TM 1-1520-240-102-14-4 CONTROLS/INDICATOR FUNCTIONHI caution lightLight comes on when heli-copter rises above altitudeon HI set index.OFF flagFlag is displayed when pow-er is removed from set,when indications are unreli-able, or when altitude ex-ceeds approximately 1,500feet.RAD ALT dimmingcontrolsOn right side of both instru-ment panels. Controls lightintensity of digital displayand 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 altimeteruntil LO index is at 100 feet. Rotate the HI set knob untilthe HI index at at 800 feet. Allow 1 minute for warmup.(2)Check for the following indications:(a)The pointer indicate between -5 and 5feet.(b)The digital displays indicate between 0and3 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 and1,100(c)Digital displays between 900 feet and1,100feet.(d)LO caution light is off.(e)HI caution light is on.(2)Release the PUSH-TO-TEST knob. Checkthat all indications return to those specified in step a.(2).NOTEThe LO caution light may appear to go out.However, the light will be visible with nightvision goggles. In bright daylight, it may benecessary to shade the indicator to verify op-eration of the dimming control.(3)Rotate the RAD ALT dimming control fromBRT to DIM. Check that the digital display dims and goesout. Check that the LO caution light dims but does not goout completely.c.Inflight operation.CAUTIONWhen operating over dense foliage, theradar altimeters will indicate the altitude tothe tops of the trees. When operating oversparse foliage, the altimeters may indicatethe altitude between the ground and abouthalf the average tree height, depending onground speed. When external cargo is car-ried, the radar altimeter may occasionallyindicate the distance between the bottomof the helicopter and the load.(1)Set the RAD ALT dimming control to desireddigital display light level.(2)Set HI and LO indexes as desired. If HI or LOindexes are not desired, set the LO index at 0 feet or theHI 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 thevisual only AN/APN-209A radar altimeter, with the opera-tion being the same as outlined in paragraph 2-14-9. Thedifference is only internal. A transmitter added internallysends out an aural warning to the pilot, copilot, and flightengineer interphone station ICS panels when the heli-copter descends below the low index setting or risesabove the high index setting (in addition to the LO or HIcaution lights).NOTEThe voice portion of the system works off a 10second 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. Bymomentarily pressing the PUSH-TO-TEST (PTT) knobonce, the message is decreased to 1/2 volume. Momen-tarily pressing the PTT knob a second time disables theaural 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 volumelevel of the aural warning message will return to Full.
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