Question to search function of FMC during decoding

[Guest stephen odgaard]

Hi,Thanks for the great tutourial Terry.One question though. It is still unclear to me in which sequence the FMC searches for a FIX/NAVAID.Does it first search in the Fixes section and subsequently in the AIRAC database or vice versa?Further could you confirm that when determining the "correct" Navaid between duplicate IDs in the AIRAC, it simply selects the one closest to the Airport in question?RegardsStephen

[downscc 1,327]

I assume you are referring to the PMDG FMC (you mentioned the tutorial).All fixes called by the keyword FIX in the procedures must be declared in the FIXES section. The FMC will not look for a fix in the AIRAC data. All fixes called by the keyword NAVAID are "external references" and the FMC will only look in the AIRAC data. I've seen procedures written with the NAVAID keyword used on fixes to spare the author the effort of including them in the FIXES section but I prefer the extra effort of declaring all fixes (including navaids) in the file although I will often use the airac data to obtain the latlon info. The effect of doing this is to reduce reliance on external references but it is just a preference.When the NAVAID refers to multiple references, the FMC seems to always pick the closest one; however, I'm not sure if it is picking the closest one to the origin, destination or previous fix. In most cases, duplicate IDs exist in different hemispheres but it is an interesting point.Hope this helps.

[Guest stephen odgaard]

Thanks Dan,Indeed the PMDG, I should have pointed this out...Thanks for your clarification. I see the definition actually can be understood that way.And just to be sure a point in the Colocated command, will allways be taken as a Navaid (external) reference?As to the Multiple Choice (eg in the Colocated command) refering to the Airport Coordinates should be sufficient.Regards

[Guest staang]

Dan is correct about where the FMC looks to find navaid coordinates. I might also note that it goes no further than those files as it will not look at the navaids within FS itself. Also, with regards to the duplicate navaid IDs. I am still trying to clarify it with PMDG but so far it seems that the FMS looks at the navaids listed in the legs page to determine what is closest. To further investigate this could you please give me an example of this duplicate navaid problem? What airport, what navaids and what terminal procedure is involved.>And just to be sure a point in the Colocated command, will>allways be taken as a Navaid (external) reference?I will get back to you on this one once I get the answer.RegardsTerry

[downscc 1,327]

>And just to be sure a point in the Colocated command, will>allways be taken as a Navaid (external) reference?The COLOCATED keyword will refer to fixes defined within the file, or not finding a local reference it will then look to the airac data. Works similar to NAVAID.I use COLOCATED very often. One use is the LNAV MAP in RNAV approaches, typically 1-2 nm from end of runway. First I define the end of the runway (using Airnav info) then I define the MAP with the COLOCATED keyword. For example:FIX RY12R LATLON N DD MM.MMM W DDD MM.MMMFIX 12R11 COLOCATED RY12R 300 1.1 (this fix is 1.1 nm from Rnw 12R)Works like a charm. One catch is to ensure the fix used by the keyword COLOCATED is defined before it is referenced (the FMC seems to be a single pass interpreter because forward references create errors).