From HXr manual - 12.3.5.1.1 Capturing the ILS with ARM

[BillinDurham]

Having done a number of ILS intercepts with the ARM feature engaged, I’ve never managed to have the system automatically switch from HDG mode to NAV mode, when ARM is engaged, as it intercepts the Localizer. Instead I’ve had to manually switch from HDG to NAV in order to have the AP do the intercept. Otherwise my AP blows right through the Localizer. It just happened again yesterday when I had a good chance to observe this situation on an actual VTF ILS approach.

Any thoughts on why that may be?

My only thought involves the EXECUTE key. I recall that originally that did not exist but it was added for some reason (?). I believe I did what was needed to enable the EXECUTE key and have used it once cleared for an approach. But over time it has become unclear exactly what it does. Originally it was not needed since ARMing an approach was enough. I’m thinking that use of the EXECUTE key may in some way interfere with the automatic switch from HDG to NAV mode when on VTF.

My config includes and HXr, a couple of HXs, an Avidyne IFD440 and a TruTrak Digiflight II AP.

Thanks in advance

[GRT_Jeff]

The EXECUTE key was added as suggested by people flying IFR approaches so they could set up the approach before final and only have to push one button to allow the final approach capture.

When one of the approach modes is armed, the EFIS will not change modes and capture the indicators until EXECUTE is pushed. EXECUTE is to be pushed after all turns are complete, as the EFIS only knows how to fly a straight in final approach. In the 12.3.5.1.1 diagram the EXECUTE button would be between steps 1 and 2.

After you push EXECUTE, the EFIS starts looking at the localizer deviation signal. When it's within 2 dots left or right for several seconds it will capture the signal and that includes switching to ENAV mode.

[BillinDurham]

So on a VTF approach, after the last vector has been given and I’m ‘Cleared for the ILS/RNAV 22L approach’, I should hit the EXECUTE key.

What I’d like to expect is that the system would keep navigating on a straight line until close enough to the intercept for a smooth, anticipated turn on to the final leg. That almost never works that way for me in practice.

Based on previous experiences, what I should expect is that I would remain flying in a straight line until within roughly 2 dots of the intercept, then, since controllers typically give you vectors until within 30deg of the final path, the system would turn to make a 45 deg intercept of the final.

In practice, I’ve found the switch from the typical 30deg intercept vector to the 45deg system intercept somewhat abrupt. I’ve often taken to manually switching from HDG mode to ENAV mode to reliably smooth things out.

Can you tell me what I should expect on a long 30deg vector to final if I hit execute before the localizer is picked up?
- will I continue in a straight line, presumably in HDG mode until the Localizer is picked up, then when within 2 dots get the 45deg intercept turn?
Can you tell me what I should expect on a long 30deg vector to final if I hit execute after the localizer is picked up at max dots?
- will I continue in a straight line in HDG mode until within 2 dots and then get the 45deg intercept turn?
- or will I get an immediate turn to a 45 deg intercept?

What I think I’ve experienced is that if I hit execute before the localizer is picked up, the system remains in HDG mode and never transitions to ENAV. Could that be the case?

Before the Execute key was implemented, on a VTF approach, I found that transition from vectors to final intercept confusing and unreliable. Therefore the Execute key seemed like a good addition but I still find myself somewhat confused by its operation in the above scenarios. That’s a hell of a thing to say given that I make approaches into busy APs with multiple parallel runways where the default approach is VTF to an ILS. Trying to use the AP to make the intercept then missing it is an easy way to generate a violation or worse. E.G. KCLT.

With all that said, once Armed and Executed, the system will fly straight to a fix and make a smooth turn on to the next leg smoothly and reliably. . But in that case, I’m already in GNAV/ENAV mode in order to navigate to a fix. It’s the VTF approaches I struggle to fly consistently

[GRT_Jeff]

If you execute without a localizer signal or with a signal beyond the outer dots then the EFIS keeps waiting and takes no immediate action. Everything waits until you are within 2 dots. In HDG mode the EFIS would keep the selected heading already set or you can change the selected heading as necessary to make an intercept. Once you travel within 2 dots then the EFIS takes over.

If you execute within 2 dots, or execute and then travel to within 2 dots, the EFIS switches to localizer capture mode within about 3 seconds. Our course capture modes are normally limited to 45 degrees, and the EFIS doesn't care what your previous angle was, so the EFIS could turn to a 45 degree intercept from any other angle if it calculates that it needs a maximum intercept angle. There is no smooth transition or special intercept mode, basically the HDG mode turns off and ENAV mode turns on. The EFIS computes autopilot commands based on the known (automatic or manually set) localizer course and the current lateral deviation.

There is one other issue when you have a combined GPS/NAV receiver like an Avidyne IFD440 or GNS430W or similar. These have automatic mode change capability in ILS approach mode so the EFIS is normally programmed to follow the GPS/NAV CDI mode (NAV Mode Source External). The CDI mode on the GPS/NAV has to change to VLOC mode, and that changes the EFIS to ILS mode, in order for the EFIS to capture a localizer. The EFIS will wait for this to happen. Typically the navigator will automatically change modes as you near the final approach fix and ILS course, but the criteria are up to the GPS/NAV unit and they have exceptions. Based on what I read in the IFD440 manual (AUTOMATIC MODE SWITCHING TO VLOC), I'm not sure if it will do an automatic mode change in VTF mode, and you would have to get closer with less than a 15 degree track intercept angle. You may need to switch the IFD CDI mode to VLOC manually.