14-13
FOR TRAINING PURPOSES ONLY
If a takeoff is made in slush or snow, the wheels
should be allowed to spin down for approximate-
ly one minute prior to gear retraction. This mini-
mizes the possibility of brake freezing by sling-
ing off accumulated slush. If frozen brakes are
suspected after the gear is extended for landing,
brakes should be applied several times to break
up any possible ice formations.
On model 25 airplanes, this requires placing the
ANTISKID switch in OFF prior to brake actua-
tion, then back to ON.
ANTISKID SYSTEMS
General
The antiskid system limits braking on each main
gear wheel independently to allow maximum
braking under all runway conditions without tire
skidding. One of two antiskid systems may be
installed. Model 25 airplanes have the Hydroair
system. Model 23 and 24 airplanes are equipped
with the Goodyear system. On all models, anti-
skid is inoperative during emergency braking.
Hydroair System
This fully modulated system consists of two
antiskid valves, four wheel speed transducers
(one on each main wheel), a control box, four
red ANTI-SKID GEN lights, and a lever-locking
ANTISKID switch on the center instrument
panel. Model 25 airplanes through –204, except
–197, have an ANTISKID TEST switch. Model
25 airplanes with a Century III wing do not have
a test switch. System malfunction is indicated by
illumination of the ANTI-SKID GEN lights
when the ANTISKID switch is on.
The antiskid system is not required to be opera-
tional for flight. However, if a malfunction is
indicated by illumination of a red ANTI-SKID
GEN light(s), it must be assumed that antiskid
protection is lost on the associated wheel(s).
Takeoff and landing data must be computed
accordingly.
Operation
Antiskid operation requires:
• The ANTISKID switch must be on.
• Each squat switch must be in the ground
mode for its respective brake to function.
• The parking brakes must be released.
• The wheels must be rotating at least
150 rpm.
Above 150 rpm, with the ANTISKID switch ON
and brakes applied, the control box receives and
analyzes wheel speed inputs from the transducer
on each main wheel (see Figure 14-11). If the
wheel deceleration rate is higher than a predeter-
mined limit, the applicable servo valve will indi-
vidually regulate braking force on the corre-
sponding brake by releasing braking pressure
into the return line.
A fault in the system is indicated by illumination
of the respective ANTI-SKID GEN light.
Cycling the ANTISKID switch to OFF then
back to ON may clear the fault. All four lights
illuminate if power to the control box is lost or if
the ANTISKID switch is OFF. An ANTISKID
TEST switch is located on the test switch panel
forward of the thrust lever on airplanes without
Century III wings. Refer to the approved AFM
for test procedures.
At low taxi speeds (wheel speed below 150
rpm), the antiskid system is inoperative. The sys-
tem is automatically disengaged when the park-
ing brakes are set; however, the red ANTI-SKID
GEN lights will not illuminate. To apply the
brakes when airborne, place the ANTISKID
switch to OFF.
Goodyear System (All Model 23
and 24 Airplanes)
This partially modulated system consists of four
solenoid-operated antiskid valves, four wheel
speed generators (one on each main wheel), a
printed circuit board, monitor lights (24-112 and
subsequent), and a lever-locking ANTISKID
switch on the instrument panel.
LEARJET 20 SERIES PILOT TRAINING MANUAL
FlightSafety
international
