Learjet 20 Series Pilot Training Manual page 309

STALL WARNING
SYSTEMS
GENERAL
One of three stall warning systems is installed,
depending on the type of wing used. Each system
provides visual and tactile warning of an impend-
ing stall, and includes the following major com-
ponents: left and right stall vanes, transducers,
computer/amplifier, stick shaker motors, angle-of-
attack indicators, L and R STALL WARNING
switches, and red L and R STALL warning lights.
All three systems use the autopilot pitch servo for
stick pusher operation. All three systems are hot
wired to the battery bus.
STALL WARNING INDICATIONS
The systems present indications of approaching
stall and full stall with ANGLE OF ATTACK
indicators, red L and R STALL lights, and the
use of control column shakers, nudgers (on air-
planes other than model 23), and a stick pusher.
Stall Vanes and Transducers
During flight, angle-of-attack vanes (Figure 15-
12) on both sides of the forward fuselage align
with the local airstream. Transducers operated
by these vanes produce voltages proportional to
airplane angle of attack. These voltage signals
are processed by the system computer/amplifier.
ANGLE OF ATTACK Indicators
The computer/amplifier translates signals from
the stall vane transducers into visual indications
of stall margin on the ANGLE OF ATTACK
indicators. The indicator face is divided into
three colored segments—green, yellow, and red.
The green segment represents the normal operat-
ing range, the yellow segment warns of an
approaching stall, and the red segment indicates
that the angle of attack is at or just above aero-
dynamic stall. On airplanes with the Century III
wing, the left and right ANGLE OF ATTACK
indicators are controlled by their respective stall
vane. On airplanes with the MKII or standard
15-18
LEARJET 20 SERIES PILOT TRAINING MANUAL
FOR TRAINING PURPOSES ONLY
Figure 15-12. Angle-of-Attack Vane
wing, each indicator is controlled by the vane on
the opposite side of the fuselage.
Stick (Control Column) Shaker
Shaker motors are attached to the front side of
each control column. Actuation of the shakers
causes low-frequency, high-amplitude vibration
in the control columns.
Stick Pusher
The stick pusher function utilizes the autopilot
pitch servo to reduce angle of attack by decreas-
ing airplane pitch attitude. Pusher activation pro-
vides elevator down motion, causing a sudden
abrupt forward movement of the control column.
Elevator servo actuation during pusher engage-
ment can result in up to 80 pounds of force
applied to the control column if activated simul-
taneously by both systems. However, the pusher
can be engaged by either system independently.
In this event, pusher force is limited to 50
pounds, which is diminished linearly by an
accelerometer to maintain 0.5 g. In the event of
inadvertent pusher engagement due to malfunc-
tion, the pilot can override the pusher.
Nudger
On airplanes other than model 23, a nudger is
incorporated into the stall warning system. As
angle of attack increases to the point of shaker
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