AMT-22 Landing Gear Notes

Landing Gear Systems – Key Concepts

Landing Gear Overview

• Landing gear endures greatest stresses in aircraft structure; hard landings can exert forces many times the aircraft weight on tires, wheels, shock absorbers, and structure.
• Regular inspection and maintenance are required due to high stresses.

Landing Gear Types and Surfaces

• Three basic surface types: water, snow/ice, hard/earth surfaces.
• Water operations historically used flying boats; today most water ops use land airplanes with twin floats; amphibious floats with retractable wheels enable dual operation.
• Amphibian: aircraft with landing gear allowing operation from both water and land surfaces.
• Snow/ice operations use skis; two main types:
  • Wheel-replacement skis (ski replaces wheel; nose movement controlled by steel cables and rubber shock cord).
  • Retractable skis (fit around standard gear; hydraulics move skis up/down).
• Hard surfaces use wheels and tires; early gear had two main wheels and a tail skid; with wheel brakes, tail skid evolved to tail wheel and then nose wheel forming the tricycle configuration.
• Drag reduction evolved from wheel pants to retractable gear for high-speed flight.

Modern Gear Configurations

• Tail-wheel gear used for unpaved operations; retractable gear minimizes parasite drag; tricycle gear popular for ground handling.
• Modern configurations are shown in typical photos (e.g., N63267, N2891X).

Shock Absorbers (Module 2)

• Purpose: convert mechanical energy to heat to absorb landing shocks.
• Main type: oleo (air-oil) shock absorbers.
  • Oil absorbs initial impact.
  • Compressed air cushions taxi shocks.
• Alternatives: spring-oil shocks with steel coil springs (small aircraft).
• Torque links (scissors/nutcrackers): hinge between piston and cylinder; allow piston movement but prevent rotation; adjustable for wheel alignment.
• Other shock-absorbing methods include bungee shock cords (used on older aircraft).
• Some aircraft use non-shock-absorbing systems like steel leaf springs or tubular steel springs that return energy smoothly; proper technique minimizes bounce.

Oleo Shock Absorber (Details)

• Structure: piston and cylinder with oil and air compartments; metering orifice controls oil transfer; air cushions taxi shocks.
• Key components (illustrative): filler plug, air valve, piston, cylinder, torque links, and metering pin.
• Servicing steps (summary):
  $1$. Jack aircraft to unload weight from wheels.
  $2$. Deflate strut via high-pressure air valve.
  $3$. Remove filler plug.
  $4$. Move strut by hand (or use exerciser jack for large aircraft).
  $5$. Fully collapse strut.
  $6$. Fill with proper hydraulic fluid up to filler plug.
  $7$. Remove AN812 valve core; bleed air via bleeder hose into hydraulic fluid.
  $8$. Move piston to remove air bubbles; re-collapse strut.
  $9$. Install proper high-pressure air valve.
  $10$. Lower aircraft; inflate with air or nitrogen to specified extension height.
• Some maintenance requires an exerciser jack to move the piston when servicing with fluid.

Shock Absorbers Alternatives and History

• Rubber ring shocks (early Piper-type) used to soften impacts with elastic bands around a braid.
• Bungee shock cords: many small rubber bands in a cotton braid; used on non-shock-absorbing gear.

Wheel Alignment

• Importance: Proper wheel alignment ensures tracking and wear life; two key checks are:
  • Toe-in/toe-out: whether front tires are closer or farther than the rear when rolling forward.
  • Camber: vertical tilt of wheels relative to the airframe.
• Checking methods:
  • For spring steel gear: use shim adjustments between axle and strut end; max shim thickness is per manual; trial-and-measurement approach.
  • For oleo-gear: use straightedge against tire fronts, then carpenter’s square to measure gap for toe-in/out; adjust via shims on torque-link arms.
  • Plate method: greased plates under wheels; rock the aircraft to relax gear; measure front vs rear rim spacing with straightedge and square.
• Camber measurement uses a bubble protractor; observed camber in positive/negative directions.
• Special notes:
  • Spring steel: adjust toe via shims between axle and end fitting; limits per maintenance manual.
  • Oleo struts: toe-in/out adjusted by shims on torque-link arms.
• Ground handling benefit: proper alignment improves steering and reduces tire wear.

Ground Steering

• Small tail-wheel aircraft: steerable tail wheel linked to rudder control; unlocks beyond ~$45^ ext{o}$ for tight turns; returns to steerable mode when straight.
• Larger tail-wheel aircraft: full-swiveling tail wheels with lock during takeoff/landing; unlocked during taxi with differential braking for steering.
• Nose-wheel steering (on nose-wheel aircraft):
  • Free-turning nose wheels steered by brakes.
  • Some nose wheels are steerable via rudder pedals (limited turning).
• Retractable landing gear nose-wheel centering: centering cam ensures nose wheel straight before retraction.
• Large aircraft use hydraulic steering cylinders for nose-wheel steering; also act as shimmy dampers during takeoff/landing. Steering is controlled by rudder pedals or a steering wheel and a steering control valve.
• Shimmy dampers prevent nose-wheel shimmy (vibration) by using two fluid compartments connected by a small orifice; hydraulic fluid movement damps rapid motion; does not affect normal steering.
• Hydraulically operated nose gear steering cylinders also serve as shimmy dampers in larger aircraft.

Nose Gear Steering and Centering (Details)

• Centering cam in the nose-gear shock strut centers the piston as the strut extends for retraction.
• On A320 and similar aircraft, nose-wheel steering involves hydraulic actuators and associated damping to prevent shimmy.

Retractable Landing Gear Systems (Module 3)

• Rationale: Parasite drag on fixed gear becomes greater than weight penalty of a retraction system at high speeds.
• Small aircraft may use mechanical systems (hand crank/lever) for gear operation; larger aircraft use electric motors, pneumatic systems, or hydraulic systems (most common in the US).
• Power Pack System: a compact hydraulic unit that combines reservoir, pump, thermal relief valve, high- and low-pressure control valves, and shuttle valve to simplify maintenance and weight.

Lowering the Landing Gear (Gear-Down)

• Sequence when gear is commanded to down:
  $1$. Gear handle activates hydraulic pump motor to drive fluid to down side actuators.
  $2$. Fluid flows around gears and to gear-down actuating cylinders.
  $3$. Pump moves gear-down piston; nose gear is easier to move; restricted flow for nose-gear to limit speed.
  $4$. Return fluid from up side to reservoir via low-pressure valve considering pressure buildup.
  $5$. Gear reaches down and locked; limit switches turn pump off.
• Gear-down locks ensure gear remains down until commanded up.

Raising the Landing Gear (Gear-Up)

• Sequence when gear is commanded up:
  $1$. Gear handle energizes pump to drive fluid to gear-up side.
  $2$. Fluid moves to gear-up side of actuators, releasing downlocks.
  $3$. First piston movement allows retraction; fluid returns via shuttle valve to reservoir.
  $4$. Fluid path back to reservoir continues; no mechanical up locks in some systems; hydraulic pressure holds gear retracted.
  $5$. When fully retracted, pressure rises and a switch shuts the pump off.
• Retractable-gear systems with wheel-well doors: sequence ensures gears retract before doors close.

Typical Hydraulic Retraction System (Figure 6-20)

• Left and right main gear actuators, uplocks, downlocks, gear-door sequence valves, and associated valves coordinate gear and door movement in proper sequence.

Emergency Extension of the Landing Gear

• All retractable-gear aircraft require backup extension:
  • Free-Fall System (simple): gear-down/up hydraulic lines connected by a free-fall valve; in emergency, gravity and air pressure drop the gear and mechanical locks hold it in place.
  • Compressed air/Nitrogen Backup (complex): shuttle valve senses hydraulic pressure loss; emergency air is introduced to actuators to lower gear; shuttle valve shifts automatically to permit air pressure to operate the actuators.
• Shuttle valve: directs normal hydraulic pressure to the gear while allowing emergency air to take over if hydraulic pressure is lost.

Aircraft Brakes (Module 4)

• Brake function: convert kinetic energy to heat through friction to slow the aircraft.
• Two basic brake types:
  $2$. Energizing brakes (servo brakes): wedging action amplifies braking force; less pilot effort.
  $2$. Non-energizing brakes: direct proportionality to pilot input; more effort required.
• Servo brakes can be single-servo or duo-servo (engage forward and backward).
• Dual-disk brakes: more braking action; lighter than multi-disk systems; involve two disks, center carrier, brake linings, and pistons with automatic adjusters.
• Operation: hydraulic pressure moves pistons to clamp disks with is on wheel hub; linings held by backplate.
• Auto Brake Systems (example: Boeing 757): preset deceleration after touchdown; armed before landing; applies brake pressure upon touchdown; compensates for thrust reversers and speed brakes; disengages if selector moved to DISARM/OFF, manual braking, thrust levers moved, or speed brakes deployed.
• Brake Maintenance: brakes endure extreme heat; thrust reversers used on jets; aborted takeoffs cause heavy brake heating; overheated brakes require inspection; seals can degrade; warped/discs can cause chattering; post-service pressure tests mandatory.
• Bleeding brakes: remove air from hydraulic fluid; bleeder valve and tube used to ensure bubble-free fluid.

Aircraft Wheels and Tires

• Wheel evolution: early aircraft had no brakes; later fixed wheels with doughnut tires; removable rims; tubeless tires led to two-piece wheels with O-ring seal.
• Two-piece wheels: aluminum or magnesium; components include outboard/inboard halves, O-ring seal, through bolt, bearing cups, tapered roller bearings, beaded wire reinforcement, and brake drive keys.
• Fusible plugs: safety devices in tubeless wheels; melt at low temperatures to vent air and prevent tire explosion when overheated.
• Wheel Maintenance: wheel care impacts tire life; inspect for corrosion; check bearing races and rollers for damage; inspect cages; reject wheels with damage.
• Wheel Removal: must deflate tires before loosening axle nut; use deflator caps for high-pressure tires to vent safely; avoid prying with tire tools.
• Tire Installation and Balancing: final balancing performed with tire mounted; do not remove factory balance weights; retread vs scrap decisions depend on bead, ply, and tread condition.

Aircraft Tires and Tubes (Module 6)

• Tire characteristics: high impact resistance, low mileage, high deflection; handle extreme landing loads and heat buildup; prone to tread wear from rapid acceleration on abrasive runways.
• Evolution: Wright Flyer used skids; first airplane tires in 1909; streamlining reduced drag; early no brakes; fat-doughnut tires for small aircraft with limited braking ability; tread evolution from diamond to rib patterns.
• Tire Construction (cross-section): bead, carcass, ply rating, tread, sidewalls, flipper, bead wires, cord body, and tires designed for high load and heat.
• Bead: high-strength carbon-steel wires securing tire to wheel.
• Carcass: layers of rubberized fabric providing body of tire.
• Ply Rating: indicates relative strength; does not equal number of plies; represents equivalent fabric strength.
• Tread: outer wear surface; grooves for traction; rib tread common for paved runways.
• Tire Inspection: tire conditions are critical; inspect for bulges, inner liner damage, bead damage, and bead seating using appropriate nondestructive testing.
• Retread: renewal of tread rubber; must assess viability of retread before applying.
• Tubes: tubes are rubber; leaks occur at holes or valve; heat from braking can deform tubes; store tubes properly.
• Tire Maintenance: keep inflated to appropriate pressure; avoid grease/oil contamination; rotate tires on long-term idle aircraft to prevent nylon flat spots.
• Bead/Seal Checks: check bead area for cracks; do not remove factory balance weights; ensure final balance with tire mounted.

Maintenance and Safety Summary

• Proper inflation and temperature correction are essential for tire life: pressure should be checked with accurate gauges; a 1% pressure change per 5°F is a guideline; aircraft weight adds ~$4 ext{ percent}$ to tire pressure.
• Dual tires must match in size, brand, and tread; large pressure discrepancies require monitoring.
• During maintenance, deflate tires before disassembly; use safety caps and deflators to prevent injury from sudden air release.

Quick Reference Facts

• Parasite drag is a major factor pushing design toward retractable gear.
• Nose-wheel centering and shimmy dampers are critical for stability during takeoff, landing, and taxi.
• Emergency extension systems (free-fall and compressed air backups) provide a failsafe for gear deployment.
• Auto brake systems automate deceleration after touchdown but require proper arming and can disengage under specific conditions.

See Also

• Figures and diagrams referenced (e.g., Figures 6-2, 6-3, 6-4, 6-5, 6-6, 6-9, 6-12, 6-13, 6-16, 6-17, 6-20, 6-27, 6-50, 6-52) correspond to specific diagrams in the study material for deeper understanding.