When to engage approach mode on autopilot?

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colohan

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Chris Colohan
I am currently training for my IFR rating. Recently I discovered that although I am good at hand-flying the plane, I can mess things up pretty badly when I try to fly using the autopilot. :smile: So I am doing some flights using the autopilot as much as possible (when legal) to make sure I am proficient.

The C172SP I am training in has a KAP140 2-Axis Autopilot. There is one thing I have not yet figured out -- when is the best time to switch from NAV mode to APR mode when intercepting an ILS?

In particular, the ILS approach I use most for practice is ILS 25R into KLVK, starting from TRACY. After passing TRACY and ensuring I am solidly on the right course (and leaving the GPS on to verify...), I switch the autopilot to HDG mode to maintain my present heading, change the #1 CDI to the ILS, and wait for the localizer needle to start to come in.

Now, my question is this: when should I switch the autopilot to APR mode and swing the heading bug to match the course of the ILS?

If I switch the autopilot to APR as soon as the needle starts to come in (or even before), the GPS makes a hard left turn towards the localizer (since it thinks it is suddenly way off course), blows straight through the localizer, makes a hard right to recover, and oscillates several times around the localizer before figuring it out. Not smooth.

If I wait until I intersect the localizer at FOOTO it is too late to turn onto the localizer course, blow straight through it, and the autopilot may refuse to go into APR mode at all.

So is there an optimal technique for making this type of transition with this autopilot? (My instructor just claims "the KAP140 sucks".) Or do I either try to smoothly intercept the localizer by playing with the heading bug in HDG mode, and then put it in APR mode -- or just disable the autopilot and hand fly the plane onto the localizer?
 
I've seen the same thing. We have a couple planes that fly a bit more smooth than the others.

What I normally do is use heading mode while vectored. If it's a full approach I may activate the approach mode much earlier. If I'm vectored to final, I'll maintain heading mode until established on the localizer. In heading mode, I can make the smoother turns that it may not do on its own.

If it's not set up well and stable on the localizer well before the FAF, I'd hand fly it. You don't want it doing S-turns that last few miles.
 
That model SHOULD allow you to be in heading mode, and ARM approach mode by pressing the APPR button. Both HDG and APPRarm annunciators should light up, and the autopilot should intercept as the localizer comes alive, based on a 45 degree intercept. Generally the autopilot should start the turn at about two dots on the CDI.

See your AFM supplement for more details.
 
The KAP 140 is designed to run its own intercept from HDG mode once you're on an intercept heading (like that 229 radial, or when the controller gives you the final vector call with a heading within 30 degrees of the final approach course). What you should be doing is as soon as you get on the 229 heading in HDG mode, hit the APR button. The APR light will com on with ARM next to it, and the HDG light will start to flash for five seconds. You have to rotate the heading bug to the ILS course during those five seconds so the autopilot has the right picture of the intercept geometry. After that, the autopilot will start running the intercept, and it should go smoothly.

I think your problem is that you're waiting too long to select APR and get the bug set to the final approach course. The autopilot thinks the approach course is more like 230, and when the needle starts to move, it tries to turn too far to make the intercept work.

See page 20 of your KAP 140 manual for the details. If it still isn't working right, the next step is the avionics shop.
 
TMetzinger said:
That model SHOULD allow you to be in heading mode, and ARM approach mode by pressing the APPR button. Both HDG and APPRarm annunciators should light up, and the autopilot should intercept as the localizer comes alive, based on a 45 degree intercept. Generally the autopilot should start the turn at about two dots on the CDI.

See your AFM supplement for more details.
Click to expand...

Tim,

The dual HDG and APR mode only work if the autopilot has an HSI, not when it has a DG.
 
colohan said:
I am currently training for my IFR rating. Recently I discovered that although I am good at hand-flying the plane, I can mess things up pretty badly when I try to fly using the autopilot. :smile: So I am doing some flights using the autopilot as much as possible (when legal) to make sure I am proficient.

The C172SP I am training in has a KAP140 2-Axis Autopilot. There is one thing I have not yet figured out -- when is the best time to switch from NAV mode to APR mode when intercepting an ILS?

In particular, the ILS approach I use most for practice is ILS 25R into KLVK, starting from TRACY. After passing TRACY and ensuring I am solidly on the right course (and leaving the GPS on to verify...), I switch the autopilot to HDG mode to maintain my present heading, change the #1 CDI to the ILS, and wait for the localizer needle to start to come in.

Now, my question is this: when should I switch the autopilot to APR mode and swing the heading bug to match the course of the ILS?

If I switch the autopilot to APR as soon as the needle starts to come in (or even before), the GPS makes a hard left turn towards the localizer (since it thinks it is suddenly way off course), blows straight through the localizer, makes a hard right to recover, and oscillates several times around the localizer before figuring it out. Not smooth.

If I wait until I intersect the localizer at FOOTO it is too late to turn onto the localizer course, blow straight through it, and the autopilot may refuse to go into APR mode at all.

So is there an optimal technique for making this type of transition with this autopilot? (My instructor just claims "the KAP140 sucks".) Or do I either try to smoothly intercept the localizer by playing with the heading bug in HDG mode, and then put it in APR mode -- or just disable the autopilot and hand fly the plane onto the localizer?
Click to expand...

From your description, it sounds as if you have a DG in your aircraft and don't have an HSI. When you press the APR button, the KAP 140 with a DG will turn to a 45 degree intercept. In your case, from Tracy, you are on a 26 degree intercept to the course, so the KAP will turn to the left and probably not have sufficient time to make the turn back to the course. Your best plan of action is to wait until the ILS is nearly centered and use heading mode to make the initial course alignment with the localizer, then engage the APR mode.

If you were doing the procedure turn, the intercept heading would already be 45 degrees and you could press the APR as you rolled out on the procedure turn intercept. In most cases it would have enough time to acquire the localizer and determine a lead for the roll out.

The KAP 140 is a good autopilot, but you must understand how it works. Your instructor probably doesn't have a good handle on how it works.

Note: The KAP works differently if there is a HSI.
 
John Collins said:
Tim,

The dual HDG and APR mode only work if the autopilot has an HSI, not when it has a DG.
Click to expand...

Not true (at least according to the book and my experience).

With an HSI, you set the course on the HSI, and the heading bug for the desired intercept.

With a DG and CDI, you set the course on the CDI and the heading bug to the course as well and you get a 45 degree intercept. That's what the supplement says, and I've made it work in the 172S models I've flown. I think from your second post you recognize this.

Now, other advice given earlier about flying the initial intercept with the heading bug and then engaging APPR after you're close is good.
 
Tim,

If you have a DG and are in heading mode, and then press APR or NAV, and reset the heading bug to the course, the intercept will be 45 degrees.

If instead, you align the aircraft with the desired intercept heading, assume using heading mode, then press HDG to select ROL mode, then press APR or NAV, and reset the heading bug to the course, the current heading is held until the course is intercepted and the autopilot extinguishes ROL mode. This in effect allows an all angle intercept capability with a DG. I think this is what you were talking about.

The autopilot needs a course reference to perform an intercept and track operation. In the case of a DG, this comes from the heading bug. In the case of an HSI this comes from the OBS pointer, so the heading bug can be used for all angle intercept. By putting the autopilot into ROL mode when a DG is present, you get double use from the heading bug, before ROL is selected it sets the intercept angle, and after ROL is selected it can be used to give the autopilot a course reference. The effect is the same in that it effectively allows a selectable intercept angle.

So, based on your information, and me RTFM, I would revise and extend my remarks as follows: To avoid the 45 degree intercept, select ROL mode when on the desired intercept course, then reset the DG heading bug to the approach course, then press APR.
 
See attached document. It's how to do it with a DG. I can scan the pages on how to do it with the HSI if anybody is interested.
 

Attachments

  • KAP140 APR mode with DG.pdf
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gprellwitz said:
The full pilots guide is available https://www.bendixking.com/servlet/...ame=/TechPubs/repository/006-18034-0000_2.pdf it's about 137 pages.
Click to expand...

Thank you! That is the link I was missing -- I had read through the (too brief) pilot's guide at the back of my 172SP POH, and assumed that was all there was. It looks like the manual you linked to is much more thorough -- I will read it now. :smile:

Thank you all for all of the tips. I will show my instructor how it is supposed to work next time we fly.:D

Chris
 
colohan said:
Thank you! That is the link I was missing -- I had read through the (too brief) pilot's guide at the back of my 172SP POH, and assumed that was all there was. It looks like the manual you linked to is much more thorough -- I will read it now. :smile:

Thank you all for all of the tips. I will show my instructor how it is supposed to work next time we fly.:D

Chris
Click to expand...
You're welcome. I tried to link to the pilot's guides of most of the complicated avionics on the newer 172's at http://www.aandmaviation.com/links.html. If someone knows of one I've missed, please let me know!

Disclosure: I'm their "webmaster", FWIW. I don't get paid! I tried to link to things that I as a pilot find useful.
 
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