the gear is fully extended and locked, illuminating the "gear down" (green) light.
  • Indicator Lights: These provide the pilot with a visual indication of the gear position. A burned-out bulb is the most common cause of a light not illuminating when the gear is in the correct position. An intermittent flickering light typically indicates an intermittent electrical connection, such as a loose wire or chafed insulation.
  • Warning Horn: This audible warning sounds when the throttles are retarded (reduced) and the gear is not down and locked. A faulty squat switch (which senses weight-on-wheels) can cause the horn to sound even when the gear is down and locked, as the system may think the aircraft is airborne. A misadjusted down-lock switch can also cause this, as it may not be making contact to signal the gear is down.
  • Mechanical Indicators: Many aircraft also have a mechanical indicator, such as a pin that extends from the wing or fuselage when the gear is down and locked. If the electrical indicator shows "down" but the mechanical pin cannot be inserted, the down-lock may not be engaged, and the aircraft must be jacked for a full retraction test.

6. Shimmy Dampers

Shimmy is a rapid, uncontrolled oscillation of the nose wheel or tailwheel. A shimmy damper is a hydraulic device designed to dampen these oscillations. If the damper is low on fluid or has internal leaks, it cannot control shimmy, leading to accelerated wear on the tire and steering linkages. The damper must be serviced or repaired to correct the issue.

Important Procedures and Regulations

1. Retraction Tests

Landing Gear Retraction Test Procedure - FAA A&P Prep LANDING GEAR RETRACTION TEST PROCEDURE 14 CFR 43 Appendix D JACK JACK HANDLE UP LOCK LOCK DECK RETRACTION CHECKLIST A/C on jacks - all wheels clear Gear handle UP - observe operation Clearance check - doors & gear Up-lock engagement - verify lock pin Gear handle DOWN - down-lock verify TEST STATUS RETRACTING EXTENDING ⚠ SAFETY NOTE Ensure proper jack capacity and safety stands per AC 43.13-1B Per 14 CFR 43 Appendix D — Retraction Test: Verify operation, clearance, and lock engagement

A landing gear retraction test is a critical functional test performed after maintenance on the gear system.

  • Procedure: The test must be performed with the aircraft on jacks to allow the gear to move freely and to prevent damage. The gear handle is placed in the UP position, and the gear operation is observed, including proper clearance and lock engagement. The gear is then extended, and the down-lock engagement is verified.
  • Regulatory Requirement: 14 CFR 43 Appendix D outlines the required inspections for annual and 100-hour inspections, which include a retraction test of the landing gear. The test must be performed by a certified mechanic, and the gear locking mechanisms must be visually inspected to ensure they are functioning correctly.

2. Maintenance Logbook Entries

Per 14 CFR 43.9, after performing maintenance, the technician must make a logbook entry that includes:

  • A description of the work performed.
  • The date of completion.
  • The aircraft total time.
  • The signature and certificate number of the person approving the aircraft for return to service.

3. Return to Service

Per 14 CFR 43.13(a), after maintenance, the aircraft must be returned to service in an airworthy condition. This means all systems, including the landing gear position indicator, must be in a condition for safe operation. An inoperative gear indicator is a safety hazard and must be repaired before return to service.

4. Torque and Cotter Pin Installation

When installing a cotter pin on a castellated nut (e.g., axle nut), the nut must be torqued to the specified value. If the cotter pin hole does not align, the nut must be loosened (never tightened beyond the specified torque) to align the hole. Over-torquing can damage the bearing or threads.

5. Corrosion Control

Per AC 43.13-1B, superficial corrosion that does not affect structural integrity can be treated by cleaning and applying corrosion prevention compounds. However, corroded down-lock springs with lost tension are a safety hazard and must be replaced, as springs are not repairable.

Common Relationships and Troubleshooting Logic

Landing Gear Troubleshooting Logic - Symptom-Based Decision Flow Landing Gear Troubleshooting Logic — Symptom-Based Decision Flow SYMPTOM 1 Gear Won't Retract Normal hydraulic pressure available at actuator LIKELY CAUSE Selector valve not energized OR up-limit switch faulty CORRECTIVE ACTION Check selector valve solenoid continuity; test up-limit switch SYMPTOM 2 Extends But Won't Lock Down Gear lowers but no downlock indication LIKELY CAUSE Weak downlock springs OR gear rigging out of adjustment CORRECTIVE ACTION Replace downlock springs; check mechanical linkage per AC 43.13-1B SYMPTOM 3 Warning Horn Sounds With Gear Down Gear down & locked, horn still active LIKELY CAUSE Downlock switch misadjusted OR squat switch not making contact CORRECTIVE ACTION Adjust downlock switch; inspect squat switch actuation linkage Reference: FAA AC 43.13-1B Ch. 7, 14 CFR 23/25 Landing Gear Systems — A&P Airframe Oral & Practical Three primary symptom paths — trace each to its corrective action
  • Gear retracts, but the "gear up" light is off: The most likely cause is a burned-out bulb. If the bulb is good, the up-lock limit switch is likely misadjusted or faulty.
  • Gear is up, but the "gear down" (green) light remains on: The down-limit switch is likely stuck closed, indicating the gear is down when it is not.
  • Gear will not retract, but hydraulic pressure is normal: The selector valve may not be directing fluid to the actuators, or the up-limit switch may not be providing the electrical signal to initiate retraction.
  • Gear extends but does not lock down: The downlock springs are weak, or the down-lock mechanism is out of rigging.
  • Warning horn sounds with gear down and locked: The down-lock switch is misadjusted, or the squat switch is faulty.
  • Vibration that increases with speed: The wheel/tire assembly is out of balance.
  • Aircraft pulls to one side during braking: A sticking brake caliper on that side is causing the brake to drag and overheat.
  • Fluid on the inside of a wheel rim: This is a critical finding indicating a leak from the wheel seal or brake assembly. It must be inspected and corrected before return to service.

Summary

The landing gear system is a complex integration of mechanical, hydraulic, and electrical components. The AME must have a solid understanding of each subsystem, its function, and its failure modes. Proper maintenance, including regular inspections, correct servicing, and precise rigging, is paramount to ensuring the safety and airworthiness of the aircraft. When troubleshooting, a logical, systematic approach—starting with the most probable and simplest cause—is essential for efficient and effective repairs. Adherence to the regulations and procedures outlined in 14 CFR and AC 43.13-1B is mandatory for all maintenance actions.

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