TM  1-1520-240-10 2-14-4            CONTROLS/ INDICATOR FUNCTION HI caution light Light comes on when heli- copter rises above altitude on HI set index. OFF flag Flag is displayed when pow- er is removed from set, when indications are unreli- able, or when altitude ex- ceeds approximately 1,500 feet. RAD ALT dimming controls On right side of both instru- ment panels. Controls light intensity of digital display and HI and LO caution light. 2-14-11.  Normal Operation - Radar Altimeter (AN/ APN-209A). a. Starting. (1) Rotate  the  LO  SET  knob  on  the  altimeter until LO index is at 100 feet. Rotate the HI set knob until the HI index at at 800 feet. Allow 1 minute for warmup. (2) Check for the following indications: (a) The pointer indicate between -5 and 5 feet. (b) The digital displays indicate between 0 and 3 feet. (c) LO caution light is on. (d) HI caution light is off. (e) OFF flag not in view. b. Testing. (1) Press and hold the PUSH-TO-TEST knob. Check for the following indications: (a) OFF flags not in view. (b) Pointers indicate between 900 feet and 1,100 (c) Digital displays between  900 feet and 1,100 feet. (d) LO caution light is off. (e) HI caution light is on. (2) Release the PUSH-TO-TEST knob. Check that all indications return to those specified in step a.(2). NOTE The LO caution light may appear to go out. However,  the  light  will  be  visible  with  night vision goggles. In bright daylight, it may be necessary to shade the indicator to verify op- eration of the dimming control. (3) Rotate the RAD ALT dimming control from BRT to DIM. Check that the digital display dims and goes out. Check that the LO caution light dims but does not go out completely. c. Inflight operation. 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 the RAD ALT dimming control to desired digital display light level. (2) Set HI and LO indexes as desired. If HI or LO indexes are not desired, set the LO index at 0 feet or the HI index above 1,500 feet. 2-14-12.  RADAR ALTIMETER (AN/APN-209(V)) Alti- tude Voice Warning System (AVWS). The  face  of  the  altitude  voice  warning  radar  altimeter (RT-1115F/APN-209) is exactly the same as that of the visual only AN/APN-209A radar altimeter, with the opera- tion being the same as outlined in paragraph 2-14-9. The difference is only internal. A transmitter added internally sends out an aural warning to the pilot, copilot, and flight engineer interphone station ICS panels when the heli- copter  descends  below  the  low  index  setting  or  rises above the high index setting (in addition to the LO or HI caution lights). NOTE The voice portion of the system works off a 10 second clock. If a transition is made thru ei- ther Preset, back to the original warning con- dition within this time frame, the voice warn- ing will stay at its original volume level. On initial transition the volume is always set to Full. By momentarily pressing the PUSH-TO-TEST (PTT) knob once, the message is decreased to 1/2 volume. Momen- tarily pressing the PTT knob a second time disables the aural warning message. Regardless of the volume level, when the aircraft transitions back through either Preset (HI or LO) (from caution light OFF to ON), the volume level of the aural warning message will return to Full.