Autopilots which use GPS to fly an ILS

I think it uses it to fly the ILS moore accurate. The problem I have with that is that if the GPS quits for some reason, either it being disabled by the army or the police or because my device doesn’t want to live anymore, I cannot use the standard instrument procedure as backup. Sure, I can handfly it, but that’s not the idea of an autopilot

I can’t judge much auto-pilots, I did not test all of them but if GPS is available GFC will keep hooking up on ILS and stay smooth even if tornado cross-wind hits the aircraft, this is not the case for Century & KAP likes on raw signal, if it hits 20deg heading correction or glitch in NAV signals you are stuffed…

To be fair it’s up to the pilots to get what they want, an AP that disconnect if GPS fails and reconnects when you press APP or an AP that disconnects in 30G40 winds and reconnect when you press APP, of course you can’t have everything…

I am happy to swap GPS rare failures with wind correction errors, I can definitely hand fly an ILS to it’s system minima if GPS fails in calm winds but I do found it tough without a robust AP help when it’s windy…

Ibra wrote:

In G1000+GFC700, I can see ILS auto-ident, even for similar ILS frequency, I imagine that’s used to build DTK for GPS-enhancement of the AP in APP mode?

In my test flying with the GFC700 in an SR22TN, the ILS control logic was similar to all other autopilots I have flown with, that is they do a poor job of handling changing crosswind velocity and direction as you descend. It would S turn with the best of autopilots. I repeated the same runway, but an identical LPV procedure. The control took advantage of roll steering and continuously held the center line with an adjusted WCA nearly instantaneously. On the GFC700, flying the LPV tracked perfectly and I did not shut off the autopilot until at 30 feet above the runway. With the ILS, it started to drift at about 500 feet and then S turned all they way to DH. This particular day had strong cross winds at the GS/GP intercept altitude which changed direction and speed below 500 feet AGL.

As far as the ILS auto-ident, if you mean the morse code, it is decoded from the audio.

Graham wrote:

Much as someone hand-flying a procedure might, when outbound from the IAF, check their course over ground on the GPS against that noted on the plate.

I do something similar, but the GPS track is usually a few degrees different than the charted value. I display and compare the DTK with the TRK and get them to match. It detects small deviations from the course long before the CDI shows a deviation, There is also a Track Angle Error that one can display, but I find the DTK/TRK easier to use. Now with a PFD on my G500TXi, ILS and LPV are much easier to hand fly. Pitch control for GS is way more precise and easier to distinguish than on my previous FD. Also with the PFD HSI showing a track indicator, the SV showing a Flight Path Pointer and the airport runway, its almost like VFR.

Ibra wrote:

To be fair it’s up to the pilots to get what they want, an AP that disconnect if GPS fails and reconnects when you press APP or an AP that disconnects in 30G40 winds and reconnect when you press APP, of course you can’t have everything…

I think it is a lame argument saying ‘if you don’t like the autopilot falling back to roll mode when the GPS is gone, tough luck, hand fly it’. If I want an RNP approach, I select the RNP approach, and I expect the A/P to fly it. If I want an ILS, I select the ILS approach, and I expect the A/P to fly it within its limits. The reason I have both on board is that if one doesn’t work, I can choose the other, independently from each other. Just I did, when I have the failure in the GPS source, and could still let George fly the approach.

I wouldn’t go so far as to call the GFC a bluff package, but in that respect it comes close IMO.Peter wrote:

It is a dramatic limitation, because it means you can’t really use an ILS as an alternate for a GPS approach

That’s exactly right. It is a limitation many pilots who have the GFC 500 aren’t even aware of. This is something that can bite you, pretty dramatically. We all know that there so and so many pilots out there who struggle hand flying in IMC, let alone the approach, don’t we. It’s a mantra of the PPL/IR seminars that we should use all available avionics to our advantage. This can just be that one hole too many in the cheese.

I an asking myself why this is a dramatic limitation?
1. More and more airfields are getting GPS only approaches.
2. We are all using GPS as a sole means of navigation for enroute AP coupled flying – and don‘t see this as a dramatic limitation … And this is a much higher percentage of the flight than the few minutes of the approach.
3. How is the probability of a GPS outage compared to a ILS outage ?
4. There is a reasonable fallback

I have been flying the GFC 500 for 1,5 years now – never had a problem – and I would not step back to one of the older APs.
I had numerous problems with the older APs – without fallback..

I think this limitation is heavily overemphasized – and you can still go the more expensive route if you like to spend the money.

Interestingly nobody is talking about the capabiltiy of doing a coupled missed approach – a huge safety factor in a very critical phase of flight.

Peter wrote:

Well, you can hand-fly it but in a real emergency when everything has turned to sh*t and you are on your “last alternate”…?

You can still use the HDG mode which helps quite a bit.