All,

 

 During GPS approaches, GPSS converts accurate WAAS signals to heading deviation signals with an AP switch set in the GPSS position (just like using the heading bug).  If your AP can receive ARINC precision data generated from a WAAS GPS, use that for GPS approaches.  If you are stuck using non GPS approaches (Like archaic Localizer, ILS, etc.), you may want to switch the AP to "approach mode" for greater sensitivity when close in.  BTW, the GPS sensitivity is stepwise automatically increased as one approaches any way point, whereas the ground radio method (localizer + glide slope) is in a cone shape that smoothly narrows as one approaches and switching to greater sensitivity decreases the measurement of each deviation needle marking.

 

In other words, if one is on an ILS/Localizer approach, one must switch off GPS management at the IAF.  If using a GPS approach (from a device that contains same), GPSS only is advisable down to the DA (DH) or beyond as it is accurate and ready to guide you to the missed approach course if you fail to land after you were expected to land (merely a single button push away).

 

Check the details in the AIM.

 

Grayhawk

 

PS I am a lover of the systematic WAAS GPS approaches that allow consistent management of airplane speeds and configurations.. 

 

Angier,
   I gave an STEC 55 with GPSS steering.  I'm not familiar with "Hi Track"
mode, but on my A/P, if you're flying an instrument approach, you need to
switch the A/P to approach mode and change the GPSS back to heading mode for
steering inside the IAF or I believe the A/P will revert to heading mode at
that point.   If you have GPS output going to your CDI, the A/P will fly the
approach just like it would tracking a localizer.  Bottom line, GPSS is not
used for flying approaches.  I'm not sure if the 30 works the same, but
check your ops manual to be sure.
   Skip Slater


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