Carbon brakes, anti-skid and autobrake

A 747 landing at weight stops using brakes on all four main-gear bogies - sixteen wheels under the -400. Anti-skid cycles the brakes to prevent locked wheels, and on the -400 an autobrake holds a precise deceleration rate automatically.

Key facts
Main-gear wheels	16 on the 747-400
Brake material	Structural carbon on the -400 vs steel on earlier 747s
Weight saving	About 1,800 lb (820 kg) of unsprung weight vs steel
Autobrake	Holds a selected deceleration rate; introduced with the -400

Carbon brakes on the 747-400 saved almost a tonne versus the steel brakes of classics while absorbing far more kinetic energy - carbon actually grows stronger into the temperature range where steel begins to fade.

The -400's autobrake and anti-skid system combines hydraulic brake power with digital monitoring and automatic pressure control to hold consistent deceleration across varying runway conditions, including reduced braking on wet or icy surfaces.

Carbon and the unsprung-mass argument

The case for carbon on the 747-400 was weight, and specifically unsprung weight. A conventional steel brake disc is heavy, and because it rotates with the wheel, that weight is carried by the landing gear and the structure supporting it. Removing roughly 1,800 lb of rotating mass across sixteen wheels was a meaningful saving on an aircraft already at the edge of what its landing gear and tyre could absorb.

Carbon also handles energy better in the relevant range. A 747 at maximum landing weight carries an enormous amount of kinetic energy, and it has to be dissipated in a few seconds. Carbon's friction behaviour stays strong through the temperatures a heavy stop produces, where steel begins to fade.

Anti-skid and autobrake

Anti-skid is the safety layer: it monitors each wheel's rotation and modulates brake pressure to keep the wheel from locking, because a locked wheel loses most of its braking and, on a heavy jet, can be structurally damaging. On the -400 this is a digital, per-wheel system.

Autobrake sits on top of it as an automation and consistency aid. Once selected, it applies the brakes on landing to hold a chosen deceleration rate, and it works out the appropriate pressure through the anti-skid system. That gives crews a repeatable, less workload-intensive stop on the long, heavy landings the type flies, and it adapts the target to the runway conditions it detects.

Related

• Landing gear
• Hydraulic systems
• Flight controls
• 747-8 systems differences