Learned Something New about GPS Approaches with the KFC200

[jlunseth]

I have a KFC200 AP in my aircraft. I also have a 430AW (WAAS) GPS and it can be connected to the AP via a GPSS, which is an Icarus SAM. I ran into a new (to me) behavior while flying the RNAV 15 (LPV) in to Joe Foss Field in Sioux Falls, SD (KFSD). I thought it was worth mentioning because many of us have a similar combination, an analog AP connected to a GPS via a GPSS.

The basic way the KFC200 works in flying, say, an ILS, is to use the heading bug to vector the aircraft onto the final approach course. The AP is put in APCH mode prior to the final approach course, the annunciator says it is “ARMED,” and when the aircraft intercepts the course the AP couples to the approach and will take things from there.  An issue is that the KFC200 and its cousin analog APs use the rate of closure to couple, meaning that when the heading bug is used to fly at the approach course from an angle, the AP uses the rate of closure to the final approach course to make its coupling calculations.  Once coupled, the AP flies to the course needle, which you have hopefully remembered to dial to the final approach course heading. If you wait to put the unit in APCH mode until you are on the final approach course and inside the FAF, it may not couple because there is no closure rate. If it does not couple, then the AP does not fly down the GS, you have to disconnect and fly the approach yourself.

The GPS and the SAM change things.  You can put the AP in HDG mode, put the GPSS in GPSS mode, and the SAM then feeds the course from the GPS, through the AP’s HDG function, and causes the analog AP to fly whatever course the digital GPS dictates. This is true right up to the final approach course and the descent down the glideslope. At some point before the FAF, the AP must be put into APCH mode, and when that is done the AP uses the course needle to fly the last part of the approach. The RWY waypoint then serves as the Localizer to direct the AP to the runway.

Typical RNAV approaches are a “T” with a racetrack at an IAF. If you start at one of the T waypoints you can typically go in without a procedure turn. If you start at the racetrack’s IAF then you need to do the procedure turn. (There are exceptions that are plated for a particular approach, but this is the general layout.) When I practice RNAV’s I have always done the racetrack because it is more complicated and, in my mind, better practice. I never thought about the fact that I needed to practice flying the T, and that it might be different.

So with that background, here is what I did. I decided to fly the RNAV 15 at KFSD from FRYRE, which is one of the T fixes, just to get the approach done faster and get on the ground. I had the AP in HDG mode with the GPSS in GPSS mode. About two miles from the right turn at the T, I decided to put the AP in APCH mode so it would arm and be ready to intercept at the right turn onto the final approach course.  Instead of doing that, it immediately began a right turn on to some kind of final approach course, but one that was well right of the real course.  I saw that and took the GPSS out of GPSS mode so I could use the heading bug to take the aircraft to the correct course, and of course right about that time Approach told me I was right of course. I told them I was correcting.  But using the heading bug to fly back to course the AP, probably now confused, did not intercept or couple, so I had to correct back. Approach came on again and right about that time we broke out so I was able to report that we had the airport in sight and the landing was uneventful.

I think what I need to do, when using the system for these T approaches, is to keep the GPSS in GPSS mode, leave the AP in HDG mode until I am on the final course, and wait to put the AP in APCH mode until on the course.  It seems to couple ok provided I switch to AP mode prior to the FAF.

This is a new one on me and I have quite a few hours flying in IMC with this equipment. Always learning something new I guess.  Anyone have any thoughts?

[donkaye]

25 minutes ago, jlunseth said:

This is a new one on me and I have quite a few hours flying in IMC with this equipment. Always learning something new I guess.  Anyone have any thoughts?

Yes.  It was all over for you when you switched from GPSS mode to APR mode before you were on the final approach course.  At that point there was no navigation to the final approach course.  Pre GPSS, HDG mode guided you to the final approach course and APR was armed.  With HDG in GPSS mode, when you switched to APR mode you lost the HDG guidance and the AP didn't know what to do.  So with the older autopilots with GPSS emulation tied the HDG mode, if in GPSS mode with either a T IAF or a hold in lieu of a PT, NEVER switch to APR until you are on the final approach course.

This does not apply to the GFC 500 where GPSS is built in.

[jlunseth]

Yes, I was pretty sure that is where I was. My next move, if we had not broken out, was going to be to re-select and activate the procedure, and hopefully I would have gotten guidance to complete the procedure by hand flying. I was a little concerned that, since I was partly into the procedure, about at the FAF I would guess, that I would still have had difficulty. If I still did not have guidance I would go missed at that point I guess. I am going to have to practice some "T's" now.

Unfortunately, there is no manual for this. It is three parts that are not the original system, trying to work together. 

Thanks for the help Don.

[N231BN]

The combination of curved flight paths and manual course pointers doesn't play well with legacy autopilots.

Another challenge is to fly a WAAS approach with a KFC200(250 in this case) that doesn't have roll steering. The CDI tracks the curved turn so I won't enter intercept mode. I end up steering the airplane with the heading bug until I'm lined up with the final approach course and then re-engage APR.

Someone should make a GPSS converter that has a third mode, auto slew. When the navigator is in GPS it can send the roll steering signals to the course input of the autopilot so it will fly all of the turns correctly in NAV or APR modes. My 231 has a Century 41 with no HSI but I have added GPSS. In NAV or APR modes the heading bug turns into the course selector. If I leave the GPSS on it acts like I have auto slew so it will make turns and give me wind corrections while in APR mode.

[donkaye]

2 hours ago, N231BN said:

The combination of curved flight paths and manual course pointers doesn't play well with legacy autopilots.

Another challenge is to fly a WAAS approach with a KFC200(250 in this case) that doesn't have roll steering. The CDI tracks the curved turn so I won't enter intercept mode. I end up steering the airplane with the heading bug until I'm lined up with the final approach course and then re-engage APR.

Someone should make a GPSS converter that has a third mode, auto slew. When the navigator is in GPS it can send the roll steering signals to the course input of the autopilot so it will fly all of the turns correctly in NAV or APR modes. My 231 has a Century 41 with no HSI but I have added GPSS. In NAV or APR modes the heading bug turns into the course selector. If I leave the GPSS on it acts like I have auto slew so it will make turns and give me wind corrections while in APR mode.

That's the way the GFC 500 seems to work, with the exception in APR mode the AP follows vertical guidance, too.

[N231BN]

That's the way the GFC 500 seems to work, with the exception in APR mode the AP follows vertical guidance, too.

That is correct, any of the modern EFIS systems have auto-slew in GPS mode. It really isn't all that new, I flew a 1993 CE-525 that had auto slew on the SPZ-5000, of course it was connected to a Universal FMS which was more difficult to use than even a KLN-90.

[carusoam]

Processor speeds and memory is in short supply on anything old like BK’s KAPs and older KFCs...

A KAP150 has a few benefits like variable intercept angles, and Turn predictive warnings...

GPSS technology came after their production... the KLN90B has the IOs to handle the GPSS... which is nice...  the KLN94, newer, color, not enough IOs...

PP thoughts only... not enough detail to be really helpful...

Best regards,

-a-

[jlunseth]

14 hours ago, N231BN said:

The combination of curved flight paths and manual course pointers doesn't play well with legacy autopilots.

Another challenge is to fly a WAAS approach with a KFC200(250 in this case) that doesn't have roll steering. The CDI tracks the curved turn so I won't enter intercept mode. I end up steering the airplane with the heading bug until I'm lined up with the final approach course and then re-engage APR.

Someone should make a GPSS converter that has a third mode, auto slew. When the navigator is in GPS it can send the roll steering signals to the course input of the autopilot so it will fly all of the turns correctly in NAV or APR modes. My 231 has a Century 41 with no HSI but I have added GPSS. In NAV or APR modes the heading bug turns into the course selector. If I leave the GPSS on it acts like I have auto slew so it will make turns and give me wind corrections while in APR mode.

My set-up will fly all kinds of lateral GPS courses as long as I leave it in HDG mode with the GPSS in GPSS mode. However, the GPSS mode does not fly the vertical course, to have the AP do that you need to get the AP in APCH mode at some point so it will couple and follow the GS. You can use the trim button on the AP to descend, but it is descent at a fixed rate that is not necessarily the GS rate.

PS I shouldn’t say “at some point.” In my experience with it, that has to happen before the FAF or it won’t couple. 

Edited August 8, 2020 by jlunseth

[N231BN]

Modern EFIS systems combine GPSS with traditional NAV guidance via auto slew in which the GPS controls the course knob for you.

For example, the GFC 500 doesn't have a GPSS button because when you select NAV and the Navigator is on GPS, it automatically knows where to go without turning the course knob.

[flysamo]

having converted to the g275 units in our Mooney, we are finding found the limits on the kap 150 ap, so we have decided to convert to GFC500, thanks for the discussions, received some explanation as to what was going on with the digital/analog interfaces, you guys are great thanks

[flyboy0681]

4 hours ago, flysamo said:

having converted to the g275 units in our Mooney, we are finding found the limits on the kap 150 ap, so we have decided to convert to GFC500, thanks for the discussions, received some explanation as to what was going on with the digital/analog interfaces, you guys are great thanks

I'm considering a G275, which will be coupled to the KAP 150. What are the limitations?

[aviatoreb]

6 hours ago, flysamo said:

having converted to the g275 units in our Mooney, we are finding found the limits on the kap 150 ap, so we have decided to convert to GFC500, thanks for the discussions, received some explanation as to what was going on with the digital/analog interfaces, you guys are great thanks

Me too - I am close to putting in GI275 to drive my KFC200 for now - and I would also appreciate specific feedback of what was not working for you please.

Erik

[flysamo]

we have been flying for while limping along with BK150 upon inspection it was a -15 unit one of the last ones produced. the main control board was burnt and unrepairable . Cost to repair was $5k, in order to keep flying we patched together a-3 unit with Mooney specked control plug ins. this allow the unit to track nav or heading , but on approaches aircraft wanted to do a hard turns, it follows shallow turns. Since the unit is analog and a temporary unit we not to worried about the box. after reading comments on the GFC 500 , people seem to be very happy.,

after flying with kap 150, which has already cost about 4k to maintain, not worried about $ spent, next month new system will be installed without a yaw damper

G275 are great, only comment is that they are different, very different from our old X BK units, its really designed to work as a system that why we are looking forward to the GFC 500, a lot of touch instead of twisting knobs

Edited August 8, 2020 by flysamo
added info

[PTK]

The GI275 installation requires verification of proper autopilot functionality prior to doing the upgrade. Mostly to check pitch and roll centering. The autopilot sometimes needs a slight readjustment after the new device driving it is replaced. These are very fine adjustments. Less than half a turn travel on the pots. They can be very easily damaged if not aware of this and/or not very careful!
@flysamo, sounds to me it wasn’t limitations on the autopilot. Rather your autopilot failed to pass the requirements.

[carusoam]

Samo,

You are approved to get the the new technology... 

The KAP150 is a digital box... but 25years old.

 

The yaw damper for the Garmin box is getting good reviews...
 

Fast microprocessors and sensors can make these things a reality...

Not just wishful thinking...

PP thoughts only, not a technical guru, or mechanic...

Best regards,

-a-