Menu Sign In Contact FAQ
Welcome to our forums

Late turn after GPS waypoint

On the G1000/KAP140 combo I fly (DA40), when flying a full procedure, the CDI changes from GPS to VLOC when turning inbound on the procedure turn.

I find it hard to get my head around how this works… do you mean NCYankee that, on an ILS, the Garmins will fly using GPS LNAV till the FAF and only then use the LOC signal?

That would produce very poor guidance.

In my plane, I press APR when approaching the LOC so the system is tracking the LOC from much earlier. There is zero “snaking” although it can take a while to take out a strong wind drift.

Roll steering on the GNS is only available on the ARINC 429 channel. It is a bank angle to fly and does not have any direct relationship to the analog CDI L/R signal. With the KFC225, NAV mode does not use CDI deviation for tracking a course when GPS is the navigation source as long as roll steering data is available. IOW it simply flies the bank angle commanded by the roll steering. This permits flying curved paths such as a DME arc, PT, Hold or turn anticipation where as long as the bank angle does not exceed the auto[pilot limits, it will roll out on the new course on center, because the bank angle needed is dynamically changes as needed.

In the case of the GNS/W or GTN series, the roll steering label is transmitted on a 100 MS basis with the bank angle it is asking the autopilot to fly. When on an ILS, the GNS/W or GTN stops transmitting a bank angle on reaching the FAF. For most autopilots using a GPSS adapter, the adapter will output a wings level (no heading bug error voltage). Interestingly enough, the legacy units would continue to output the roll steering bank angle all the way to the MAP. This behavior exists only on ILS or localizer procedures and not on VOR procedures. There are two cases for how the KFC225 would track on an ILS, case 1, VLOC is selected on the GNS/W or GTN. Case 2, GPS is selected on the GNS/W or GTN. In either case, the CDI deviations will guide the autopilot, but in the latter case, there will not be a GS on the HSI and the GS won’t be tracked.

The KFC225 does a good job on the ILS in no or low cross wind conditions, but it will S turn if the winds change close in to the runway like I described. I have flown enough time with this autopilot to verify this, although in general it does a better job than most adjusting to an updated wind correction angle.

KUZA, United States

I thought the later KLN94 with KAP140 had roll steering:

-0102 includes with DC roll steering output for use with KAP-140 and KFC-225 autopilots
(click Here for diagram)

ekbr ekbi, Denmark

Yes, the KLN94-KFC225 combination does offer an analogue form of roll steering.

The KLN94-KFC225 analog roll steering connection requires a KLN94 P/N 069-01034-0102 (which can be done as a software upgrade from the 069-01034-0101 most commonly installed) and is only a few wires, shown at the top of here plus a config change in the KFC225.

I have only ever flown with one pilot who had this wired up, but he was not able to get it to do anything obvious. It was, I believe, installed by Air Touring who despite being CAA Level 2 approved could not even install a KI229 RMI – they had to have 2 or 3 goes at it. However there is no reason why it should not work; normal “dumb” steering (whereby the autopilot uses the HSI/EHSI course pointer as the heading, and uses the deviation bar to tweak the tracking) is good enough for everything I have ever seen including the T-shaped GPS approaches. And this is why I never bothered to wire it up… also in my case there is difficult access to get those particular wires in place. Also the KLN94 cannot fly any curved paths and has no holds or arcs in its database so again there was no point.

The wider issue with roll steering is that if used with a mechanical HSI, the HSI course pointer is totally ignored by the autopilot (it still functions as usual in VOR/LOC modes) which is not ideal for situational awareness which is compromised by having a “dead” HSI course pointer, which makes GPSS in conjunction with a mechanical HSI a slightly dodgy idea…

If I got a penny every time I met a pilot with a GNS or GTN box who is less than clear about what it does on an ILS, I would have enough for a second TB20 Somehow, Garmin have managed to really confuse people with the terminology.

Shoreham EGKA, United Kingdom

Before we installed the GTN650 we had the KLN94 and KAP140 – without the roll steering.
We need to align the heading indicator, set the heading bug and then set the cdi correctly to run the Autopilot in NAV mode. Still so with the GTN650.
One pilot told me he has experience with a newer machine where – with the same avionics – he was able to set a course on the GPS and get the autopilot to track that without setting the heading indicator and CDI.
Has that anything todo with the roll steering connection?

ekbr ekbi, Denmark
25 Posts
Sign in to add your message

Back to Top