Landing gear, steering and ground loads

A 747's landing gear is a support structure, a shock absorber, a steering system and a retractable mechanism. Its eighteen wheels are only the visible end of a much larger assembly of struts, braces, actuators, locks, sensors and hydraulic lines.

This guide uses the 747-400 as its main example. The classic 747 and 747-8 retain the recognizable five-leg arrangement, but dimensions, loads, control logic and installed equipment differ. The companion brakes guide explains how the aircraft stops.

Five legs, eighteen wheels

Assembly	Number of legs	Wheels per leg	Main role
Nose gear	1	2	Forward support and steering
Wing main gear	2	4	Main support beneath the wing-root region
Body main gear	2	4	Main support beneath the fuselage; steering assistance in turns

The four main legs carry sixteen wheels on four-wheel trucks, also called bogies. Their distribution matters as much as the total wheel count. Wheel spacing, tyre pressure, pavement construction and aircraft loading all affect the stresses transmitted into a runway or taxiway. More wheels alone do not establish that an airport can accept an aircraft.

Boeing's 747-400 airport-planning manual, sections 4 and 7, separates ground manoeuvring from pavement loading. Those are different compatibility questions: a pavement may support the aircraft while a particular turn still lacks sufficient clearance.

The load path from aircraft to pavement

The aircraft's weight reaches the pavement through its structural attachment points, landing-gear legs, wheel axles and tyres. During touchdown the gear must also accommodate vertical motion and transient forces. During braking, longitudinal forces act through the wheels and gear structure; during a turn, lateral forces enter the same structure.

The shock strut controls the landing impact. In a typical air/oil strut, compressed gas behaves like a spring, while oil forced through a restricted passage dissipates energy and controls the movement. The gas and oil perform different jobs. A spring by itself would store energy and return it, encouraging a bounce; damping controls how quickly that energy is released.

This is general landing-gear theory described in the FAA Airframe Handbook, chapter 13. The illustrated principle should not be mistaken for a servicing diagram or a specification for the gas pressure in a particular 747 strut.

Original illustration: Boeing-747.com. Conceptual load path and shock-strut functions; not a cutaway of a particular 747 gear leg.

Retraction is a sequence, not one movement

A gear lever sends a command. Hydraulic actuators provide the energy for movement. Doors, locks and position sensing allow the structure to travel into or out of its stowed position in the required sequence. The mechanism has to end in a secure state, not merely arrive near the desired angle.

This is why selected down, moving down and down and locked are different statements. A lever position records the crew's request; a position or lock indication reports what the sensing system has detected. An unexpected indication can involve a mechanical problem, an unavailable power supply or an indication fault. Looking only at the lever cannot distinguish those possibilities.

The FAA handbook's chapter 13 explains retraction, downlocks, uplocks and landing-gear safety devices. Exact 747 normal and alternate-extension arrangements belong to the applicable aircraft manual. An alternate system provides another means of completing an extension; it should not be assumed to restore every door function or normal retraction capability.

Why the body gear steers

In a tight turn, wheels in different positions on a long aircraft follow different arcs. If every main-wheel truck stayed aligned with the fuselage, some tyres would be dragged sideways. That tyre scrub produces wear and unwanted loads rather than useful forward rolling.

The 747's body gear can turn opposite to the nose gear, improving alignment with those arcs. Boeing describes its 747-400 body-gear steering as hydraulically actuated and electrically programmed in proportion to nose-gear angle. The same source states that the body gear is centered, mechanically locked and depressurized during takeoff and landing. See Boeing, section 4.1.

The benefits include reduced tyre scrub, reduced gear stress and improved manoeuvrability. This is not the same as independently steering every wheel, and the wing gear does not perform the same counter-steering function. A photograph of angled body-wheel trucks on a taxiway can therefore show normal operation.

Steering authority and clearance are different

The flight-deck tiller provides nosewheel-steering input for ground manoeuvring. Rudder-pedal steering has a smaller range on the 747-400. Body-gear steering follows its own enabling logic; it is not continuously active throughout the flight and landing roll.

Even when steering is available, a turn requires clearance for the complete aircraft: the nose, main gear, engines, wing tips and tail do not follow the same path. A nosewheel remaining on pavement does not establish that a main-wheel truck will remain clear of its edge. Nor does a drawing of a theoretical minimum turning radius guarantee the same result on a slippery surface.

Boeing lists aircraft weight, centre of gravity, ground speed, surface conditions, thrust and braking among the factors affecting manoeuvring. Its planning diagrams are therefore a starting point for airport assessment, not a taxiing instruction. This corrects a common oversimplification: retaining the familiar gear layout did not make every airport automatically suitable for the larger 747-8.

Support on the ground still depends on balance

Four main legs provide a wide and distributed support arrangement, but cargo loading still requires an approved sequence and centre-of-gravity limits. There is no general guarantee that a 747 cannot tip onto its tail. The relationship between the aircraft's centre of gravity and its support points changes as passengers, cargo and fuel are added or removed.

Likewise, multiple wheels do not make an unavailable tyre, brake or gear leg harmless. The effect depends on what failed, the remaining equipment, aircraft weight and the relevant procedures. Redundancy creates options; it does not remove structural limits.

Watch the hardware move

Captain Joe's body-gear video below connects the steering actuators to the visible counter-steering motion. The maintenance swing-test video shows a 747-400 gear extension/retraction check. A jacked aircraft is a useful way to see sequencing, but its unloaded motion is not a demonstration of touchdown loads.

For the energy absorbed after touchdown, continue to brakes, autobrake and anti-skid. For the power supply behind gear movement, see hydraulic systems.