TM  1-1520-240-10 2-14-2            warning symbol will appear on the 10,000 foot counter. Power to operate the AIMS altimeter is supplied by the No. 2 DC bus through the NAV AIMS ALT circuit breaker on the No. 2 PDP. A barometric pressure setting knob is provided to insert the desired altimeter setting in inches of Hg. A vibrator Figure  2-14-2.  AIMS Altimeter powered by No. 2 DC bus is contained in the altimeter and  requires  a  minimum  of  1  minute  warmup  before checking or setting the altimeter. If DC power to the alti- tude  encoder  is  lost,  a  warning  flag  placarded  CODE OFF appears in the upper left instrument dial. The flag indicates that the altitude encoder is inoperative and that the system is not reporting altitude to ground stations. The CODE OFF flag monitors only the encoder function of the altimeter. It does not indicate transponder condition. The AIMS altitude reporting function can be inoperative without the CODE OFF flag showing, as in case of transponder failure or improper control settings. It  is  also  possible  to  get  a  good  Mode  C  test  on  the transponder control with the CODE OFF flag showing. Display of the CODE OFF flag only indicates an encoder power failure or a CODE OFF flag failure. In this event, check the DC power is available and the circuit breakers are in. If the flag is still visible, radio contact should be made with a ground radar site to determine whether the AIMS  altitude  reporting  function  is  operative.  The  re- mainder of the flight should be conducted accordingly. 2-14-7.  Preflight Operation - Altimeter If  the  AIMS  altimeter  encoding  function  is  to  be  used during flight, perform the following steps before takeoff: CAUTION If the baroset knob binds or sticks, do not use excessive force to set the altimeter. Excessive   force   can   damage   altimeter gears, resulting in altimeter error. Settings can sometimes be made by backing off the knob and turning at a slower rate. a. Set  the  pilot’s  altimeter  to  the  field  barometric setting. b. Check that the pilot’s altimeter indicates within 70 feet of field elevation. If the altimeter error is greater than 70 feet, do not use the altimeter IFR flight. 2-14-8.  In Flight Operation - Altimeter. Operate the AIMS altimeter function as follows: a. Be sure the IFF set is on and set to the proper code. b. Be sure the altimeter is set to the local altimeter setting. c. Set the M-C (mode c) switch on the IFF control panel to ON. d. Check that the red CODE OFF flag is not visible in the pilot’s altimeter. The  copilot’s  altimeter  is  a  pneumatic  counter-drum- pointer type which displays altitude in the same manner as the pilot’s altimeter. It also incorporates a barometric pressure setting knob and an internal vibrator powered by the No. 2 DC bus. A minimum of 1 minute of vibrator operation is require before setting or checking the altime- ter. At ambient pressure, both altimeters should agree within 70 feet of the field elevation when the proper barometric pressure  setting  is  set  in  the  altimeter.  If  the  internal vibrator of either altimeter becomes inoperative due to DC  power  failure,  the  pointer  drum  may  momentarily hang up when passing from  9 through  0 (climbing) or from 0 to 9 (descending). This will cause a lag of magni- tude  which  will  depend  on  the  vertical  velocity  of  the aircraft and the friction in the altimeter. 2-14-9.  Radar Altimeter (AN/APN-209A). Radar altimeters are provided for the pilot and copilot (fig. 2-14-3). The altimeters provide a continuous indication of the height of the helicopter above the surface from 0 to 1,500 feet. Altimeter indications are reliable with pitch and roll attitude up to 45_. Altitude is displayed by a dial, pointer, and a digital dis- play. Each altimeter has HI and LO caution lights. The caution lights on each altimeter can be set independently of the other altimeter. The caution lights are set by rotat- ing the LO SET and H SET knobs until the L index and H index on the perimeter of the altimeter are at the de- sired altitudes.