TM 1-1520-238-10 2-76 Section XIV. FLIGHT INSTRUMENTS 2.50 FLIGHT INSTRUMENTS. The instruments discussed in this section are, for the most part, those that directly measure flight performance. Cau- tion, warning, audio systems, and some flight instruments are common to both crew stations. The instruments are grouped as common, pilot, and CPG flight instruments. 2.50.1 Common Flight Instruments. The flight instru- ments found in both the pilot instrument panel (fig 2-9) and the CPG instrument panel (fig 2-10) are the pressure (barometric) altimeter, instantaneous vertical speed indi- cator, airspeed indicator, attitude indicator, and the clock. a. Pilot Barometric Altimeter. The pilot has an AAU-32/A encoding barometric altimeter. This altimeter is the same as the CPG’s except the AAU-32/A interfaces with the IFF for Mode C operation. b. CPG Barometric Altimeter. The CPG has an AAU-31/A barometric altimeter. The altimeter is graduat- ed in 50-foot increments and marked at 100-foot intervals (0 – 9 x 100). Just left of center is a 100-foot drum and a 1000-foot drum to supplement the scale pointer. The scale window, at the lower right section of the instrument face, indicates barometric pressure setting in inches of mercury. It is adjustable by use of the barometric pressure set knob on the lower left corner of the indicator case. Maximum allowable altimeter error is 70 feet. c. Vertical Speed Indicator (VSI). The VSI mea- sures the rates of change in static air pressure resulting from climbs and descents. An adjustment screw on the lower left corner is used to zero the pointer, if necessary, prior to flight. d. Airspeed Indicator. The airspeed indicator mea- sures the difference between pitot pressure and static pressure. Instrument range markings and limitations are contained in Chapter 5, Section II, System Limits. At low airspeeds and high power settings, indicated airspeeds may be unreliable and fluctuate greater than 10 KIAS. e. Pilot Standby Attitude Indicator. The pilot standby attitude indicator provides an independent dis- play of helicopter attitude. The indicator can display 360 of roll and 85 of pitch. A PULL TO CAGE knob at the lower right corner has two functions. Pulling it out with power applied to the instrument will cage the motor-driven internal gyroscope and level the background horizon line to 0 in pitch and roll. The knob may be turned to adjust the pitch of the artificial horizon relative to the fixed aircraft symbol. The indicator receives 28 vdc from the emergen- cy dc bus through the STBY ATTD circuit breaker on the pilot overhead circuit breaker panel. f. CPG Remote Attitude Indicator. The CPG re- mote attitude indicator (RAI) displays helicopter attitude from information obtained from the heading and attitude reference system (HARS). The indicator can display 360 of roll and 90 of pitch. HARS input drives roll and/or pitch servos, which results in the appropriate roll or pitch of the artificial horizon. If HARS input ceases or becomes unreliable, 28-vdc power will cause an OFF flag to appear at the window on the left side of the instrument face. A pitch trim knob at the lower right corner may be turned to adjust the pitch of the artificial horizon relative to the fixed aircraft reference symbol. The RAI receives 115 vac from the No. 1 essential ac bus through the ATTD IND circuit breaker on the CPG No. 1 circuit breaker panel. g. Clock. The clock combines the features of a standard clock and a stopwatch by displaying normal and elapsed time in hours, minutes, and seconds. Once wound, the clocks will run for eight days. The elapsed- time pushbutton control is on the upper right corner of the case. The clock is wound and set with a knob at the lower left corner of the case. 2.50.2 Pilot Flight Instruments. The flight instruments in the pilot instrument panel (fig 2-9) are the video display unit, standby magnetic compass, free air temperature gage, accelerometer, and radar altimeter. a. Video Display Unit (VDU). The VDU is a multi- purpose instrument that provides the pilot with flight, navi- gation, and targeting information. A turn-and-slip indicator is located below the face of the cathode ray tube. The sig- nals for the turn rate indicator are provided by the DASE. b. Standby Magnetic Compass. The standby magnetic compass is attached to the pilot glareshield. Pri- mary heading information is taken from the horizontal situ- ation indicator, which is discussed in Chapter 3, Section III.