Q about autopilots and GPS approaches

rcaligan

rcaligan

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rcaligan
I've never flown a GPS approach with an autopilot, but I had a student who asked me this question:

You're flying a standard "T" approach, approach has been activated in the GPS, and you're approaching the final approach course at a right angle. Will the autopilot turn you 90 degrees onto the final approach course?
 
Yes, it will. So sweetly, it can make you overly dependent on the AP. And, some do.

I've flown several approaches on AP with headings as precise as I can ask for. It's just a matter of selecting Nav on the AP and having GPS selected as the source after having the correct approach entered. My local favorite is watching it fly the overlay DME Arc WDR involving a few tight turns greater than a T-approach. All are "fly by" points rather than "fly to." With the T-approaches usually being 90 degrees onto final, I suspect most all those are fly-bys as well so you'll be on final course as early as possible.

Edit: After reading Chris and Kent's posts, I should add I have very little experience outside the KAP-140. The King flies beautifully with either the KLN-94 or the G-1000. I've used the S-Tec 55 and hated it, particularly altitude hold.
 
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It depends on the A/P.

With the DigiTrak in our RV, the A/P will fly through the point, then come back to the next track line. Usually it will cross over the point, cross through the desired track the first time, then be dead-on when coming back the second time.

IIRC, the new DigiTrak's use GPSS that will actually precess the turn and won't fly through the point the first time.

But, yes, most "tracking" autopilots will fly a GPS approach, as well as follow the localizer, VOR track, etc.
 
rcaligan said:
I've never flown a GPS approach with an autopilot, but I had a student who asked me this question:

You're flying a standard "T" approach, approach has been activated in the GPS, and you're approaching the final approach course at a right angle. Will the autopilot turn you 90 degrees onto the final approach course?
Click to expand...

Like many things in aviation, "it depends."

If your autopilot has GPSS (GPS Steering), it will anticipate the turn and make a beautiful standard-rate turn that puts you smack on the final approach course.

If your autopilot simply has nav mode, it may either fly all the way to the point of the T, way overshoot final, and maybe make it back... I know before we added the GPSS to ours, by the time it realized what was going on, it had completely overshot final and gone full deflection so it'd just get confused and make a very slow turn.

Also, knowing how the GPSS works is essential. On the S-Tec, you put the autopilot in *Heading* mode, not Nav mode. Then, you flip the GPSS to "GPSS" and it feeds headings to the autopilot (as opposed to "HDG" which passes through the input from the DG bug). Every so often, someone will squawk the GPSS as not working, and every time when I go test it, it works fine. The problem is they're putting the autopilot in Nav mode, so even though the little GPSS light is on, the autopilot is ignoring its commands.

If your student owns a plane, s/he should have the autopilot manual. If it says nothing about GPS, chances are it doesn't have GPSS. Otherwise, many autopilot manuals are available for download on the internet.

Hope this helps! :yes:
 
CJones said:
It depends on the A/P.
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Very true. My GPS is coupled to the AP but I do not have GPSS so it will only really track a course. If I am flying the T of the approach with AP when it comes time to turn I have to manually turn the heading bug. Even then it is still better to do the turn manual and then reengage the AP to the FAF and then disconnect there.
 
smigaldi said:
Very true. My GPS is coupled to the AP but I do not have GPSS so it will only really track a course. If I am flying the T of the approach with AP when it comes time to turn I have to manually turn the heading bug. Even then it is still better to do the turn manual and then reengage the AP to the FAF and then disconnect there.
Click to expand...

If I'm actually letting the A/P fly the approach, that's what I do as well. When the 430 says "Next DTK XXX", I disengage the A/P and begin the turn myself. Once I get within 20 degrees or so, I'll re-engage the A/P and let it pick up and I go back to sleep. :D
 
KFC-200 will intercept up to 120 degrees, but be careful, it can go the wrong way on the localizer if the intercept angle is too great (as me how I know <g>).

On the standard T approach to a gps; it's a bit sloppy. I have the Garmin 430 coupled to the KFC-200. But, it will fly it! One can improve it using the heading mode until turning to final; then, fly in the approach mode.

Lance recommended getting GPSS installed when I did the WAAS upgrade. Here's a place where it would have made a difference, but, we'd spent enough for now and stayed with the auto slew we currently have.

Best,

Dave
 
CJones said:
It depends on the A/P.
Click to expand...
Right. If it's a certified installation, it will follow the turn anticipation from the GPS and start the turn far enough ahead to roll you out on course on the final segment. If not (like your DigiTrak), it's anyone's guess. And you cannot have a 120-degree intercept angle of the final segment on one of those TAA SIAP's unless you did not enter via an authorized IAF.
 
One other note. Most TAA GPS approaches have a depicted HILPT at the center of the 'T' that's required (sans vectors to final) if you use the center IAF from the airport side. Normally you'd use one of the two IAFs at the ends of the arms so using this PT is a pretty rare event. But the problem is that some GPS navigators will try to guide you around the PT even if you come at the center IAF from the side away from the airport where the PT is supposed to be skipped.
 
lancefisher said:
One other note. Most TAA GPS approaches have a depicted HILPT at the center of the 'T' that's required (sans vectors to final) if you use the center IAF from the airport side. Normally you'd use one of the two IAFs at the ends of the arms so using this PT is a pretty rare event. But the problem is that some GPS navigators will try to guide you around the PT even if you come at the center IAF from the side away from the airport where the PT is supposed to be skipped.
Click to expand...
Is that so even if you've programmed the IAF to be out on the end of the T"?

Such as WESUV in this approach?
http://download.aopa.org/ustprocs/20070802/SE-4/csg_rnav_gps_rwy_06.pdf

It seems like I've seen an approach with more than one IAF out to one side of the T but not including an HILPT.
 
lancefisher said:
One other note. Most TAA GPS approaches have a depicted HILPT at the center of the 'T' that's required (sans vectors to final) if you use the center IAF from the airport side. Normally you'd use one of the two IAFs at the ends of the arms so using this PT is a pretty rare event. But the problem is that some GPS navigators will try to guide you around the PT even if you come at the center IAF from the side away from the airport where the PT is supposed to be skipped.
Click to expand...
If you start an approach at an IAF with a HPILPT, or one that is not a "NoPT" route but goes through a HPILPT, the Garmin 430/530 asks if you want to hold at the depicted HPILPT (and the answer should be "yes" unless ATC has cleared you straight in), but if you start at one of the "NoPT" IAF's, it will skip the hold without asking. Others may not be that sophisticated.
 
Ron Levy said:
If you start an approach at an IAF with a HPILPT, or one that is not a "NoPT" route but goes through a HPILPT, the Garmin 430/530 asks if you want to hold at the depicted HPILPT (and the answer should be "yes" unless ATC has cleared you straight in), but if you start at one of the "NoPT" IAF's, it will skip the hold without asking. Others may not be that sophisticated.
Click to expand...
For my GPS, King KLN90B, also the 89B and the 94 it does not matter if you start on a non-PT waypoint or not if you are in the leg mode the waypoint will sequence to the next. If you want and/or need the PT you need to slect OBS and that will prevent the waypoint sequence so that you can do the PT.
 
Ron Levy said:
If you start an approach at an IAF with a HPILPT, or one that is not a "NoPT" route but goes through a HPILPT, the Garmin 430/530 asks if you want to hold at the depicted HPILPT (and the answer should be "yes" unless ATC has cleared you straight in), but if you start at one of the "NoPT" IAF's, it will skip the hold without asking. Others may not be that sophisticated.
Click to expand...

Actually the answer should usually be "no" since most approaches have the entire 180 degree sector opposite the airport listed as NoPT on the center IAF and there's not much point in using the center IAF when coming from the airport side.

FWIW, the GNS-480 for all it's FMS like abilities always assumes you will be flying the HILPT if you select the center IAF to start the approach from. Garmin claims this is because Jeppesen doesn't put anything in the DB to indicate where the PT is or isn't required directionally.
 
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