Autobrake Tutorial
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
Landing Mode
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 deceleration:
1. MIN
2. MED
3. MAX
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 deceleration rate.
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 enabled.
Some studies
suggest that brake life can be extended by the use of autobrakes.
Takeoff Mode
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.. |
