Inserting step down fix on LNAV approach?

Checks

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LNAV approaches sometimes contain step down fixes. My FMS has a vertical path angle (VPA) that will clear this step down fix if the temperature is ISA (+15 C at sea level) or above. When below ISA,there is no way to say for sure whether my VPA will clear the step down fix since the VPA will be shallower.
With all that said....When I pull up the LNAV procedure from the database it does not contain any fixes between the FAF and the Runway. I can insert the named step down fix from the database and insert an altitude next to it and my FMS will fly the LNAV VPA and compute a path that will incorporate the step down fix altitude. The GPS will show GPS APPR from the FAF to the Runway.

Is there any legal issues with modifying the approach this way to match exactly what the instrument procedure shows?

Thanks in advance!
 

Seadogrun

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Which FMS are you using and why is it taking the fixes out? Are you inputting the temp?

I would say it is bad news to manually put in the fixes. What is likely to happen is that you will not have the appropriate sensing (RNP) for the approach and the self monitoring to alerting in case you can't maintain the proper accuracy (approach armed).

It would seem better to fudge the temp than lose the lateral accuracy, but don't quote me on that.
 

Checks

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FMS-3000. The scaling stays at .3 and GPS APP is on throughout the approach with the step down fix inserted.
 

j41driver

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When we fly GPS approaches we are not allowed to modify the approach as it is in the database. Don't know if that's just our Ops or FARs though...
 

Checks

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Thats what I have always been told also. Just looking for a reference.
 

Seadogrun

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I've seen it referenced in training videos/CTS a bunch of times. I think your answer will be in the FMS Manual.
 
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