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E. The primary mode of navigation in the horizontal plane uses two independent measurements of DME slant range. These measurements are optimally filtered with IRS position, heading, and velocity information to produce an accurate computation of the airplanes present position. There is also a secondary mode of navigation in the horizontal plane, based upon bearing and slant range data obtained from a single VOR/DME facility. Position is computed in terms of latitude and longitude for display on the CDU. The system also provides a variety of other navigation output variables. These navigation output variables include track angle, drift angle, ground speed, and wind information. The navigation outputs are available for display on the CDU, or on the flight instruments.
F. The HSI receives FMCS navigation data from the Digital to Analog Adapters (DAA). The following FMCS Navigation interface signals are converted by the DAA for display on the HSI:
(1)
Desired Track
(a) FMCS desired track is displayed on the HSI, by the course arrow read against the azimuth card. The course arrow responds to drift angle plus track angle error signals from the FMC.
(2)
Waypoint Bearing, Waypoint Bearing Valid
(a) The FMCS bearing to waypoint data and waypoint bearing valid signal are applied to the HSI. When the signal is valid the waypoint bearing pointer displays bearing to the next waypoint. If the signal is not valid, or VOR/ILS is selected on the HSI transfer switch, the waypoint bearing pointer is stowed at 90 degrees (3 o'clock position).
(3)
Cross Track Deviation
(a) The course deviation bar indicates the deviation from the desired track. The course deviation bar is read relative to the course deviation scale.
(4)
Vertical Deviation, Vertical Deviation Valid
(a) The vertical deviation pointer indicates the deviation from the desired desent. The vertical deviation pointer is read relative to the vertical deviation scale. A VERT flag comes into view when desent data is not computed or unreliable.
(5)
Ground Speed
(a) Ground speed information is displayed in the upper right numerical display.
(6)
Distance
(a) Distance (miles-to-go) is displayed in the upper left numerical display.
(7)
Distance and Ground Speed Valids
(a) The distance (MILES) and ground speed (GND SPEED) numerical displays are each covered by a flag, if FMCS-valid input signals are lost.
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737-300/400/500MAINTENANCE MANUAL
(8)
Drift Angle, Drift Angle Valid
(a) The FMCS drift angle data and drift angle valid signal are applied to the HSI. When the signal is valid, the drift angle pointer displays the FMCS computed drift. If the signal is not valid, the drift angle pointer is stowed behind the mask at the bottom of the indicator.
(9)
Annunciators
(a)
The navigation source annunciator indicates the system supplying information to the HSI: namely FMCS (NAV) or VOR/ILS.
(b)
A navigation warning annunciator, located behind the two windows (at the center of the display), provides a warning in the windows when navigation data is not computed or invalid.
(c)
The to-from annunciator (also located behind the two windows), provides to or from waypoint information in the windows when navigation data is valid.
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