(2) Displays. The servoed altimeter provides
the following displays:
1. Counter drum display of altitude, in
2. Pointer display of altitude between
annunciated by a black and white
crosshatch on the left digit position of
the counter display.
barometric pressure counter, manually set by the
BARO knob, displays barometric pressure in inches of
mercury (Hg.), and millibars (Mb.), and provides this
information to the 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
(4) Altitude Alert Annunicator. The Altitude
Alert Annunciator (ALT), located on the upper bezel,
illuminates to provide a visual indication of 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
copilots 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 that 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.), and millibars (Mb.).
Below an altitude of 10,000 feet, a diagonal symbol will
appear in the 10,000-foot counter. A circuit breaker,
placarded COPILOT ALTM, located in the flight group
on the right side circuit breaker panel protects the
3C-28. RADIO ALTIMETER INDICATOR.
a. General. Displays radio altitude information
from 2500 feet to touchdown with an expanded linear
scale under 500 feet.
Refer to Figure 3C-20.
(1) Decision Height Annunciator Lights to
alert that aircraft is at or below selected DH.
decision height bug is manually set by knob.
1. DH (Decision Height) Annunciator
2. Decision Height Bug
3. Failure Warning Flag
4. DH (Decision Height) SET Knob
5. TEST Push Button
Figure 3C-20. Radio Altimeter
(3) Failure Warning Flag A failure warning
flag in view indicates the system information may be
(4) Decision Height Set Knob Sets the
radio altimeter decision height marker to a decision