TM 1-1520-240-102-14-3When the helicopter descends below the low index set-ting or rises above the high index setting, the correspond-ing HI or LO caution light will illuminate. If helicopteraltitude exceeds 1,500 feet, pitch or roll angle exceeds45_, or the system is unreliable, the following will occur.The OFF flag will appear, the pointer will move through1,500feet behind the dial mask, and the digital displayand LO and HI caution lights will extinguish.If power to the system is lost, the following will occur. TheOFF flag will appear, the digital display and caution lightswill go out, and the pointer will remain at the last validindication when power was lost. The altimeters have aself-test feature. Pressing the PUSH-TO-TEST knob willcause the pointer and digital display to indicate between900 and1,100feet. If LO set and HI set are indexedbelow900feet, the LO caution light goes out, and the HIcaution light comes on. Power to operate the radar altim-eter is supplied by the No. 2 DC bus through the NAVRAD ALT circuit breaker on the No. 2 PDP.Figure 2-14-3. Radar Altimeter (AN/APN-209)2-14-10. Controls and Functions, Radar Altimeter(AN/ APN-209A).CONTROLS/INDICATORFUNCTIONLO SET knobEither pilot’s LO SET knobapplies power to altimetersystem. LO set index onboth altimeters can be setindependently. Both LO setindices must be masked toturn the set off.HI SET knobSets position of HI set indexand tests altimeter systemwhen pressed.CONTROLS/INDICATORFUNCTIONLO set indexIndicates altitude trip pointfor LO caution light.HI set indexIndicates altitude trip pointfor HI caution light.Indicator pointerIndicates absolute altitudefrom 0 to 1,500 feet.Digital indicatorProvides direct reading fourdigit indication of absolutealtitude from 0 to 1,500.LO caution lightComes on when helicopterdescends below altitude onLO set index.
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