TM 1-1510-225-10
3B-176
3B-28. ALTIMETERS.
a. Pilot's Servoed Altimeter.
(1) Description. The pilot's altimeter provides
a
servoed
counter
drum/pointer
display
of
barometrically corrected pressure altitude, derived
from the air data computer system.
(2) Power. Power is provided through a
28-Vdc 1-ampere circuit breaker, placarded PILOT
ALTM, located on the right hand circuit breaker panel
and 26 Vac power is provided through a 1-ampere
circuit breaker, placarded PILOT ALTM, located on
the
circuit
breaker
panel
in
the
nose
gear
compartment.
(3) Operations. The servoed
altimeter
provides the following display.
1. Counter drum display of altitude, in
20-foot increments.
2. Pointer display of altitude between
1000-foot
levels
in
20-foot
graduations.
3. Latitudes
below
10,000
feet
are
annunciated by a black and white
crosshatch on the left digit position of
the counter display.
4. A
barometric
pressure
counter,
manually set by the BARO knob,
displays barometric pressure in inches
of mercury (Hg), and millibars (Mb),
and provides this information tot he air
data computer. A red warning flag in
view indicates the altitude information
is unreliable. However, the MODE C
(altitude reporting) information may be
valid.
In the event of a total aircraft ac and dc
electrical power loss, the warning flag will
be in view, the altimeter will be inoperative,
and the indicated altitude will remain as
existed at the time of failure. A dc power
loss only (retains ac power) usually results
in the altimeter spooling down toward a
lower altitude instead of retaining the
altitude
at
which
the
power
failure
occurred.
The altitude (ALT) alert annunciator, located on
the upper bezel, illuminates to provide a visual
indication when the Aircraft is within 1000 feet of the
pre-selected altitude during the capture maneuver and
extinguishes as the aircraft is within 300 ± 50 feet of
the pre-selected altitude. After capture, the light will
re-illuminate if the aircraft departs more than 300 ± 50
feet from the selected altitude, and extinguish when
the aircraft has departed more than 1000 feet from the
selected altitude.
b. Copilot's Pneumatic Altimeter.
(1) Description. The copilot's altimeter is a
pneumatic instrument equipped with a 28-Vdc internal
vibrator. The vibrator is required to overcome friction
and assure instrument accuracy. Altitude is displayed
on the altimeter by 10,000, 1,000, and 100-foot
counter drums and a single needle pointer which
indicates hundreds of feet in 20-foot increments. The
barometric pressure setting knob is provided to
simultaneously adjust the baro counters display in
inches of mercury (Hg) millibars (Mb). Below an
altitude of 10,000 feet, a diagonal symbol will appear
in the 10,000 foot counter.
(2) Power. Power is provided to the copilot’s
pneumatic altimeter through a 28-Vdc 1-ampere circuit
breaker, placarded COPILOT ALTM, located in the
Flight group on the right side circuit breaker panel and
protects the vibrator circuit.
3B-29. RADIO ALTIMETER INDICATOR.
a. Description. The radio altimeter in the
C-12T3 is integrated into the EFIS system. Digital
readout of the radio altitude is displayed in white
above the decision height readout area in the bottom
right hand corner of the EADI. The display appears
automatically when within the radio altitude range
(2500 feet) and changes to 5-foot increments between
0 and 99 feet, 10-foot increments between 100 and
999, and 50-foot increments between 1000 and
2500 feet.
NOTE
Loss of Radio Altitude Data will cause loss
of the TCAS II due to the absence of
required power.
b. Power. Power to the Radio Altimeter is
provided through a 28-Vdc 2-ampere circuit breaker,
placarded RADIO ALT, located on the copilot's right
hand circuit breaker panel.
c. Controls and Functions.
(1) Test Button – A PUSH TST button
located on the EFIS control panel as the center button
of the DH SET knob. Pushing the test button causes
WARNING
