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A Detailed Technical Description of the KFC 150 found ... finally


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In the course of troubleshooting a problem with the trim servo in my KFC 150 I attempted to find an engineering system description.  I knew this had to exist somewhere but it was not to be found in any resource I could locate for free.

The codex for the logic of KFC 150 operation is King Manual 006-5539-00 entitled "King KC 190,191,192 Autopilot Computer Maintenance / Overhaul".  There are no free sources that I could locate.  eFlightManuals.com has them available for .1 amu  or perhaps you can give your local shop a couple of six packs and take theirs to Staples.

However you acquire access to the manual, turn to Section 4 for your model computer and begin to learn how the system actually works in detail.  Expect that this will take a while and in some places be a bit of a steep learning curve if you are not an engineer and/or electronics enthusiast.  Here is a crib if you are interested in the trim servo.  The King terminology for what I would call the "trim system" is the "autotrim system".  I thought initially that was an option I did not have installed.

Even for the non-engineer, the Theory of Operation section of this tome is full of useful and interesting things not covered in the Pilot's Guide, which is primarily concerned with what button to push when rather than how the system actually functions.  Why King did not make this information easily available is a mystery to me.  Perhaps they thought only engineer pilots would care.  Here you can discover the one true answer to such mysteries as:

-  How to calculate the lead for level off when you hit altitude hold.  Answer: Lead the desired altitude by 15% of the rate of climb in feet.  I tested it yesterday, it works perfectly.

-  How does the system recapture after a losing the glideslope signal?  In distinctly different ways I discovered depending on the delay before the glideslope signal is again recieved.

- How in the world does it decide when to engage NAV mode once it is armed?  Well ... it depends on exactly how you engage NAV mode and the offset error from the desired course at the time it is engaged.  There is a whole section on just this topic.

- And the ultimate and final answer to that old chestnut:  Does the glideslope capture from above and below?  Answer:  Absolutely yes it does but it requires a null in the glideslope error signal to do so.  If you approach from above and then level off just at or above the glideslope capture never occurs.  (Guess now I know this ...)  Fly though the glideslope, like you do when level approaching from below, and it captures every time.

It is my hope that some other technical knowledge seeker will someday find this post and discover the path to KFC 150 enlightenment thus preserving their sanity.  Not to mention that you will be able to definitively determine the difference between how your KFC 150, or someone else's, is presently working and how it is SUPPOSED to work.

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12 hours ago, Ragsf15e said:

Now for the Garmin equipment…

Good luck. The King manuals are available because shops used to repair avionics in the field and techs needed an in depth understanding of how products are supposed to work in order to fix them. But Garmin products are all LRU (Line Replaceable Units) which means that in the field troubleshooting is limited to identifying an LRU that needs to be replaced. This probably makes sense because it is very difficult to field repair SMT boards at the component level so the most common repair would be a board swap, and at that point you might just as well replace the entire unit. There is also a shortage of avionics technicians capable of doing unit-level repairs in the field. 

The downside is that there is little information available about how this stuff actually works -- especially failure modes -- that is available to the owners and operators of this equipment. Just look at all the threads where people are trying to figure out how the AHRS works and what the failure modes are. Most pilots won't care -- but some of us do.

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50 minutes ago, N201MKTurbo said:

I have it, what do you want to know?

I can't publish it, it says their lawyers will descend on me if I do.

Is there an offset setting for the OAT? My 275 reads 2 degrees cooler than a half dozen other sources in the hangar with the doors closed.

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