RNAV Approach Procedure with GTN 750

[kdub]

How does one fly an RNAV approach with the TDS 750?  I can't get the plane to fly a published hold on the approach.  The 750 announces the turn to enter the hold but the plane turns in the opposite direction to go straight to the FAF.

 

Kdub

[amyinorbit]

That's because in this avionics installation, the GTN is not connected to the autopilot in such a way that it can give the autopilot a bank angle command.

There are two ways an autopilot can follow a signal sent by a FMS or GPS unit:

1. deviation steering: the autopilot knows nothing about the GPS's path, just what is shown on the HSI, and steers like it would for a VOR signal, based solely on the selected course and CDI deflection
2. roll steering: the autopilot receives a bank angle computed by the GPS, and just follows that rather than computing a steering solution itself.

Solution (2) is superior, because it allows the autopilot to follow complex paths that a HSI cannot show, such as arcs and holding patterns. The SPZ-5000 site (which the 525 is equipped with) only has one input capable of (2), that's the one marked "FMS" on the EHSI source panel, and it's fed by the UNS-1. The GTN 1 and 2 units are connected to the NAV sources, which as far as the autopilot is concerned are just nav radios, so they use deviation steering (solution 1). That means that you cannot follow arcs or holding patterns on them because the HSI course pointer for a holding pattern just points along the inbound course.

The way to use the GTN for RNAV approaches is then:

• Fly the flight plan using the FMS. Make sure you load the same STAR and approach (and any directs you get given along the way) in both the FMS and the GTN
• once established on the final approach course, sync the heading bug, switch to heading mode, then change nav sources to the GTN (and make sure the GTN is sending GPS, nor VLOC, signals)
• once you see the correct lateral and vertical deviation from the GTN on the EHSI, engage APPR mode on the autopilot