G1000 - CDI switch comes late sometimes

[olasek]

When I practiced a few ILS approaches on my G1000 PC trainer it became obvious that counting on automatic CDI switch from GPS to LOC may not be a good idea. It does happen (it is permanently enabled on the Cirrus Perspective) but sometimes happens too late. For example at KSAC it works beautifully because altitudes are so structured that GS intercept occurs almost exactly at FAF. So when the CDI switch happens 2-3 miles outside of FAF you are well below the GS and have plenty of time to arm APR and down you go. But at KPRC when you 2-3 miles outside of FAF you are already at or even above the GS - so when the switch happens there is no chance to catch GS unless you want to dive for it. So the moral of the story - it pays to do it manually and not counting on G1000 to do it for you.

 

[TMetzinger]

I think this is a rare occurence, but absolutely one to be aware of.

Looking at the KPRC ILS/DME 21L chart, I see you should cross I-PRC 10.1 at or above 7900. But the altitude constraint of 7100 at HUMTY is ONLY when the GS is out (hence the LOC ONLY note).

So this is clearly a case where you want to be switching the CDI to LOC/GS early. Thanks for pointing it out. As you said, it may be good to develop the habit of switching the CDI manually as soon as you're established inbound, no matter how far out you are.

 

[bbchien]

All my stus have this item on the "within eight miles of the IAF" checklist.

 

[TMetzinger]

All my stus have this item on the "within eight miles of the IAF" checklist.

Not sure I understand. I much prefer to use GPS for guidance to the IAF and towards the FAF, it doesn't vary the way a VOR might. Now, that said, when getting vectors, or between the IAF and the FAF, once I've got a good localizer signal and am established or close to it, I switch the CDI.

But I don't switch the CDI before the IAF, or even right after the IAF. It all depends on the approach.

 

[olasek]

Not sure I understand.

Neither do I. "Within 8 miles" casts a rather large net and may be difficult to measure if your IAF is where it is in PRC case (HUMTY). And Like Tim said using GPS guidance as long as you can has its advantages - again in this very case it guides you through the whole procedure turn (if not using vectors to final). I rather 'feel' the right moment to do it - in the case of PRC ILS you could simply do it when you are aligned with the final approach course.

 

[bbchien]

'When within 8 miles of your IAF make certain the box is displaying VLOC or GPS, whichever is appropriate."

Is that so hard?

As is, looking on the waypoints page to make certain the waypoints you THINK you will be flying on the approach are all there.

AS is, looking up the ATD of the waypoint that is not on the screen and taking note that you will be using ATD to identify a waypoint, per 1-1-19.

 

[taters]

I absolutley agree that it is something you should not count on. ...As soon as I am being vectored(recieve first heading off course) for the approach...I set ILS course in HSI, check freq...and confirm VLOC/green needles...I have (in Cirrus) seen the cdi fail to go from pink to green...plus I like to see the loc working well before I get close to it.