Figure 3-26. Pilots Altitude Indicator (BA-141).
routed to the Air Data Computer. The ADC recomputes
all data on hand, sends corrected altitude pressure
information to the Flight Director and autopilot, servo
commands to correct the display on the pilot's altimeter,
and applies altitude information to the transponder (for
transmission to ground stations).
Controls/Indicators and Functions.
(1) OFF Flag. Presence indicates loss of power
to instrument and unreliable readings.
(2) ALT alert annunciator. Illuminates when
aircraft is within 1000 feet of preselected altitude during
capture maneuver and extinguishes when aircraft is
within 250 feet of preselected altitude. After capture,
annunciator will illuminate if aircraft departs more than
250 feet from the selected altitude.
(3) Altitude Scale. Used with needle to indicate
aircraft altitude in hundreds of feet. Subdivided into 20
(4) IN HG Indicator. Indicates local barometric
pressure in inches of mercury. Adjusted by use of
(5) MB Indicator. Indicates local barometric
pressure in millibars. Adjusted by use of BARO knob.
(6) BARO Knob. Used to manually set
barometric pressure displayed in the MB and IN HG
(7) Needle indicator. Used with altitude scale to
display aircraft altitude in hundreds of feet.
(8) Altitude counter drums. Indicates aircraft
altitude in tens of thousands, thousands, and hundreds
of feet above sea level.
If the OFF flag is visible, either DC
power is off, the fuse has blown, or
there is an altimeter encoder failure.
Since the OFF flag monitors only the
encoder input to the altimeter and
altitude reporting function may be
inoperative without the OFF flag
showing, in the case of transponder