Modern Hydro-Aire Automatic Braking (Autobrake)
systems provide two modes of operation:
1. Landing Mode
2. Refused Take-Off (RTO) Mode
An Autobrake Selector Switch (in the cockpit)
allows the pilot to arm the RTO feature prior to takeoff or to select from
several deceleration levels for landing. The autobrake system generally consists
of the following:
- Cockpit-mounted selector switch
- Hydraulic manifolds containing a solenoid and a
servo valve (certain systems have a dual, two manifold arrangement)
- Control electronics, which in many cases, is
included in the brake system control unit
In Landing Mode, metered pressure is automatically
applied to the brakes after touchdown independent of the pilots brake pedals.
The system regulates brake pressure to compensate for the effects of aircraft
drag, thrust reversers, and spoilers to maintain the selected deceleration
level. Aircraft manufactures usually request a system with three levels of
Although five levels (1, 2, 3, 4, and MAX) can be
provided, the lower settings produce moderate braking levels from 2 to 8 ft/sec.
The maximum settings are more aggressive and can be arranged to use all the
available runway friction provided by the antiskid system. In some applications,
different deceleration levels can be selected by an external wiring
configuration to permit differing deceleration levels for airlines with unique
operational requirements. The pilot can select a different deceleration level
during landing roll-out if so desired.
During the roll out, application of pedal braking
transfers control back to the pilot. An advanced feature is available that
compensates for pilot braking technique and makes the transfer exceptionally
smooth. A Landing Autobrake System has significant advantages compared with
standard pedal brakes. The smooth and steady application of brakes symmetrically
across the aircraft improves lateral stability and ensures a constant
This greatly enhances passenger ride comfort during
the landing. The brakes are always applied at the same point in the touchdown
sequence and the deceleration rate is constant which makes stopping distances
consistent and predictable. The pilot can pre-select a runway turnoff with a
high degree of confidence. Operation under Cat IIIb auto-land conditions is
Some studies suggest that brake life can be
extended by the use of autobrakes.
RTO autobrakes have been certified as the primary
means of stopping the aircraft during a rejected take-off. With the system armed
during takeoff, movement of the thrust levers back to the fully retarded
position or operation of the thrust reversers will trigger full brake
application. This automatic feature reduces crew workload during an emergency
and frees resources for dealing with the cause of the rejected take-off.
The prompt, sustained application of full system
pressure to the brakes minimizes stopping distance and reduces the probability
of runway excursion. As with the Landing Mode, application of pedal braking
transfers control back to the pilot.