What is aircraft landing gear, and how does it work?
Learn what aircraft landing gear means, its parts and types, how retractable gear works, when it is lowered, and how pilots handle faults.
Aircraft landing gear is the complete undercarriage that supports and controls an aeroplane on the ground. It includes the legs, wheels or other surface-contact devices, shock absorbers, brakes and steering, together with retraction, locking and position-indication equipment where fitted. It carries weight, absorbs touchdown loads and enables taxiing, take-off and landing.
What does landing gear mean on an aircraft?
Landing gear means the aircraft’s entire ground-support system, not just its wheels. In our Aviation & Real-World Flying coverage, we use landing gear and undercarriage as equivalent terms; the latter is especially common in British aviation.
Aircraft landing gear is the formal phrasing, while plane landing gear and airplane landing gear mean the same thing. The term is normally collective: an aeroplane with three legs has one landing gear system. “Landing gears” is appropriate when discussing several aircraft or different gear designs, but not usually for the individual legs. The standard spelling is two words.
Most aeroplanes use wheels, but floats, skis and helicopter skids also qualify as landing gear. Amphibious aircraft may combine floats or a boat-shaped hull with retractable wheels.
Which parts make up aircraft landing gear?
A landing gear system combines load-bearing structure, energy absorption, ground control and, on retractable installations, machinery that moves and locks the gear.
| Component | What it does |
|---|---|
| Legs, trunnions, axles and braces | Transfer vertical, braking and sideways loads between the runway and the airframe. Drag braces and side braces also maintain alignment and may form part of the locking mechanism. |
| Shock strut or spring | Absorbs landing and taxi loads. An oleo-pneumatic strut uses compressed gas as its spring and metered hydraulic fluid for damping; lighter aircraft may use spring steel, composites, rubber bungees or other arrangements. |
| Wheels and tyres | Carry the aircraft and provide the runway contact patch. Pressure and construction are specific to the aircraft and operating surface. |
| Wheel brakes | Convert motion into heat during deceleration. Brakes are usually fitted to the main wheels; some aircraft also have braked nose wheels. Anti-skid systems regulate pressure to prevent sustained wheel locking. |
| Steering and shimmy damping | Provide directional control and suppress rapid nose-wheel or tailwheel oscillation. Steering can use rudder pedals, a tiller, differential braking or a combination. |
| Actuators, doors and locks | Move retractable gear, streamline the wheel bays and secure each leg in its selected position. |
| Position and weight-on-wheels sensors | Report gear position and distinguish flight from ground operation. Their signals may also affect spoilers, braking, thrust reversers and other aircraft systems. |
The tyres and shock absorbers handle much of the vertical energy, but the structure must also tolerate braking loads and crosswind side loads. Flaps, spoilers and thrust reversers help the aircraft land or stop, but they are not parts of the landing gear.
How does the landing gear of an aircraft work?
Fixed gear remains exposed throughout the flight, while retractable gear folds into reinforced bays to reduce aerodynamic drag. Larger aircraft commonly use hydraulic actuators; lighter designs may use electric motors, manual mechanisms or, less commonly, pneumatic power.
Hydraulic systems are suited to landing gear because they can transmit large forces through compact actuators. Our explanation of how aircraft hydraulic pressure operates high-load systems covers the pumps, valves, fluid and actuators behind that movement.
How does retractable landing gear extend and retract?
A typical retractable system releases its locks, moves the legs in a controlled sequence and mechanically secures them before confirming their position to the crew.
- The pilot selects the gear: a lever or switch sends an electrical, hydraulic or mechanical command. A weight-on-wheels interlock commonly inhibits accidental retraction while the aircraft is on the ground.
- Doors and uplocks release: doors open where required, and each uplock releases the retracted leg. Door arrangements vary; some remain open whenever the gear is extended.
- The actuators move the legs: hydraulic rams, electric motors or mechanical linkages drive the gear. Gravity and airflow may assist movement on some designs.
- The downlocks engage: hooks, locking pins or over-centre braces secure the extended gear. On most aircraft, hydraulic pressure alone is not relied upon to keep the gear safely down.
- Sensors confirm the result: cockpit indications change when the required position and lock switches agree. A leg that is moving or not positively locked produces an in-transit or unsafe indication.
- Retraction reverses the sequence: after take-off, the downlocks release, the gear retracts into its bays and uplocks hold it there. Enclosing doors then close if fitted.
The exact order differs by aircraft. Some doors are attached directly to the legs; others have their own actuators and sequencing valves. The worked Airbus A320 gear sequence for simulator users shows how doors, uplocks, downlocks, sensors and gravity extension interact on a specific transport-category design.
What happens to plane landing gear at touchdown?
At touchdown, the tyres spin up to runway speed and the shock struts compress, transferring the impact into the aircraft over a controlled stroke instead of one abrupt load.
On most tricycle-gear aeroplanes, the main wheels touch first because they are designed to carry the primary landing load. As lift decays and weight settles onto the wheels, braking becomes more effective. Weight-on-wheels logic may then enable or command ground spoilers, anti-skid braking, nose-wheel steering and thrust-reverser functions, depending on the aircraft.
The pilot lowers the nose wheel without forcing it onto the runway. A nose-first touchdown, excessive sink rate or unresolved sideways drift can overload tyres, braces and attachment points even when the gear does not immediately collapse.
When should pilots lower the landing gear?
Pilots lower the gear at the point specified by the aircraft procedure and early enough to verify a down-and-locked indication before the approach must be stable.
There is no universal altitude or distance. Extension creates considerable drag, so crews may use it to help slow the aircraft and establish the descent. Selecting it too early wastes energy and fuel; selecting it late can leave the aircraft fast, unstable or vulnerable to a gear-up landing.
Two published limits govern the decision. VLO is the maximum landing-gear operating speed and may have separate values for extension and retraction. VLE is the maximum speed with the gear extended. The aircraft flight manual, operating handbook and checklist always control; a limit from another model must not be assumed to apply.
What are the main types of plane landing gear?
The principal wheeled layouts are tricycle, conventional tailwheel and tandem gear, while floats, skis and skids serve specialised operating surfaces.
| Type | Arrangement | Main characteristics |
|---|---|---|
| Tricycle | Main gear behind the centre of gravity and a nose gear at the front | Used by most modern trainers and airliners. It offers good forward visibility, stable braking and relatively straightforward ground handling. |
| Conventional or tailwheel | Main gear ahead of the centre of gravity and a small wheel or skid at the tail | Provides good propeller clearance and can suit rough surfaces, but is more prone to directional instability and ground loops. |
| Tandem or bicycle | Main units arranged along the fuselage centreline, usually with wingtip outriggers | Used on certain slender-wing military and specialist aircraft. Outriggers prevent the aircraft tipping sideways. |
| Floats, skis or skids | Surface-specific supports replacing or supplementing wheels | Allow operation from water, snow or prepared landing areas, but may limit ordinary ground movement. |
A bogie or truck is a beam carrying several wheels on one leg; it is not a separate whole-aircraft layout. Heavy aeroplanes use bogies and multiple main legs to distribute weight, reduce pavement loading and provide sufficient tyre and braking capacity.
What is the difference between main gear, nose gear and tailwheel gear?
The main gear carries most of the aircraft’s weight and landing load, while the nose gear or tailwheel supports the other end and assists with directional control.
On a tricycle aircraft, the main legs sit close behind the centre of gravity and normally carry the wheel brakes. The nose gear supports the forward fuselage and provides low-speed steering, but it is not intended to absorb the main touchdown impact.
A tailwheel aircraft has its centre of gravity behind the main wheels. Any developing swing therefore tends to increase unless corrected promptly, which is why taildraggers demand accurate rudder and brake inputs during take-off and landing.
Why are some landing gears fixed and others retractable?
Fixed landing gear is chosen when simplicity, low weight and durability matter more than drag; retractable gear is used when the performance gain justifies added weight, cost and maintenance.
- Fixed gear suits many trainers, utility aircraft and rough-field machines. It has fewer actuators, locks, doors and warning circuits to inspect or fail. Wheel fairings can reduce drag but may be unsuitable in mud, snow or rough conditions.
- Retractable gear reduces drag and can improve climb, cruise speed and fuel efficiency. Its disadvantages include extra mass, more complex inspections and the possibility of an extension fault or inadvertent gear-up landing.
At the speed of a basic trainer, permanent gear may be the better engineering trade-off. On a fast piston aircraft, turboprop or jet, leaving the wheels exposed would impose a much larger drag penalty.
How do pilots know the landing gear is down and locked?
Pilots confirm landing gear position through dedicated indicators, warnings and the landing checklist rather than by relying on the position of the cockpit lever.
A green light commonly means that a downlock switch reports a safe condition. On aircraft with three independently indicated legs, “three greens” means all three report down and locked. Colours, symbols and system logic vary, however; modern aircraft may use a synoptic display or text message instead of three separate lights.
An unsafe indication can mean that a leg is still moving, not fully extended or not locked. A defective switch, broken wire or incorrect sensor rigging can produce the same warning even when the external model appears normal. The gear must still be treated as unsafe until the aircraft-specific procedure establishes otherwise.
Gear-position sensors and weight-on-wheels sensors have different jobs. A leg can be down and locked while the aircraft is airborne, while a failed weight-on-wheels signal can make brakes, spoilers or steering behave as though the aircraft were still in flight.
What can go wrong with aircraft landing gear?
Common faults affect extension, locking, indication, tyres, brakes, steering or the structure itself, and one symptom can have several possible causes.
| Fault | Typical causes and implications |
|---|---|
| Unsafe or disagree indication | The leg may be between positions or unlocked, but a switch, wiring or rigging fault can produce the same indication. |
| Failure to extend or retract | Possible causes include lost hydraulic pressure, electrical failure, a defective actuator, jammed door, damaged lock or mechanical obstruction. |
| Tyre damage or blowout | Under-inflation, foreign-object damage, overheating, excessive braking, a locked-wheel skid or a hard landing can damage the tyre. |
| Brake fade or overheating | Repeated heavy braking or a high-energy rejected take-off can reduce braking performance and damage wheels, tyres or nearby components. |
| Steering fault or shimmy | Wear, incorrect tyre pressure, poor alignment or defective damping can cause weak steering or rapid wheel oscillation. |
| Hard-landing or side-load damage | High sink rate, a nose-first arrival or sideways motion at wheel contact can bend links, overload attachments or collapse a leg. |
What happens if the landing gear will not come down?
If the landing gear does not extend normally, pilots stabilise the aircraft and follow its approved abnormal checklist and alternate-extension procedure.
- Control the aircraft: remain within the applicable gear speed and preserve enough time and fuel to complete the checklist.
- Interpret the indications: determine which legs, locks or doors are reporting an abnormal condition and check the required electrical and hydraulic configuration.
- Use the alternate system: depending on the design, this may release uplocks for gravity extension or use a hand pump, backup pressure source or separate electric motor.
- Confirm what can be confirmed: use cockpit indications and mechanical tell-tales where fitted. An external observation can show gross position but may not prove that an internal downlock is engaged.
- Prepare for the indicated condition: if any gear remains unsafe, the crew plans the landing, emergency response and evacuation considerations according to the aircraft manual.
Repeatedly cycling the gear without checklist authority can exhaust hydraulic fluid, worsen a jam or move a partly secured leg. Our detailed explanation of landing-gear failure causes and the handling logic pilots use covers this abnormal case without replacing an aircraft’s approved flight manual.
How do you fix landing gear problems in a flight simulator?
In a flight simulator, apparent landing gear faults are most often caused by aircraft state, speed, missing system power, assistance features or conflicting control assignments.
- Check the aircraft and operating condition: fixed gear cannot retract, and many retractable aircraft correctly inhibit retraction while weight-on-wheels logic says they are on the ground.
- Compare the lever, indicators and external model: establish whether the command was received and whether the gear is moving. On a detailed aircraft, external animation alone is not proof that a downlock is engaged.
- Inspect every controller assignment: look for duplicate gear-toggle, gear-up and gear-down bindings. A latched hardware switch can instantly reverse a keyboard or virtual-cockpit command.
- Disable unwanted automation: AI piloting, landing assistance and automated checklists may operate the gear without an obvious manual input.
- Restore the required systems: confirm that electrical power, hydraulic pumps and relevant controls are configured correctly. Avoid resetting simulated circuit breakers at random when the aircraft models a specific procedure.
- Respect the speed limits: slow below the published operating limit before repeating an authorised action. Detailed aircraft may simulate overspeed damage or prevent unrealistic movement.
- Check failures and persistent state: an enabled random failure, saved maintenance condition or damage state may survive a flight reload, depending on the simulator and aircraft.
Use the scope of the problem to choose the next check. If every retractable aircraft is affected, suspect a global binding, assistance option or hardware switch. If only one aircraft is affected, suspect its procedure, power configuration, failure state or model-specific systems.
If the gear extends normally but collapses during touchdown, examine sink rate, sideways drift, braking, premature retraction commands and simulated damage rather than the extension system alone. Our guide to diagnosing a landing-gear collapse in a flight simulator separates handling errors and overstress from controller conflicts or aircraft-model faults.