(d)  
An ALERT annunciator is located in the lower left portion of the instrument face. When the FMCS signals that the airplane is within a predetermined time to a leg change, the ALERT annunciator comes on.

 G.  For vertical navigation, the navigation function provides computed values of altitude and Flight Path Angle (FPA). Altitude computations operate upon input of inertially smoothed altitude from the IRS, corrected by barometric altitude and uncorrected pressure altitude from the DADS. Flight path angle is derived from IRS inputs of vertical speed, and north/south and east/west velocity components.
 (1)  IRS Selection
 (a)  
The preferred source of inertial reference data for the FMCS is defined as IRS No. 1, for either dual or triple IRS installations. The FMCS continuously monitors the IRS operating mode, and the IRS ARINC 429 output Sign/Status Matrix (SSM) indicators. The FMCS will attempt to use data from an IRS that provides fully valid SSM indications, when the IRS is in the NAV (navigate) mode. Data from the IRS in the ALIGN or ATT (attitude) modes is not acceptable for navigation.

 (b)  
The FMC applies a reasonableness test to the data received from each IRS. The reasonableness test is to ensure that the present position of each IRS is within 4 nautical miles of a position estimate determined solely from radio navigation systems.

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 (c)  
AIRPLANES WITHOUT SB 34-1226; If an IRS fails to agree with the radio position estimate, it is not used for FMCS navigation processing, until the IRS is within the 4 nautical mile limit. Also present position of an IRS must compare within 10 nautical miles of the present position data of the other IRS. If an IRS does not meet the radio position reasonableness test or the IRS comparison test, its data is excluded from the systems computations, until it meets the tests.

 (d)  
AIRPLANES WITH SB 34-1226; If an IRS fails to agree with the radio position estimate for more than 120 seconds, it will not be used for FMCS navigation processing until such time as it recovers and passes the selection criteria. Also present position of an IRS cannot exceed 10 nautical miles for more than 40 seconds, the position data of the other IRS. If an IRS does not meet the radio position reasonableness test or the IRS comparison test, its data is excluded from the systems computations, and a VERIFY POSITION message shows on the CDU.

 (2)  DADC Selection
 (a)  The primary source of air data for the FMCS is by definition DADC No. 1. The validity of air data is checked using the information coded in the Sign/Status Matrix (SSM) of the ARINC 429 outputs of both DADS. If the FMC program detects an invalid condition in the SSM of any digital word from DADC No. 1, the FMC will then use DADC No. 2 as its source of air data.
 H. Optimum Navigation Mode Determination
 (1)  The precedence of navigation modes for the FMCS navigation function is as follows, with the inputs to the FMCS as shown:
 Navigation Mode_______________  Inputs
 DME/DME/IRS  Two DME ranges. IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed.
 DME/VOR/IRS  One DME range and one VOR bearing. IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed
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737-300/400/500MAINTENANCE MANUAL
 Dead-Reckoning  IRS latitude and longitude. IRS
 (D/R) with IRS  north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude and true airspeed.
 Airplane on Ground  IRS latitude and longitude. IRS north and east velocity. IRS altitude and vertical speed. IRS true and magnetic heading. Pressure altitude.
　
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