TM  1-1520-240-10 2-14-6            CAUTION When  operating  over  dense  foliage,  the radar altimeters will indicate the altitude to the tops of the trees. When operating over sparse foliage, the altimeters may indicate the altitude between the ground and about half the average tree height, depending on ground speed. When external cargo is car- ried, the radar altimeter may occasionally indicate the distance between the bottom of the helicopter and the load. (1) Set  RAD  ALT  dimming  control  to  desired digital display light level. (2) Set  HI  and  LO  indexes  as  desired.  If  the helicopter exceeds one of these indexes, an aural warn- ing message will be heard at full volume initially, but the volume  level  can  be  decreased  by  one-half  (1/2)  by pressing the PUSH-TO-TEST once or inhibited by press- ing PUSH-TO-TEST a second time. If HI or LO indexes are not desired, set the LO index at 0 feet or the HI index above 1,500 feet. 2-14-15.  VERTICAL SPEED Indicator. Two VERTICAL SPEED indicators are in the instrument panel  (fig.  2-1-7  and  2-1-9).  They  indicate  the  rate  of climb based on the rate of change of atmospheric pres- sure. The indicator is a direct-reading pressure instru- ment requiring no electrical power for operation. 2-14-16.  Attitude Indicator. Two attitude indicators are on the instrument panel (fig. 2-1-7 and 2-1-9). The attitude indicators have been spe- cifically tailored for the flight characteristics of a helicop- ter by the inclusion of an electrical trim capability in the roll axis in addition to the standard pitch trim. Normal flight attitudes of helicopters, defined by fixed amounts of roll as well as pitch, are easily trimmed into this indicator, and optimum operation of the helicopter in an attitude such as hover is facilitated. Degrees of pitch and roll are indicated by movement of a universally mounted sphere painted optical black and light gray to symbolize earth and sky, with a horizon line separating the two colores. To adjust the miniature aircraft in relation to pitch, use the lower knob. The pitch adjustment range is about 5_ nose up and 10_ nose down minimum. To compensate attitude in the roll axis, use the upper knob. The roll adjustment range is about 8_minimum in either direction. NOTE Rapid rotation of the pitch and roll trim knobs on  the  attitude  indicator  may  cause  abrupt pitch and roll attitude changes with AFCS on. The indicator incorporates integral lighting. The pilot and copilot attitude indicators should erect within 90 seconds after electrical power is applied. Power to operate the attitude indicator gyros is supplied by the No. 1 and No. 2 AC bus through the NAV PILOT VGI and NAV COPLT VGI circuit breakers on the No. 1 and No.2 PDP. 2-14-17.  Pilot  and  Copilot  Attitude  Indicator  (VGI) Switch. A VGI switch is on the instrument panel below each atti- tude indicator (fig. 2-1-7 and 2-1-9). The switch is labeled NORM and EMER. When the switch is at NORM, each attitude indicator operates from a separate gyro. If either the pilot or the copilot gyro fails, signaled by the OFF flag on the indicator, manual switching to the remaining gyro is accomplished by placing the respective VGI switch to EMER. The switching of the gyros from NORM to EMER operation is accomplished by a gyro transfer relay. Fail- ure of the gyro will also result in failure of the associated AFCS. Power is supplied by the No. 2 DC bus through the NAV CONT VGI circuit breaker on the No. 2 PDP. 2-14-18.  Turn and Slip Indicator (4-Minute Type). Each turn and slip indicator (fig. 2-1-7 and 2-1-9) is con- trolled by an electrically actuated gyro. The instrument has a pointer (turn indicator) and a ball (slip indicator). Power to operate the gyros is supplied by the DC essen- tial bust through the NAV TURN & SLIP circuit breakers on the No. 1 and No. 2 PDP. 2-14-19.  Magnetic Compass. The  magnetic  compass  is  mounted  on  the  top  of  the center instrument panel glareshield (fig. 2-1-3). It is a direct reading instrument requiring no electrical power. It consists of compass card mounted on a magnetic ele- ment in a liquid-filled bowl. 2-14-20.  Free Air Temperature Gauge. The free air temperature gauge is on the exterior of the pilot’s eyebrow window (fig. 2-1-3). The unit is calibrated in degrees from -70 to + 50_C.