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 B.  Tuning Commands from the FMC go to the VORs and DMEs by way of the DAA. The receiver/interrogator pairs may be operated in either of two basic modes: manual or automatic. A MODE selector switch is located on each of the VHF NAV control panels. If a MODE switch is in the MAN position, the frequency command for tuning the VOR/DME pair comes from the control panel and is the frequency manually selected by the crew. When the switch is in the AUTO position, the frequency command originates in the FMC. The FMC automatically generates tuning commands, as a function of the available navigation aid ground stations relative to the navigation position estimate and the airplane altitude, by the station selection processes in the FMC software program. When both VHF NAV control panels are in the MAN mode, the FMC will use its normal navigation mode hierarchy as long as navaid geometry and range criteria are met.
 C.  Autotuning Control
 (1)  Autotuning of the airplane's VOR/DME receiver/interrogator pairs is accomplished by the operational program's tuning management processes within the navigational function. The radio tuning management operates in conjunction with the Kalman filter used to process and mix sensor data. Normally, the tuning process will assign highest priority to generating frequency commands for the selected best DME/DME station pair. The process will operate in this mode unless no acceptable DME/DME pair is available. If this is the case, the system will attempt to tune the closest VOR/DME station.
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737-300/400/500MAINTENANCE MANUAL
 (2)  Frequency commands are furnished to the VOR/DME receiver/interrogator pairs in the form of ARINC 429 outputs to the DAA, which converts the digitally encoded data to the form of 2X5 discrete outputs from the DAA. The autotune discrete from the control panel controls the directional buffer/receiver/transmitter in the DAA. The discrete is only present when auto mode has been selected through the control panel and the auto manual relay is held in the auto position.
 D. Automatic Station Selection
 (1)  
When the AUTO mode of VOR/DME tuning is selected, the FMC station selection process is responsible for the tuning of navigation aids that can be used to provide navigation solutions when combined with data from the IRS.

 (2)  
The navaids considered to be available for selection are those contained in what is called the local data base. Indexing techniques draw from the FMC navigation data base those navaids within 130 nautical miles north, south, east, and west of the estimated current airplane position. This group of navaids, combined with any additional navaids defined by entry into the temporary navigation data base, make up the local data base.

 (3)  
The best navigation aids are defined as the 10 navigation aids closest to the airplane's computed present position. From the best navaid list, the FMC determines the best navaid pair as a function of the crossing angle. The crossing angle must lie between 30 degrees and 150 degrees. The best navaid pair is defined as the combination which produces a crossing angle closest to the ideal value of 90 degrees. Reselection of a best navaid pair will normally occur after the service range or geometry requirements for the navaids in use are no longer met, or when data for one or both of the selected stations are declared unreasonable or can no longer be received. Additionally, after using DME/VOR navigation for 2 minutes, the FMC will attempt to reselect a best navaid pair for the preferred DME/DME mode of navigation.

 E. Frequency Agility DME Operation
 (1)  If manual tuning is selected for one of the two DME interrogators, the FMCS will use this manually tuned DME station. However, the FMCS must then find a complementary station with the remaining automatically tuned DME interrogator. The complementary station must maintain an acceptable Line-Of-Position (LOP) crossing angle (Fig. 14). When this is not possible the remaining automatically tuned interrogator is operated by the FMC in a frequency agility mode, to keep the primary DME/DME navigation solution. In the frequency agility mode the DME interrogator is automatically tuned at five-second intervals, between two DME stations, selected by the FMCS as the best pair.
 11. Guidance Computation (Fig. 14)
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