The nine essential components of aircraft landing gear are the shock strut (an oleo-pneumatic oil-and-air absorber), torque links, drag braces, side braces, wheels, tires, brakes, axles, retraction actuators, and the trunnion mounting assembly. Together, these parts absorb up to 600,000 pounds of impact force during heavy commercial touchdowns while comprising only 4-approximately 5%[1] of the aircraft’s empty weight.
FAA Part 25.473 certifies main gear to handle sink rates up to 10 feet per second.
The core components of aircraft landing gear include the shock strut (which is an oleo-pneumatic absorber, basically a combined oil and air cushioning device), the torque links, the drag and side braces, the wheels, the tires, the brakes, the axles.
And the retraction actuators that pull the gear up and down.
And then there’s the trunnion mounting assembly too.
This guide walks through each part by what it does, how it tends to fail.
And where it should sit on your inspection priority list. It draws on FAA airworthiness directives and maintenance data pulled from Airbus A320 and Boeing 737 fleets.
So you’ll see exactly what keeps a 200-ton machine rolling safely from 150 knots all the way down to a full stop.
Quick Takeaways
- Inspect the oleo-pneumatic shock strut first—it absorbs approximately 80%[2] of touchdown impact energy.
- Check torque links for wear; misalignment causes shimmy above 100 knots taxi speeds.
- Verify main gear handles FAA Part 25.473 sink rates up to 10 ft[3]/s.
- Monitor tire pressure weekly—tires absorb only 5%[4] of vertical landing impact forces.
- Prioritize trunnion assembly inspections during C-checks to prevent catastrophic gear collapse failures.
How Aircraft Landing Gear Works as a Load Path
Think of landing gear as a Load path. It’s essentially a chain that channels forces from the runway into the airframe.
It handles vertical, horizontal, and rotational forces without breaking anything. When a typical airliner touches down, it delivers a vertical sink rate of 2,3 feet per second.
But the FAA Part 25.473 actually certifies main gear to absorb up to 10 ft[5]/s.
So where does all that energy go? It doesn’t just disappear. It flows through every component in a specific sequence.
Here is the force trail at touchdown:
- The Tire compresses against the runway, which absorbs the first ~approximately 5%[6] of the impact energy.
- The Wheel and bearings spin up from 0 to ~150 knots in under one second. This generates a brake torque reaction.
- The Axle bends slightly under the vertical load. On a Boeing 737, that’s often 25–approximately 40 tons[7] per main gear.
- The Shock strut, which is the oleo-pneumatic cylinder, absorbs roughly 80–approximately 90%[8] of the remaining vertical energy. It does this through hydraulic fluid orificing.
- The Drag and side braces resist fore-aft and lateral loads. They lock the leg geometry in place.
- Finally, the Trunnion fittings transfer any residual loads into reinforced wing-spar or fuselage hardpoints.
Understanding this sequence is why the components of aircraft landing gear are engineered as a complete system. A weak torque link can cascade into axle failure. A worn oleo seal can raise peak loads on the trunnion.
For the certification basis, you can see the FAA 14 CFR §25.473 ground load criteria.
Wheels and Tires — The First Contact Point
Tires and wheels are basically the first components of aircraft landing gear to soak up the runway impact, and honestly they do it under pretty brutal conditions. Picture a Boeing 737 main tire slamming into the runway at roughly 140 knots while carrying around 20,000 lb[9] per wheel.
All of that sitting on a nitrogen-filled bias-ply carcass pumped up to 200,220 psi[10].
Modern transport-jet tires are tubeless these days, with nylon or aramid cord plies wrapped around steel beads. The ply ratings, usually somewhere around 22,32 PR on narrow-bodies, actually reflect strength rather than a literal count of the plies inside.
Tread grooves run only around the tire, never sideways.
Because they need to channel water away and stop the tire from skating on a film of it. Goodyear data ties that risk to the classic 9 × √tire pressure speed threshold, which works out to about 127 knots at approximately 200 psi.
FAA AC 25-7 covers the certification flight-test conditions behind those numbers.
Wheel halves come in three flavors:
- Forged aluminum (2014-T6): the default option, cheap, repairable, and roughly 30 lb[12] per half on a 737.
- Magnesium alloy: about 30%[13] lighter, though it corrodes fast. Mostly you only see it on older military jets and business aircraft now.
- Carbon-fiber composite halves: just starting to show up on light aircraft, shaving another 20–approximately 25%[14] off the weight that hangs below the suspension.
On a walk-around, you want to keep an eye out for three failure clues. Cord exposure, meaning white aramid threads showing through the tread, equals an immediate scrap.
Reverted-rubber streaks left behind from a hydroplaning event. And Discolored heat-fuse plugs on the wheel hub.
If a plug has melted, that tells you the brake went past roughly 390°F[15] (approximately 199°C[16]) and deflated the tire on purpose so the rim would not burst.
Brake Assemblies and Anti-Skid Systems
Among the components of aircraft landing gear, brake assemblies do the hardest thermal work, a Boeing 737 rejected takeoff can dump over 50 megajoules into each brake stack within seconds, peaking near 1,500°F[1] at the rotor surface.
Modern brakes use a multi-disc stack: stationary stators keyed to the torque tube alternate with rotors keyed to the wheel. Hydraulic pistons clamp the stack, converting kinetic energy into heat. Two material choices dominate:
| Property | Steel Rotors | Carbon-Carbon |
|---|---|---|
| Weight | Baseline | ~approximately 40%[2] lighter |
| Service life | ~1,000 landings | 2,000+ landings |
| Heat capacity | Lower | ~2.5x higher |
| Cost per ship-set | Cheaper upfront | Lower lifecycle cost |
Anti-skid valves sample wheel speed via tachometers and modulate brake pressure when deceleration exceeds the runway’s available friction, releasing pressure within milliseconds to prevent lockup, similar in concept to automotive ABS but tuned for far higher slip ratios. The FAA Airplane Flying Handbook covers operational behavior in detail.
On walk-around, check three things: wear pin protrusion (flush with the housing means the stack is at limits), rotor surfaces for heat-bluing or warping, and brake temperature monitoring readouts before turnaround. Spongy pedal feel after a hot landing usually signals brake fade, let the stack cool before dispatch.
Axles, Torque Links, and Wheel Attachment Hardware
The axle carries every pound of wheel load into the strut.
While torque links (often called scissors) stop the lower strut from spinning on its own axis as the oleo compresses. Together they form the rotational and vertical interface among the components of aircraft landing gear, and they fail in predictable, inspectable ways.
An axle is typically a forged 4340 steel stub press-fit or bolted to the lower strut cylinder. Wheel bearings ride on it, preloaded by a castellated axle nut torqued to the manufacturer’s expected level (often approximately 15,20 ft[3]-lb on a Cessna 172, then backed off to the next cotter pin slot per the FAA Airframe Handbook).
Over-torque crushes the bearing; under-torque lets the wheel wobble and elongate the bore.
Torque links sit in two halves joined at an apex bolt. At every 100-hour inspection, mechanics check three things:
- Bushing slop — grab the lower link and twist; lateral movement over 0.015 in[4]. Means the bushings are out of limits.
- Apex bolt torque — must be tight enough to clamp but loose enough to articulate; a dry, squeaking joint signals lost preload.
- Grease fitting condition — Zerks must accept MIL-PRF-81322 grease without bypassing; a blocked fitting starves the bushing and accelerates wear by roughly 4x in field data from regional fleet operators.
Skip these checks and shimmy follows, often misdiagnosed as a tire balance issue when the real culprit is a worn apex bushing.
Shock Struts — Oleo-Pneumatic Absorbers Explained
The shock strut is really the energy-absorbing heart of the components of aircraft landing gear. It uses what’s called an Oleo-pneumatic design, which basically means hydraulic oil (the “oleo” part) on one side and compressed nitrogen on the other.
Together they soak up the loads from touching down that would otherwise just snap the airframe in half.
So here’s how the two-stage damping actually works. The moment the wheels hit, the piston shoves hydraulic fluid through a tiny metering hole, which is often shaped by a tapered metering pin sitting inside.
That throttling action essentially burns off the kinetic energy as heat, somewhere around 80 to approximately 90%[5] of the landing impact. The nitrogen charge, which is pressurized to about 150 to approximately 200 psi[6] static on a light twin (much higher on the bigger transport jets.
Since a 737 main strut sits closer to approximately 1,800 psi[7]), then acts like a spring.
It holds up the static weight of the airplane and lets it rebound smoothly.
Servicing these things is pretty unforgiving, though. Each strut has an Exposed-chrome “X” dimension printed right on the placard.
For example, it’s 3.25 inches on a Cessna 210 nose strut. If you inflate it past that number you end up sending jolts straight into the fuselage.
And if you underinflate it, the strut bottoms out on hard landings.
Three failure modes really dominate the hangar squawks:
- Seal leaks, which is basically red hydraulic fluid weeping down the chrome, usually coming from a nicked scraper ring
- Low nitrogen charge, where the strut sags overnight and often gets misdiagnosed as a hydraulic leak
- Chrome pitting, which is corrosion hiding under the paint that shreds the seals on the very next compression stroke. The FAA actually flags this in AMT Airframe Handbook Chapter 13
A collapsed strut during rollout has caused quite a few runway excursions logged in the NTSB aviation database, and almost every single one traces back to somebody skipping the 100-hour servicing.
Drag Braces, Side Braces, and Down-Locks
Drag braces and side braces are the structural triangulation that keeps the strut from folding under landing loads, and the down-lock is what guarantees they stay rigid. Drag braces resist fore-aft forces (typically 0.5,0.8g during a firm touchdown on a transport jet).
⚠️ Common mistake: Technicians often blame worn tires or wheel bearings for shimmy above 100 knots taxi speeds, replacing them without resolving the vibration. This happens because torque link wear mimics tire-related symptoms but actually causes the misalignment driving the oscillation. The fix: measure torque link apex bushing play during every A-check and replace bushings before radial clearance exceeds manufacturer limits (typically 0.020 inches).
While side braces handle the lateral kick from crosswind landings and high-speed taxi turns, which the FAA Airplane Flying Handbook cites as a leading cause of side-load gear damage.
The locking trick is geometric, not hydraulic. Once the brace extends, the upper and lower links pass roughly 2,5° beyond a straight line, called Over-center, so any compressive landing load actually tightens the joint instead of collapsing it.
A small spring-loaded link or hook holds that over-center position; hydraulics only unlock it for retraction.
Position is confirmed by redundant Micro-switches (proximity sensors on newer fleets) reading the lock link, not the actuator. That distinction matters: in service data reviewed across narrow-body operators, a misrigged or contaminated down-lock switch, not actuator failure, accounts for the majority of “unsafe gear” cockpit indications, which is why the checklist response is always a visual confirmation or alternate extension before assuming the gear is actually unlocked.
Among the components of aircraft landing gear, the down-lock is the cheapest part that can ground the most expensive airplane.
Retraction Actuators and Sequencing Mechanisms
Among the components of aircraft landing gear, retraction actuators really do the heavy lifting. Literally.
They swing a approximately 1,500 lb[8] main gear assembly up into the wheel well in about 8 to 12 seconds. Most large transports run hydraulic actuators at approximately 3,000 psi[9], though the Boeing 787 and Airbus A380 push that up to 5,000 psi[10].
Business jets are leaning more and more on electromechanical actuators, basically electric motor-driven units. And a few older types like the Cessna 421 still rely on a pneumatic blow-down system as a backup for getting the gear down.
The whole retraction sequence is choreographed by Sequence valves, Uplocks, and Gear doors working together. A typical order goes like this: doors open, then the downlock releases, the strut retracts, the uplock engages, and finally the doors close.
If a sequence valve drifts even 0.3 to 0.5 seconds out of timing, you end up with the door smacking the strut. That is actually a pretty common write-up on aging A320s.
So what happens when things go wrong? Emergency free-fall extension dumps the hydraulic pressure, releases the uplock mechanically using either a cable or an electric solenoid, and then gravity plus the airloads swing the gear down and locked.
Crews confirm three greens on the panel. If the nose gear hangs partway, a brief yaw maneuver often seats the downlock the rest of the way.
During C-checks, you want to inspect the actuator rod chrome for pitting beyond approximately 0.002 in. You also measure internal bypass leakage, which the FAA AC 43.13-1B caps at around 3 to 5 cc/min for most types.
And honestly, you have to verify sequence valve timing on a gear-swing rig too. Have a look at the FAA AC 43.13-1B for acceptable practices.
Configuration Differences — Tricycle, Tailwheel, Fixed, and Retractable
Direct answer: Configuration changes which components of aircraft landing gear you actually need. Tailwheel designs swap a nose strut for a small steerable tailwheel and often replace oleo struts with leaf springs or bungee cords.
Tricycle nose gear adds a shimmy damper and centering cam that tailwheels don’t need. Fixed-gear aircraft delete actuators, uplocks, sequencing valves, and wheel-well doors entirely, saving roughly 3-approximately 5%[12] of empty weight on light singles, per typical Cessna 172 vs.
172RG comparisons.
Tailwheel mains carry 80-approximately 90%[13] of aircraft weight on two wheels far ahead of the center of gravity. That geometry makes oleo struts overkill, so a Piper Cub uses bungee cords and a Cessna 180 uses a tapered steel leaf spring, cheap, light, no seals to leak.
The tailwheel itself needs a steering chain linked to the rudder pedals and an unlock detent for full-castering ground turns.
Tricycle nose gear sits ahead of the CG, so any side load wants to flutter the wheel left-right at 30+ knots. That’s why every retractable tricycle includes a shimmy damper (hydraulic piston resisting rotation) plus a centering cam that aligns the wheel before it tucks into the well.
See FAA Airplane Flying Handbook for ground-handling specifics.
| Configuration | Best mission | Components deleted |
|---|---|---|
| Fixed tricycle | Training, paved fields | Actuators, uplocks, doors |
| Retractable tricycle | Cruise speed > 140 kt | None — full assembly |
| Fixed tailwheel | Bush, grass, gravel | Nose strut, shimmy damper |
Materials and Weight Trade-offs Across the Assembly
Material choice across the components of aircraft landing gear is a four-way fight between strength, weight, cost, and inspection burden. No single alloy wins, engineers pick per part.
Struts. 300M ultra-high-strength steel dominates main strut cylinders at roughly 280,300 ksi tensile strength. It’s cheap to forge but corrodes aggressively if the cadmium plating cracks, which is why FAA AMT guidance mandates regular plating inspections.
Titanium Ti-approximately 10V[14]-2Fe-3Al, used on the Boeing 777 main gear beam, cuts strut weight by about 20%[15] versus steel but triples raw material cost. 7075-T6 aluminum appears mostly in lighter business-jet gear, where loads stay under the alloy’s fatigue ceiling.
Brakes. Carbon-carbon brakes replaced steel on most narrow-bodies after 2000. A 737NG carbon conversion saves approximately 550,700 lb[16] per shipset; widebody conversions like the 747-400 saved over 1,800 lb[1]. Carbon also lasts 2,3× more landings, but a single rotor costs approximately $8,000[2],approximately $15,000.
Wheels. Forged 2014-T6 aluminum is standard. Composite wheels remain experimental; certification fatigue testing is brutal because wheel failures are catastrophic.
The honest trade: every pound saved aloft costs you in either dollars, inspection hours, or fatigue-life uncertainty.
Frequently Asked Questions
How often is landing gear overhauled?
Most commercial gear assemblies get a full overhaul every 10 years or 20,000 cycles, whichever comes first, per the manufacturer’s CMM (Component Maintenance Manual). A Boeing 737NG main gear overhaul runs roughly $400,000[3],approximately $700,000 per ship-set and includes NDT (non-destructive testing) of every forging, chrome strip-and-replate of strut chrome.
And replacement of all seals and bushings.
What causes a gear collapse on landing?
Three culprits dominate NTSB reports: failure to fully extend and lock (pilot or hydraulic fault), side-load fracture of the drag or side brace, and corrosion-driven fatigue cracks in the trunnion. The NTSB accident database shows down-lock issues account for a disproportionate share of retractable single-engine incidents.
Why nitrogen instead of air in struts?
Nitrogen is dry and inert. Shop air carries moisture that corrodes the strut bore from the inside, and oxygen plus hot hydraulic fluid can flash-ignite, the so-called “tire-and-wheel explosion” risk. Servicing expected level is typically approximately 95%[4]+ pure N2.
How do pilots verify gear-down when a light fails?
Standard procedure: pull the bulb and swap it with another position, check the circuit breaker, then use the mechanical viewers (small mirrors or sight glasses on the floor or wing) that show the down-lock pins among the components of aircraft landing gear are physically engaged. If still unsure, fly the tower for a visual.
Putting the Nine Components Together
Every landing transfers roughly 1.5 to 2 times the aircraft’s weight through one continuous chain: tire contact patch → wheel → axle → torque link → shock strut → drag/side brace → trunnion → airframe. Break any link and the rest fails within milliseconds.
That’s why the FAA’s Aviation Maintenance Technician Handbook (Airframe) treats the gear as a single system, not nine parts.
Use this walkaround checklist before every flight. Each item ties back to a failure mode covered earlier:
- Tires — check tread depth, sidewall cuts, and cord exposure. Under-inflation by approximately 10%[5] doubles sidewall flex heat.
- Wheels — look for cracks at bolt holes and a popped thermal fuse plug (signals a hot brake event).
- Brakes — measure wear pin extension; a flush pin means pads are at limit.
- Axle and torque links — grip the scissor and shake. Any lateral play means worn apex bushings.
- Shock strut — measure exposed chrome. Less than expected level (often 4 inches on a 737) means low nitrogen or oil.
- Drag/side braces and down-lock — confirm the over-center lock is fully seated and lock pin springs are intact.
- Actuator and hydraulic lines — wet seals or red fluid streaks indicate seal failure before retraction loss.
- Wiring and proximity sensors — chafed looms near the trunnion cause false “gear unsafe” indications.
Treat the components of aircraft landing gear as a load path, not a parts list, and inspection time drops while catch rates climb.
References
- [1]sciencedirect.com
- [2]en.wikipedia.org
- [3]safefly.aero
- [4]studyaircrafts.com
- [5]skybrary.aero
- [6]baspartsales.com
- [7]wikipedia.org
- [8]sciencedirect.com/topics/materials-science/landing-gear
- [9]sheffield.com/2023/parts-of-aircraft-landing-gear.html
- [10]skybrary.aero/articles/landing-gear
- [11]youtube.com/watch
- [12]en.wikipedia.org/wiki/Landing_gear
- [13]safefly.aero/aircraft-landing-gear-system-complete-guide-to-components-types-…
- [14]baspartsales.com/landing-gear-brakes-wheels/
- [15]studyaircrafts.com/landing-gear-system
- [16]slideshare.net/slideshow/aircraft-landing-gear-system/29517894
