(4)  
Vertical flight path is calculated by combining the ground velocity magnitude and the airplane velocity. This value is calculated at a 5 Hz rate.

 (5)  
Drift angle is calculated by using computed track angle and the true heading. It is processed a rate of 20 Hz.

 (6)  
Magnetic variation is computed by using the true heading and magnetic heading from the IRS. It is processed at a 5 Hz rate.

 (7)  
North and east wind velocity components are only valid when:

 (a)  
Vertical flight path angle is valid

 (b)  
Airplane velocity is valid

 (c)  
True heading is valid

 (d)  
Airplane ground speed is greater than 80 knots

 (e)  
True air speed (TAS) is valid

 (8)  
Calculations of the wind components are accomplished through taking the difference between the TAS along the true heading and the ground speed along the track. These calculations will produce the raw north and east components. These components are then filtered to produce filtered north and east wind velocity components.

 (9)  
Wind magnitude, bearing, tailwind and crosswind are then calculated by combining the filtered north and east wind components at a 1 Hz rate.

 E. Altitude Computation
 (1)  FMC's airplane altitude is computed by filtering the ADC 's barometric pressure and altitude inputs and combining this with the IRS pressure altitude. If both the ADC altitude inputs are valid, then the baro correction is derived by taking the difference of the two inputs. The derived baro correction is then filtered and added to the IRS's pressure altitude if valid or to the ADC pressure altitude if the IRS's pressure altitude is invalid. The altitude valid flag is then set to true.
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 ALL  ú ú 22 Page 64 ú Jun 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 F.  Local Earth Radius Computation
 (1)  Calculations of the earths radius will be done at a rate of 0.1 Hz when the IRS position has been entered. The initialization radius or base radius will be 3437.74677 nm. The computed earth radius will be used in the complementary position filter described previously.
 G.  Tuning management
 (1)  
In order to improve airplane position, the FMC's navigation function combines VHF NAV radio information and IRS's derived position to enhance airplane position.

 (a)  
ILS receivers provide angular deviation measurements from a fixed bearing. This data has the highest accuracy but is only used on airplane approach due to its short range.

 (b)  
DME interrogators provide distance information from the airplane to ground facilities. These VHF NAV radios provide the second highest accuracy and are usable at distances from 9 to over 200 nm.

 (c)  
VOR receivers provide bearing measurements between the airplane and the ground facilities north reference. There VHF NAV radios provide the poorest accuracy with a 130 nm range. These receivers will operate in conjunction with the DME interrogators.

 (2)  
DME tuning (Fig. 31)

 (a)  
The DME interrogator has the capability of free scanning 252 separate frequencies. Five of these frequencies will be used by the FMC for VHF NAV radio updating. The frequency command for these five frequencies originates in the FMC. The FMC automatically generates tunning 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 process in the FMC navigation data base.

 (b)  
The DME will tune the FMC's selected stations. These stations will be stored in the memory slots within the DME in accordance with the FMC.

 (c)  
There are two station types which are required by the FMC to be tuned and placed into one of the five memory slots if possible. These are listed in order of priority. 1) Manually tuned stations are those stations which are

 manually entered into the CDU. These stations will be used for displaying distance data on the IDS displays and have top tuning priority.
 2)  Procedure tuned stations are associated with a particular flight plan procedure. The tuning frequency will be supplied by the FMC navigational data base.
　
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