(2)  
The ILS is considered valid by the FMC if:

 (a)  
Raw data passes the input processing validation.

 (b)  
The localizer deviation is less than 1.25 degrees.

 (c)  
Received frequency matches the frequency stored in the NDB for the selected runway.

 (3)  
The VOR and DME go through two validation processes. These are short-term and long-term validation. The long term validation will be dependent upon the short-term validation. These processes are described as follows:

 (4)  
Short-term validation

 (a)  The DME is short term valid if: 1) Raw data passes the I/O process validation. 2) The frequency is between 108.00 and 117.95 mega Hertz. 3) The DME ground distance does not exceed a rate of change of
 1nm per second with respect to the FMCs calculated ground
 distance. 4) The tuned frequency is the same as the received frequency. 5) The airplane is not within the zone of confusion (inverted
 cone with an apex of 60 degrees). 6) The actual DME ground distance does not differ from the
 FMC's estimated distance by a predetermined tolerance. 7) The ground range is less than 300 nm. 8) There are two DME/DME pairs which are considered for
 updating and the position of the airplane given by these two pair does not differ by 0.8 or 0.5 nm in the case where one is a procedure tuned navaid.
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A
747-400
MAINTENANCE MANUAL

NOTE: M= MANUAL, P= PROCEDURE, R= ROUTE TUNED STATIONS
THE ABOVE ALGORITHM PERTAINS ONLY TO THE LEFT DME AND VOR. THE RIGHT DME AND VOR OPERATE THE SAME EXCEPT CHANNEL 4 WILL CONTAIN THE DISPLAYED INFORMATION. THIS IMPLIES THAT THERE CAN BE TWO SIMULTANEOUS DISPLAYS ON THE CDU, ONE FROM THE LEFT SIDE AND ONE FROM THE RIGHT.
ALL THE MANUAL STATIONS HAVE BEEN ENTERED ON THE LEFT CDU, IF THEY WERE TO BE ENTERED ON THE RIGHT CHANNEL 4 WOULD CONTAIN THEIR INFORMATION.
Autotuning Logic
Figure 31 (Sheet 1)

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747-400
MAINTENANCE MANUAL

TWO

YES
DME/DME PAIR AVAILABLE NO ONE

YES
DME/DME PAIR
AVAILABLE

NO CHANNLES 1 & 2 WILL BE USED FOR DME/DME POSITION UPDATING REGARDLESS IF THE PAIR IN 3 & 4 IS BETTER. THIS IS DUE TO RE-EVALUATION TIMING.
CHANNLEL 3 WILL BE DISPLAYED.

THE CLOSEST TUNED NAVAID IN CHANNELS 1 & 2 WILL BE TUNED CHANNELS 3 & 4 WILL BE USED CHANNLES 3 OR 4 WILL BE USED TO THE PAIRED STATIONS AND BE FOR POSITION UPDATING IN THE FOR RADIO UPDATING. USED FOR POSITION UPDATING. DME/DME MODE.
CHANNEL 3 WILL BE DISPLAYED. CHANNEL 3 WILL BE DISPLAYED. CHANNEL 3 WILL BE DISPLAYED.

IF THERE IS TWO DME/DME PAIRS THEN THE PAIR IN CHANNELS 1 & 2 WILL BE USED FOR POSITION
ANY

UPDATING.

DME/DME PAIR
YES  
NODME/DME PAIR DOES NOT INVOLVE IF THE PAIR INVOLVES THE TWO
AVAILABLE
A M,P OR R  DIRECTED STATIONS THEN CHANNELS 3 & 4 WILL BE USED FOR POSITION UPDATING.
NO  YES CHANNELS 3 WILL BE DISPLAYED.
RADIO UPDATING WILL BE ACCOM-PLISHED THROUGH THE CLOSEST NAVAID IN EITHER CHANNEL 3 OR 4.
CHANNEL 3 WILL BE DISPLAYED.
THIS PAIR WILL GO INTO CHANNELS 1 & 2 AND BE USED FOR POSITION UPDATING.
CHANNEL 3 WILL BE DISPALYED.
Autotuning Logic
Figure 31 (Sheet 2)

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