TM  1-1520-240-10 3-3-4            3-3-8.  Gyromagnetic Compass Set (AN/ASN-43). The gyromagnetic compass set (fig. 3-3-3) is a direction sensing system which provides a visual indication of the heading of the helicopter with respect to the earth’s mag- netic field or referenced to a free directional gyro. Head- ing information is displayed on both HSIs. The display is used  in  a  navigation  to  maintain  flight  path  direction. Also, any heading selected with the heading bug on ei- ther HSI can be automatically maintained by the AFCS. Power to operate the system is supplied by the No. 1 AC bus and the AC instrument bus through the NAV CMPS circuit breakers on the No. 1 PDP. 3-3-9.  Controls and Function, Gyromagnetic Com- pass Set (AN/ASN-43).  (fig 3-3-3 and 3-3-4) CONTROLS/ INDICATOR FUNCTION LATITUDE Switch Two-position switch on the directional gyro lo- cated on the avionics shelf. This switch is used to correct for gyro precession for the hemi- sphere of operation: N for northern hemisphere and S for southern hemisphere. LATITUDE Control Switch Rotary control on direc- tional gyro labeled 0 to 90. It is used to set the local latitude into the di- rectional gyro for free gyro operation. SLAVED-FREE Switch On compass control panel. When switch is at SLAVED, the compass cards on both HSIs are slaved to the output of the remote compass and-stabilized by the di- rectional gyro. At FREE, the cards are referenced to the directional gyro only and act as turn indi- cators. FREE is used primarily in polar regions where magnetic indica- tions may be unreliable. PUSH TO SET synchro- nizing knob Rotated in ( ) or (+) di- rection as indicated by annunciator pointer to align compass cards with gyro-compass. CONTROLS/ INDICATOR FUNCTION HDG Flag A red flag on both HSIs indicates failure of the gyromagnetic compass system when displayed. 3-3-10.  Operating    Procedures    —    Gyromagnetic Compass Set.  (fig. 3-3-3) a.  Slaved  Gyro  Operation.  With  compass  slaving switch at SLAVED, the system operates in the slaved mode  and  the  directional  gyro  precesses  to  align  the compass cards on the HSIs with the magnetic heading of the helicopter. During the first 2 minutes after power is applied, the system operates in a fast slave mode while the gyro attains its speed. After this initial alignment peri- od is complete, the gyromagnetic compass will return to the normal slaved mode. During this mode of operation, the compass cards will remain aligned with the magnetic heading of the helicopter. The annunciator pointer will occasionally point to a dot ( ) or a plus sign (+) indicat- ing that corrections are automatically being made. b.  Free Directional Gyro Operation.  If the compass slaving switch is at FREE, the system operates in the free directional mode. In this mode, the compass cards can be set to any heading by pressing the PUSH TO SET knob  and  turning  it  until  the  cards  reach  the selected setting.  Normally,  the  free  directional  gyro  mode  is employed only in polar regions of the earth where mag- netic references are unreliable. However, it can be useful if the slaving system malfunctions. For proper operation, the latitude controls on the directional gyro (fig. 3-3-4) must be set to the local latitude. Figure  3-3-3.  Gyromagnetic Compass Set Control Panel