Can someone help me understand when I can arm VNAV?

[jcarne]

Okay, I have read the manual and some other threads and still don't understand. I'm doing instrument training and me and my CFII don't seem to fully understand when Dynon will let me arm VNAV on the approach. It seems as though I cannot even arm it until I get the glideslope indicator on the HSI.

An example; we were flying the RNAV 16 into KWRL and inbound on the hold to COVGO (IAF) the HDX wouldn't even let me arm VNAV. Right around COVGO we picked up the glideslope indicator and armed and intercepted the glideslope for the LPV. However, there was a pretty small window in which it would let me press the VNAV button on my AP module.

So long story short, can someone explain the absolute soonest I can arm VNAV? Sorry for my ignorance here but I simply don't know the answer. Thanks for the help!

P.S. A GNS-430W is feeding the HSI.

 

[kurtfly]

You have to be on the final inbound course, and below glide slope. The glide slope needle needs to be present and above you. Also, if you are in a decent, like descending to a FAF (PEYIP in this case) and you hit the VNAV button the decent will stop. It will level off until it intercepts glide slope.

Kurt

 

[midlifeflyer]

Are you seeing this behavior on other approaches? This one, as you point out has a distance/altitude loss combination between the IF and FAF that would have you descending on the glidepath immediately after crossing COVGO. Any excess altitude and you'd be above glidepath in a jiffy.

If it's happening on other approaches too... although the glidepath for an RNAV approach does not appear until the FAF becomes the active waypoint.the AP should arm before a valid glide slope/path indication so it can smoothly transition (manual says so).

One possibility off the top of my head. Perhaps the AP is looking for a pre FAF VNAV profile that would give guidance for stepdowns? That's a feed a later GPS model would give but one the 430 does not. Maybe the AP is looking for that feed before the FAF becomes active - which might be a reason arming is suppressed? Dunno.

An experiment and perhaps a workaround for you and your CFI. Once inbound toward COVGO, activate the COVGO PEYIP leg. That will make the FAF the active waypoint and enable the glidepath early so you can arm it, but (unlike VTF) retain COVGO as a waypoint to make sure you don't descend too soon.

 

[jcarne]

Thanks for the replies guys. Makes a little more sense now. I’ll do some more experimenting and midlifeflyer we will try your workaround, can’t see why that wouldn’t work. Just seems odd the thing won’t let us arm it sooner.