TM 55-1520-228-10
Change 11 2-46.1
side the altimeter whenever the aircraft power is on. The
vibrator is powered through the ALT AAV31/32A[A] or
ALT VIB[C] circuit breaker. The encoder is DC powered
with the vibrator through the ALT AAU31/32A circuit
breaker [A] or separately AC powered through the ALT
ENCDR circuit breaker [C]. If power to the altitude en-
coder is lost, a warning flag placarded CODE OFF will
appear in the upper left portion of the instrument face
indicating that the altitude encoder is inoperative and
that the system is not reporting altitude to ground sta-
tions. The CODE OFF flag monitors only the encoder
function of the altimeter. It does not indicate transponder
condition. The altitude reporting function may be inop-
erative without the AAU-32/A CODE OFF flag showing,
in case of the transponder failure or improper control
settings. It is also possible to get a “good” MODE C test
on the transponder control with the CODE OFF flag
showing. Display of the CODE OFF flag only indicates
an encoder power failure or a CODE OFF flag failure. In
this event, check that encoder power is available and
that the circuit breakers are in. If the flag is still visible,
radio contact should be made with a ground radar site
to determine whether the altitude reporting function is
operative, and the remainder of the flight should be con-
ducted accordingly.
b. Operation.
(1) Normal Operation. The ALT AAU31/32[A]
or ALT VIB and ALT ENCDR[C] circuit breakers should
be in prior to fight. The Mode C switch (M-C) on the
transponder control should be switched to ON for alti-
tude reporting during flight. The AAU-32/A altimeter in-
dicates pneumatic altitude reference to the barometric
pressure level as selected by the pilot. At ambient pres-
sure, altimeters should agree with $ 70 feet of the field
elevation when the proper barometric pressure setting
is set in the altimeter. A red flag marked CODE OFF is
located in the upper left portion of the altimeters face. In
order to supply Mode C information to the IFF trans-
ponder, the CODE OFF flag must not be visible. A vibra-
tor, powered by the DC essential bus, is contained in the
altimeter and requires a minimum of one minute war-
mup prior to checking or setting the altimeter.
(2) Abnormal Operation.
(a) If the altimeters internal vibrator be-
comes in-operative due to internal failure of DC power
failure, the pointer and drum may momentarily hang-up
when passing from 9 through 0 (climbing) or from 0
through 9 (descending). This hang-up will cause lag, the
magnitude of which will depend on the vertical velocity
of the aircraft and the friction in the altimeter.
(b) If the CODE OFF flag is visible, the en-
coder power is not available, the circuit breaker is not in,
or there is an internal altimeter encoder failure.
(c) If the altimeter indicator does not corre-
spond within 70 feet of the field elevation (with proper
local barometric setting) the altimeter needs rezeroing
or there has been an internal failure.
(d) If the baroset knob binds or sticks, ab-
normal force should not be used to make the setting as
this may cause internal gear failure resulting in altitude
errors. Settings can sometimes be made by backing off
and turning at a slower rate.
2-63. PRESSURE ALTIMETER.
NOTE
Not applicable after MWO
55-1520-228-50-24 is applied.
A The pressure altimeter (figure 2-13) furnishes direct
readings of height above mean sea level when properly
adjusted.
2-64. ATTITUDE INDICATOR.
A The attitude indicator (figure 2-13) displays the atti-
tude of the helicopter. The indicator is self contained and
is connected through circuit breakers on the overhead
console to the 115-volt AC bus.
C The attitude indicator (figure 2-14) displays the heli-
copter pitch and roll attitudes in relation to the earth
horizon. Pitch attitude is displayed by the motion of the
sphere with respect to the miniature airplane. Roll atti-
tude is displayed by the motion of the roll pointer with
respect to the fixed roll scale. The sphere can be ad-
justed to zero indication by the pitch trim knob. The rate
of turn pointer indicates in which direction and at what
rate the helicopter is turning. The inclinometer indicates
when the helicopter is in trim, either in a coordinated turn
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