TM 55-1510-221-10
Figure 3-28. Pilot’s Altimeter Indicator (BA-141)
3-33. PILOT’S ALTIMETER INDICATOR.
The pilot’s altimeter, on the upper left side of
the instrument panel (fig, 3-28), is a servoed unit
under control of the Air Data Computer and is part
of the Flight Director/Autopilot system. Altitude is
displayed by a 10,000 foot counter, a 1000 foot
counter, a 100 foot counter, and a single needle
pointer (coupled with the 100 foot counter) which
indicates hundreds of feet on a circular scale in 20
foot increments. Below an altitude of 10,000 feet, a
diagonal striped symbol will appear on the 10,000
foot counter. The barometric pressure knob allows
ground supplied pressure values to be adjusted and
displayed in inches Hg or millibars. If AC power to
the altimeter is lost, a warning OFF flag will appear
in the upper counter drum display window to indi-
cate power loss, unreliable altimeter readings, and
possible loss of encoder transmissions to ground sta-
tions. Circuits are protected by a 3-ampere fuse in a
junction box.
When the BARO knob is adjusted to ground
supplied instructions, the updated altitude pressure
is routed to the Air Data Computer. The ADC
recomputes all data on hand, sends corrected alti-
tude pressure information to the Flight Director and
autopilot, servo commands to correct the display on
the pilot’s altimeter, and supplies altitude informa-
tion to the transponder (for transmission to .
a.
Controls/Indicators and Functions.
(1.) ALT alert annunciator. Illuminates
when aircraft is within 1000 feet of preselected alti-
tude during capture maneuver and extinguishes
when aircraft is within 250 feet of preselected alti-
tude. After capture, light will illuminate if aircraft
departs more than 250 feet from the selected alti-
tude.
(2.) Altitude counter drums. Indicates air-
craft altitude in tens of thousands, thousands, and
hundreds of feet above sea level.
(3.)
IN HG Indicator. Indicates local baro-
metric pressure in inches of mercury. Adjusted by
use of BARO knob.
(4.) Needle indicator. Indicates aircraft
altitude in hundreds of feet with subdivisions at 20
foot increments.
(5.) BARO Knob. Used to manually set
barometric pressure displayed in the MB and IN
HG windows.
3-83
