With anti-skid operative, full pedal braking results in a deceleration rate of 8 knots-per-second to 10 knots-per-second on a dry runway.
Optimum Use of Braking Devices
Page 2
Autobrake System:
Autobrake system is designed to provide a selected deceleration rate for enhanced passenger comfort, typically between 3 knots-per-second and 6 knots-per-second, depending on the selected mode.
When a low deceleration rate ( i.e., LOW mode ), the brake pressure is applied after a specific time delay to give priority to the thrust-reversers effect high speed (Figure 3).
Thrust reversers:
Thrust reversers provide a deceleration force that is independent from the runway condition.
Thrust reverse efficiency is higher at high speed (as illustrated by Figure 3); thrust reversers therefore should be selected as early as possible after touchdown (in accordance with applicable SOPs).
Thrust reversers should be returned to reverse idle at low speed ( to prevent engine stall and/or foreign objet damage caused by exhaust gas re-ingestion ) and stowed at taxi speed.
Nevertheless, maximum reverse thrust can be maintained down to a complete stop in an emergency.
Reverse(d) thrust t t t t
Runway conditions
Runway contamination increases:
.
Impingement drag (i.e., the drag resulting from the spray of contaminant striking the landing gear and airframe); and,
.
Rolling drag (i.e., the displacement drag).
Runway contamination also affects the braking efficiency.
The following landing distance factors are typical on wet or contaminated runway :
Factors
Wet 1.3 to 1.4
Standing water or slush 2.0 to 2.3
Compacted snow 1.6 to 1.7
Ice 3.5 to 4.5
Table 1
Landing Distance Factors
Wet or Contaminated Runway
Optimum Use of Braking Devices
Page 3
Understanding a Typical Landing Roll
Figure 3 illustrates a typical landing roll and shows
the contribution of the different deceleration forces
to the total stopping force, as a function of
decelerating airspeed (i.e., from touchdown speed
to taxi speed).
The ground spoilers are armed and the autobrake
system is selected with the LOW mode (i.e., for time
delayed brake application).
The autobrake demand in LOW mode (typically a
3 knots-per-second constant deceleration-rate) is
equivalent, at given gross weight, to a constant
stopping force ( pink dotted line ).
At touchdown, the ground spoilers automatically
extend and the thrust reversers are selected with
the maximum reverse thrust.
Typical Decelerating Forces during Landing Roll
Stopping Force Autobrake LOW Mode
Total Stopping Force
The resulting total stopping force ( red curve ) is the combined effect of:
.
Aerodynamic drag ( green curve );
.
Reverse thrust ( blue curve ); and,
.
Rolling drag ( purple curve ).
During the initial landing roll, the total stopping force already exceeds the autobrake demand.
Autobrake activation is, thus, inhibited:
.
as long as the total stopping force exceeds the autobrake demand; or,
.
until the autobrake time delay has elapsed.
As airspeed decreases, the total stopping force decreases because of a corresponding decrease in:
.
aerodynamic drag; and,
.
reverse thrust (i.e., decreasing reverse efficiency).
When the total stopping force becomes lower than the autobrake demand (or when the autobrake time delay has elapsed) the wheel brakes begin contributing to the total deceleration and stopping force.
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