Altitudes below 10,000 feet are annunciated by a
black and white crosshatch on the left digit position of
the counter display.
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 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 re main 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
The altitude alert annunciator (ALT), located on
the upper bezel, illuminates to provide a visual
indication when the aircraft is within 1,000 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 1,000 feet from 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 count 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 bare 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 T3 F3 COPILOT ALTM and R COPLT VS
& ALTM, located in the Flight group on the right side
circuit breaker panel, protects the vibrator circuit.
Refer to Figure 2-6, Sheets 1 through 5.
c. Radio Altimeter Indicator F3 . Displays
radio altitude information from 2,500 feet to touchdown
with an expanded linear scale under 500 feet.
(1) The radio altimeter provides the following
displays. Refer to Figure 2-37.
1. Radio Alt Pointer display of radio
altitude from 0 to 2,500 feet.
manually set by knob.
3. Annunciator light to alert that aircraft is
at or below selected DH.
4. A failure warning flag in view indicates
5. Test button to check indicator R/T unit
and flag operation.
Operating the test button causes the flag to
come into view and altitude pointer to indicate
approximately 100 feet. Release of button causes
pointer to return to existing altitude and flag to retract.
The radio altimeter also provides altitude and
decision height information to the ground collision
avoidance system (GCAS). Setting the radio altimeter
decision height marker to a decision height altitude
with the DH SET knob will cause the GCAS to
announce the "Minimums, Minimums" aural warning
when the aircraft transitions through the selected DH
1. Decision Height Annunciator
2. Decision Height Bug
3. Decision Height Flag
4. Decision Height Set Knob
5. Test Push Button
Figure 2-37. Radio Altimeter Indicator F3