Recently I was flying a PA28 with a KAP 140 autopilot, OBI (not HSI) and DG.
During the cruise I had the NAV1 source selector set to GPS, with an active leg on the KLN89B GPS. I hand flew the a/c to intercept the GPS course by following the CDI on NAV1.
Next I selected HDG to engage the autopilot followed by ALT. I checked that the heading bug was aligned with the aircraft's current heading and as expected, the A/P maintained altitude and direction.
Finally, I selected NAV, hoping that the aircraft would track the GPS course. As the CDI was already centred, I wasn't expecting the aircraft to make any large corrections. HDG extinguished on the display and it was now showing NAV / ALT.
At about 5 miles from my turning point, I decided to move the heading bug to align with the course for the next leg and to my surprise, the A/P started to roll the aircraft as if it was following HDG. I checked the A/P display and it was still showing NAV, and the CDI was centred. I had a little play about with the heading bug and sure enough, the aircraft was following it.
For the remainder of the flight I kept the A/P in HDG mode because I was't sure if it was actually tracking properly in NAV.
I've since checked the KAP 140 manual and it says for A/C equipped with an OBI as opposed to HSI, you should immediately set the HDG bug to the OBS course after selecting NAV. Why does the heading bug affect the A/P in NAV mode when you are using a DG as opposed to an HSI???
Last edited by TotalBeginner; 28th May 2012 at 20:44.
When flying an intercept to a NAV or GPS course, the autopilot sees two signals - cross-track deviation (CDI bar off-centre) and couse error - the difference between the heading bug and lubber line when you don't have an HSI. The first is used to determine how far off track you are and the second tells the autopilot which way to turn to capture the desired track when the deviation diminishes at capture. Once on track, the autopilot will use predominantly cross-track deviation to stay on course and the course error will be washed out by the cross-wind correction circuit. If you turn the course pointer once captured, you introduce an error signal to oppose the deviation signal and hence turn when you may not want to.
In the installation you describe, the heading selector is supplying the OBI signal to the autopilot. The CDI in this system does not output the necessary signal for where you've set the OBI card. When you have an HSI the functionality is much more logical.
What about when you're intercepting a course? Let's say the A/P is flying HDG (30deg intercept) with NAV armed. The CDI comes alive and at 2 dots it captures. Does that mean the A/P will not steer to maintain track until the heading bug matches the desired course?
Almost correct. You set the CDI and HDG bug to the desired radial. Assuming you have full scale devaition, the autopilot will turn towards the desired course until a point where course error and devaition are equal and opposite - this is scaled in the autopilot to be 45 degrees. As you fly towards the radial and deviation diminishes, the NAV autopilot will go from NAV ARM to capture and the aircraft wil turn to the pre-set course.
I have an HSI (EHSI actually) and on that the autopilot (in NAV mode) follows the course pointer, using it as the "heading", and then it adjusts this "heading" to minimise the error shown on the deviation bar.
I suppose, as wigglyamp says, if you don't have an HSI then the autopilot must be using the heading bug as the "heading".
I don't know the KAP140 but on the KFC225 you get undocumented behaviour in that the NAV mode must not be selected if the deviation bar is deviated by less than about 3 divisions (just over 1/2 full scale). If you do select NAV with a smaller deviation, the autopilot just turns onto the course pointer setting immediately, without intercepting the track. It does gradually, slowly, converge onto the track. Usually this is not what you want
When I bought the plane, it took me ages to suss out what was happening. Honeywell denied it, of course
I had the very same issue when I got my plane with a Cessna Navomatic 300A autopilot :-) Show me a flight instructor that actually understands those autopilots! Once I finally managed to understand its logic, I replaced it with a new model
In my setup, the DG was not slaved (and worn out) so it required adjusting using the mag compass. This had a very bad side effect.
In NAV mode, you set the heading bug + OBS to the desired radial, it will then intercept the radial, once intercepted turn onto the heading of the radial (i.e. where the heading bug is) and then establish a wind correction angle.
Now if you adjust the DG because of drift, the autopilot would sense a new setup, immediately turn onto the selected heading and slowly reestablish the wind correction angle. Imagine you got 35kt at 90° -- I had ATC ask me about my consumption of alcoholic beverages once...
I have come to the conclusion that there is no alternative to thoroughly understanding the internal logic of your autopilot. The user manuals are usually rather poor and rarely cover the complete logic.
Much of the time, in some complicated procedure, one ends up flying in HDG mode (and using the "track ahead" line on the GPS ..... if you have one) because then it does exactly what you want
Exactly. I now have a S-TEC 60-2 (similar to the KAP140) and an Aspen EFD1000Pro with GPSS. Ever since I got GPSS, I have never used anything but HDG mode -- only exception when flying an ILS using the autopilot.
The Aspen (more specifically its Air Data Computer) has a lot more information available than the autopilot and can do a much better job calculating the appropriate heading. I consider GPSS the biggest improvement of GA autopilots ever.
digital technology (such as the GFC700, DFC90 and DFC100) adds an entirely new layer of very good.
Yes, unfortunately mostly restricted to new airplanes. GFC700 and DFC100 are not available as a retrofit, and DFC90 is only certified for Cirrus and Piper Malibu and only for N-reg. S-TEC pretty much has a monopoly today for retrofits. Outdated technology at a premium price.
Once Aspen add altitude preselect to their PFD, they could provide everything a modern digital AP offers such as constant IAS climb/descent, a straight & level button, envelope protection, etc.
Avidyne are very very slow in certifying the DFC90 for "new" aircraft types. I get the feeling, watching them and speaking to the same faces at shows etc, that they have lost their way. Most of their business got wiped out when Cirrus (etc) moved from Avidyne to G1000. Some of their MFDs are barely visible in sunlight - totally useless.
Garmin are also very slow with the GFC700 for other aircraft models, which is otherwise a very good (if not yet long term reliability proven) autopilot, flying correct intercepts etc.
STEC are a big retrofit market player but their AP has poor performance because it doesn't have a decent source of pitch and roll information.
Avidyne are using STEC servos for the DFC90 because it saves them developing their own, and developing and certifying the multitude of different servo mounts.
All these systems are a poor solution however because they use brush DC motors which wear out after X hours and then you have to hand fly... and pay out 4 figures for a fix. They should have gone brushless... the model airplane world did so yonks ago, and anyway one would probably use a stepper motor for an autopilot servo.
This is a rather poor example of the use of brush motors, and crap electronics design, by Honeywell, in autopilots