The probe extends through the skin of the aircraft
indicator responds to movement of the transmitter
into the airstream.
potentiometer. Figure 6-32 shows the relationship
of the indicator to the lights and stall warning.
The exposed end of the probe contains two
The AOA indexer on the pilot's glare shield has
parallel slots (ports). These slots detect the
two arrows and a circle illuminated by colored
differential airflow pressure (fig. 6-30). Air from
the slots passes through two separate air passages
lamps to provide approach information. The cam-
to separate compartments in a paddle chamber.
actuated switch contacts in the angie-of-attack
Any differential pressure, caused by misalignment
indicator also control the angle-of-attack indexer.
The upper arrow is for high angle of attack
of the probe to the direction of airflow, causes
(green). The lower arrow is for low angle of attack
the paddles to rotate. The moving paddles rotate
(red). The circle is for optimum angle-of-attack
the probe, through mechanical linkage, until the
pressure differential is zero. This occurs when the
(amber). An arrow and a circle together show an
slots are symmetrical with the airstream direction.
intermediate position.
The indexer lights function only when the
Two potentiometer wipers, rotating with the
landing gear is down. A flasher unit causes the
probe, provide signals for remote indications.
Probe position, or rotation, is converted into an
indexer lights to pulsate when the arresting hook
electrical signal by one of the potentiometers that
is up with the HOOK BYPASS switch in the
is the transmitter component of a self-balancing
CARRIER position.
bridge circuit. When the angle of attack of the
STALL WARNING SYSTEM
aircraft changes, the position of the transmitter
potentiometer alters. This causes an error voltage
to exist between the transmitter potentiometer and
Many aircraft have stall warning indicators to
warn the pilot of an impending aerodynamic stall.
the receiver potentiometer in the indicator.
In the past, stall warning indicators were of a
Current flows through a sensitive polarized
pneumatic control type. These devices activated
relay to rotate a servomotor located in the
indicator. The servomotor drives a receiver
either warning horns or flashing lights. Later,
research found that a stall relates directly to the
potentiometer in the direction required to reduce
the error voltage. This action restores the circuit
angle of attack, regardless of airspeed, power
setting, or aircraft loading. The stall warning
to a null or electrically balanced condition. The
devices of most aircraft now in the fleet operate
polarity of the error voltage determines the
at a specified angle of attack. The devices operate
resultant direction of rotation of the servomotor.
The indicating pointer is attached to, and moves
through cams in the angle-of-attack indicator. The
with, the receiver potentiometer wiper arm to
cam-driven switch activates a vibrator motor
connected to either a rudder pedal or the control
show on the dial the relative angle of attack.
Adjustable cam switches (fig. 6-31) control the
approach light, approach indexer, and stall
warning. The cams (switches) operate as the
Figure 6-30.-Mechanical schematic of airstream direction
detector.
Figure 6-31.-Angle-of-attack indicator cams.
