Overview

This chapter covers the design, operation, inspection, and maintenance of aircraft landing gear systems. Landing gear is a critical airframe system that supports the aircraft on the ground, absorbs landing impact loads, and enables safe taxi, takeoff, and landing operations. The chapter encompasses shock struts, retraction mechanisms, braking systems, anti-skid systems, wheels and tires, steering systems, and the associated hydraulic and electrical control systems. Understanding the interrelationship between these subsystems and the regulatory requirements for their maintenance is essential for the Aircraft Maintenance Engineer (AME).

Key Concepts Explained

Landing Gear Configurations and Functions

The primary purpose of landing gear is to support the aircraft's weight on the ground, absorb and dissipate landing impact energy, and provide a means for ground maneuvering. Two main configurations exist:

Tricycle Gear: Features two main wheels (or sets of wheels) located behind the center of gravity and a nose wheel forward. This configuration provides superior ground stability, improved forward visibility during taxi, and better directional control. It is the standard configuration on virtually all modern aircraft.

Conventional (Tailwheel) Gear: Features two main wheels forward and a smaller wheel at the tail. Common on older aircraft and some bush planes, this configuration requires more pilot skill for ground handling due to its inherent instability.
Bogey (Bogie) Gear: A multi-wheel truck assembly that distributes the aircraft's weight over several wheels. Common on large transport aircraft, bogies reduce ground pressure and improve load distribution on runways.
Trailing Arm Design: A levered suspension where the wheel is mounted on a pivoting arm that trails behind the main strut. This design improves shock absorption and reduces structural loads.

Oleo-Pneumatic Shock Struts

The oleo strut is the most common type of landing gear shock absorber. It combines hydraulic fluid for damping with compressed nitrogen for spring action.

Operation: When landing impact occurs, the piston compresses, forcing hydraulic fluid through small orifices. This controlled fluid displacement absorbs energy and provides damping. The compressed nitrogen acts as a spring, returning the strut to its extended position. The combination prevents rebound and ensures smooth ground operation.

Servicing: Oleo struts are serviced with hydraulic fluid (typically MIL-H-5606 or Skydrol, depending on the system) and dry nitrogen. Typical nitrogen pressure ranges from 200 to 1000 psi, depending on aircraft weight and strut design. Exact values are specified in the maintenance manual.

Inspection Criteria:

Strut Extension: Must be within manufacturer-specified limits. Low extension indicates low nitrogen charge.
Fluid Leakage: Minor seepage (oil residue on the upper barrel) is acceptable if strut extension is within limits. Active dripping requires corrective action per the manufacturer's instructions.
Piston Condition: The piston surface must be free of corrosion, nicks, and scoring.
Torque Links (Scissors Links): These prevent the lower piston from rotating within the upper cylinder, maintaining correct wheel alignment and absorbing torsional loads. They must be free of cracks and properly lubricated.

Common Defects:

Low Nitrogen Charge: Causes the strut to not extend fully when the aircraft is on jacks. The strut may compress too easily under load.
Over-servicing with Fluid: Results in a hard strut with insufficient cushioning.
Seized Piston: Prevents any movement; requires overhaul.
Leaking Seals: Causes fluid loss and reduced damping.

Retractable Landing Gear Systems

Retractable landing gear reduces aerodynamic drag, improving aircraft speed, fuel efficiency, and climb performance. Systems are typically hydraulic, though electric and mechanical systems exist.

Retraction Sequence: Hydraulic systems use sequence valves to control the order of operations. A typical sequence is:

Gear doors open (if applicable)
Landing gear retracts
Gear doors close

Key Components:

Uplock Mechanism: Secures the landing gear in the fully retracted position to prevent it from falling out during flight.

Downlock Mechanism: Secures the gear in the fully extended and locked position to prevent collapse during landing and ground operations. Downlocks are often spring-loaded or hydraulically actuated.
Sequence Valves: Control the order of hydraulic operations. A faulty sequence valve can cause the gear to retract but doors to remain open, or doors to fail to close after retraction.
Squat Switch (Ground Safety Switch): Activated by the weight of the aircraft on the gear. It interrupts the electrical or hydraulic circuit to prevent inadvertent retraction while the aircraft is on the ground.
Position Indicating System: Uses microswitches to indicate gear position. A red "unsafe" light indicates the gear is not in the correct locked position. A green "down and locked" light confirms proper extension.

Troubleshooting Common Issues:

Gear retracts but doors remain open: Likely a faulty sequence valve failing to direct pressure to the door close circuit.
Gear retracts but red unsafe light remains on: Indicates a position switch not making contact; check microswitches first.
Slow retraction: Often caused by a restriction in the hydraulic system. Low pressure would also cause slow movement.
Gear fails to retract with normal pressure: Likely a mechanical obstruction or binding.
Emergency extension failure: Often due to a discharged emergency accumulator or blow-down bottle.

Braking Systems

Modern jet transport aircraft predominantly use multiple-disc brakes, which provide high braking torque and heat absorption. Carbon discs are lighter and handle higher temperatures than steel discs.

Brake System Components:

Master Cylinders: Convert pedal force into hydraulic pressure.
Brake Assemblies: Contain rotating discs (attached to the wheel) and stationary discs (attached to the axle or torque tube).
Brake Wear Indicator Pins: Protrude when linings are new and recede as linings wear. When flush with the housing, linings have reached minimum allowable thickness and must be replaced.
Parking Brake: Applies hydraulic pressure to the brake assemblies, holding the brakes applied even after the pilot releases the brake pedals.
Fusible Plugs: Designed to melt at a specific temperature (e.g., 300°F/150°C) caused by overheated brakes. They release tire inflation pressure to prevent tire burst due to heat.

Brake System Troubleshooting:

Spongy Pedal with Full Reservoir: Classic symptom of air in the hydraulic system. Air is compressible, unlike brake fluid, leading to a soft pedal feel. Corrective action is bleeding the brakes.
Hard Pedal with Little Travel: Indicates excessive pressure or a blockage, such as a restricted return line.
Continuous Air During Bleeding: Indicates air is being drawn into the system from an external source, most likely a leak.
Locked Wheel on Landing: Strongly indicates the brake remained applied when it should have released. The anti-skid system is designed to prevent this.

Anti-Skid Systems

Anti-skid systems monitor wheel speed and modulate brake pressure to prevent wheel lockup, allowing maximum braking without skidding, especially on wet or icy runways.

Operation: Wheel speed sensors send signals to the anti-skid control unit. If a wheel begins to decelerate too rapidly (indicating impending lockup), the control unit commands a valve to release brake pressure momentarily, allowing the wheel to spin up again.

Troubleshooting:

Rapid Cycling During Landing Rollout: Indicates erroneous speed signals or control valve malfunction. A faulty wheel speed sensor is a common cause.
Rapid Cycling at Low Speed: A faulty wheel speed sensor can send erratic or low-speed signals, causing unnecessary cycling.
Inoperative System with Normal Brakes: If the Minimum Equipment List (MEL) allows dispatch with the anti-skid inoperative, the AME can defer the defect by following MEL procedures (placarding, logging).

Wheels and Tires

Tire Construction: Modern transport aircraft predominantly use radial-ply tires, where cords run radially from bead to bead. This design provides longer tread life, better heat dissipation, and lower rolling resistance.

Inspection Criteria:

Tread depth (minimum wear indicators)
Cuts, abrasions, bulges, and foreign object damage
Sidewall condition
Uneven wear patterns (may indicate alignment or pressure issues)

Wear Patterns and Causes:

Center Tread Wear: Over-inflation causes the tire to crown and contact the runway primarily in the center.
Shoulder Wear: Under-inflation wears the edges.
Feathered or Diagonal Wear: Toe-in misalignment.
Flat Spots: Seized brake or locked wheel.

Wheel Assembly:

Fusible Plugs: Melt at a specific temperature to release tire pressure and prevent burst due to brake heat.
Bearings: Must be inspected for discoloration (blue indicates overheating and loss of hardness; requires replacement).
Balancing: Wheel and tire assemblies must be balanced per manufacturer's instructions using approved balancing weights.

Steering and Shimmy Dampers

Shimmy Damper: A hydraulic or friction-based device that reduces or eliminates rapid side-to-side oscillations (shimmy) of the nose wheel. Shimmy can cause structural damage and loss of control. A leaking shimmy damper is a defect that must be addressed per the manufacturer's instructions.

Torque Links (Scissors Links): Prevent the lower strut piston from rotating within the upper cylinder, maintaining correct wheel alignment and absorbing torsional loads.

Important Regulations and Procedures

Regulatory Framework (CARs Standards)

Standard 571.02: Maintenance must be performed in accordance with the manufacturer's recommendations. Tools and test apparatus must be in a condition and accuracy to ensure work meets manufacturer's standards. Substituting different materials (e.g., grease) without authorization is not permitted.
Standard 571.03(1): Any maintenance or elementary work must be recorded in the technical records. Adding fluid to a strut is considered elementary work and must be recorded.
Standard 571.06: Repairs must use acceptable or approved data. If the manufacturer provides no repair for a cracked component, replacement with a new or serviceable part is required. If the manufacturer allows a repair (e.g., welding), that is the correct action.

Leak Evaluation and Action

When a hydraulic leak is found on a landing gear actuator:

Check fluid level in the reservoir.
If the level is within limits and the leak is a slow drip, clean the area, cycle the gear to check for active leakage, and then decide if a repair is needed per the manufacturer's instructions.
Minor seepage (oil residue) on an oleo strut with extension within limits is acceptable; clean and document.
Simply tightening fittings may damage seals. Immediate replacement without diagnosis is excessive.

Hard Landing Inspection

After a hard landing, the manufacturer's maintenance manual specifies a special inspection to check for damage to the landing gear, attachments, and airframe structure. Hidden damage is possible, so a thorough inspection is required.

Torque and Fastener Procedures

Under-torqued bolts with no evidence of movement or damage can be re-torqued to specification.
If an axle nut is below specified torque, it can be tightened to the correct value, but a new cotter pin must be used. Cotter pins should not be reused.
Out-of-calibration tools must not be used. Follow the AMO's calibration control procedures.

Minimum Equipment List (MEL) and Deferrals

If the MEL permits operation with a defect (e.g., inoperative anti-skid), the AME can defer the defect by following MEL procedures (placarding, logging). A maintenance release can be issued for the aircraft in its current condition as long as the MEL is complied with.

Common Relationships Between Concepts

Hydraulic System and Landing Gear Operation

The hydraulic system is the heart of retractable landing gear. Understanding the relationship between components is crucial:

Sequence Valves control the order of operations. A faulty sequence valve can cause gear retraction without door closure, or vice versa.
Squat Switches prevent ground retraction by interrupting the control circuit.
Emergency Accumulators provide a dedicated pressure source for emergency extension.
Hydraulic Pressure must be within specified limits. Low pressure affects all functions, not just one.

Shock Strut and Tire Relationship

The oleo strut and tire work together to absorb landing impact:

Low Nitrogen Charge: Strut does not extend fully; tire may absorb more impact.
Over-servicing with Fluid: Hard strut; tire may absorb more impact, potentially causing damage.
Tire Pressure: Affects braking performance and wear patterns. Over-inflation causes center wear; under-inflation causes shoulder wear.

Brake System and Anti-Skid Relationship

The anti-skid system modulates brake pressure to prevent lockup:

Faulty Wheel Speed Sensor: Causes rapid cycling or erratic anti-skid operation.
Locked Wheel on Landing: Indicates brake remained applied; anti-skid should have prevented this.
Spongy Pedal: Air in the system affects brake feel but not anti-skid logic.
Hard Pedal: Blockage or restriction; anti-skid cannot modulate pressure effectively.

Inspection and Maintenance Interdependencies

Minor Seepage on an oleo strut with correct extension is acceptable; replacing seals is excessive.
Cracked Components: If the SRM provides a repair, it can be performed. If not, replacement is required.
Worn Brake Linings: Indicator pins flush with housing means replacement is required.
Corroded Downlock Springs: Must be replaced; temporary fixes are not acceptable.
Blue Discoloration on Bearings: Indicates overheating; bearings must be replaced.

Troubleshooting Logic

When troubleshooting landing gear systems, follow a systematic approach:

Verify Hydraulic Pressure: Normal pressure eliminates many causes.
Check Sequence Valves: If one function works but another does not, suspect sequencing issues.
Inspect Position Switches: If gear appears mechanically correct but indicators show unsafe, check microswitches.
Evaluate Mechanical Condition: Binding, obstructions, or worn components can prevent proper operation.
Consult Manufacturer's Manual: Always follow approved data for troubleshooting and repair procedures.

Chapter 7: Landing Gear Systems