The airspeed indicator (figure 2-13 and figure 2-14) displays the air speed of the helicopter indicated in knots. The airspeed is obtained by measuring the difference between impact air pressure from the pitot tube and the static air pressure from the static ports.
2-62. AAU-31/A PNEUMATIC ALTIMETER.
1. Description. The AAU-31/A pneumatic
counter-drum-pointer altimeter is a precision pressure altimeter (figure 2-14). Pressure altitude is displayed by a 100-foot drum and a single pointer indicating hundreds of feet on a circular scale, with 50’ center markings. Below an altitude of 10,000 feet, a diagonal warning symbol will appear on the 10,000-foot counter.
A barometric pressure setting knob is provided to insert the desired altimeter setting in inches of Hg. A DC powered vibrator operates inside the altimeter whenever aircraft power is on.
2. Operation.
a. Normal Operation. The ALT VIB C or
ALT AAU 31/32A A circuit breaker must be in. The altimeter indicates pneumatic altitude reference to the barometric pressure level as selected by the pilot. A vibrator, powered by the DC essential bus, is contained in the altimeter and requires a minimum of one minute warmup prior to checking or setting the altimeter.
b. Abnormal Operation. If the
altimeter's internal vibrator becomes inoperative due to internal failure or 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. Pilots should be especially watchful for this type failure when the minimum approach altitude lies within the "8" - "1" part of the scale (800-1100, 1800-2100, etc.).
2-62.1. ALTITUDE ENCODER/PNEUMATIC ALTIMETER AAU-32/A.
a. Description. The AAU-32/A pneumatic
counter-drum-pointer altimeter is a self-contained unit which consists of a precision pressure altimeter combined with an altitude encoder (fig. 2-18.1). The display indicates and the encoder transmits, simultaneously, pressure altitude reporting. Altitude is displayed on the altimeter by a 10,000 foot counter, a 1,000 foot counter and a 100 foot drum. A single pointer indicates hundreds of feet of a circular scale, with 50 foot center markings. Below an altitude of 10,000 foot a diagonal warning system will appear on the 10,000 foot counter. A barometric pressure setting knob is provided to insert the desired altimeter setting in inches of Hg. A DC powered vibrator operates in-
Figure 2-18.1. AAU-32/A Altitude Encode/Pneumatic Altimeter
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.