Figure 6-25.-AAU-19/A altimeter.
power must be on. During standby operation, a
pointer shows hundreds of feet on a circular scale.
red STBY flag appears on the dial face. The
The barometric pressure setting (baroset) knob is
altimeter automatically switches to standby
used to insert the local pressure in inches of
operation during an electrical power loss or when
mercury. The baroset knob has no effect on the
the altimeter or altitude computer fails. The
digital output (mode C) of the ADC. This digital
standby operation is selected by placing the mode
output is always referenced to 29.92 inches of
switch to STBY. An ac-powered internal vibrator
mercury.
automatically energizes in the standby mode to
The altimeter has a servoed mode and a
lessen friction in the display mechanism.
pressure mode of operation. The mode of
With the local barometric pressure set, the
operation is controlled by a spring-loaded,
altimeter should agree to 75 feet of field
self-centering mode switch, placarded RESET and
elevation in both modes.
STBY. In the servoed mode, the altimeter displays
altitude, corrected for position error, from the
synchro output of the air data computer. In the
AAU-21/A ALTIMETER. -- A A U - 2 1 / A
standby mode, the altimeter operates as a
altimeter is used in low/slow aircraft. It has a
standard altimeter. In this mode, it uses static
counter-drum-pointer display similar in appear-
pressure from the static system that is uncorrected
ance to the AAU-19/A. The altimeter contains
for position error.
a servo-driven encoder. The encoder provides an
altitude signal to the aircraft transponder for
The servoed mode is selected by placing the
transmission to a ground station.
mode switch to RESET for 3 seconds. The ac
6-23
