Heading to intercept an approach course

[N2843R]

I'm trying to figure out how I can get the autopilot to fly an established heading to intercept a nav course. On an instrument approach, ATC will often provide a vector to intercept a final approach course. When on that vector, I want the autopilot to maintain the heading until the course is intercepted. However, when I push NAV, it will fly a heading that it "thinks" is the best to intercept the course, which often is not the heading assigned. Other autopilot systems have this feature. Does it exist with the Dynon A/P? If so, how do I enable it?

 

[airguy]

When you are within range of the final course, NAV will grab it and intercept at it's own calculated angle. When you are further away, it will hold the heading/track value until it gets within a certain distance of the final course, then it will grab it and intercept at its calculated angle. In my experience ATC usually turns you onto the final approach course close enough that if you press NAV at that point, you are already within the range where the autopilot will activate GPSS and turn itself in for its own calculated intercept.

If this really bugs you, just delay pressing "NAV" until you are on top of the final approach course and then turn in - but ATC isn't going to care if your heading wanders a few degrees on the turn in - remember they are used to watching students learn how to hand-fly it every day. Much more important, in my mind, is monitoring that the autopilot is indeed on GPSS mode rather than HDG/TRK mode and monitoring that it is doing what is expected - even if that means an intercept angle that is more or less than you would have had with a true HDG/TRK intercept.

 

[skysailor]

Skyview will absolutely do this. This is all done on the autopilot ROLL page. You simply need to put the airplane in HDG mode on the vector you want to maintain until you interecept. This will show green. Now, press the NAV button. The HDG button will stay green and the NAV button will show gray (armed). At the top of your PFD you will see HDG->GPS (if an RNAV GPS approach or just Skyview flight plan for this example). When the airplane captures the course your will hear "Autopilot Mode", the green HDG button will go out and the gray NAV button will become green and the airplane will begin the turn. At the top of the PFD you will see HDG->GPS has changed to now show GPS.

 

[airguy]

As Skysailor says - that's the way Skyview works - but Skyview will want a particular intercept angle to join. If your HDG selection gives a relatively shallow intercept angle, when Skyview switches from HDG to NAV you may actually see it turn you in toward the localizer at a steeper angle before turning back to intercept - it's all in how the autopilot is trying to get you back on the localizer.

Remember when it switches to NAV - it doesn't know that you're trying to do a smooth intercept, it just knows that it is supposed to be on the centerline and you are left/right of it, so it turns you in to get back on the centerline.

 

[BradThePilot]

Skyview is goofy in this respect - other autopilot systems don't do this and if I let Skyview do it's natural thing when doing an approach into MSP/DTW/ORD like I do in the work airplane I think there'd be some questions. I would expect that it would hold the heading until intercepting the course and then switch to nav, but as has been pointed out, it doesn't work like that. So I just switch to nav mode manually at the appropriate time.

 

[rjones560]

I'm trying to figure out how I can get the autopilot to fly an established heading to intercept a nav course. On an instrument approach, ATC will often provide a vector to intercept a final approach course. When on that vector, I want the autopilot to maintain the heading until the course is intercepted. However, when I push NAV, it will fly a heading that it "thinks" is the best to intercept the course, which often is not the heading assigned. Other autopilot systems have this feature. Does it exist with the Dynon A/P? If so, how do I enable it?

Well, it would be interesting to know what navigation source you are using for the approach in question. I have 3 HDX screens in my RV10 and the Dynon autopilot as well as a Garmin GTN650 for IFR work. I wonder if any of what you are describing has ever happened before ATC cleared you for the approach? Once you are cleared for the approach you can adjust heading as necessary for a good intercept. As a professional pilot, I could not even count how many times ATC gave me a lousy vector for the intercept. As a result, I would adjust the heading as necessary to get on the approach path. If it was too shallow I would increase the angle. Some times this was necessary to intercept before the final approach fix. Too steep and I would decrease the intercept angle as I approached the approach path. The Dynon autopilot is just doing all that for you. Cleared for the approach from a vector on an ILS I select HND, then NAV and VNAV. The NAV will remain grey (armed) until the approach path is captured. VNAV will do the same until the glide slope is captured. If the autopilot does not like the intercept angle, the heading will change a little before the localizer is captured. That is within your authority when cleared for the approach.

 

[BradThePilot]

That is within your authority when cleared for the approach.

But not within your authority when you are given a heading and told to join the final approach course (no approach clearance given). That is where the Dynon AP does it wrong.

 

[rjones560]

But not within your authority when you are given a heading and told to join the final approach course (no approach clearance given). That is where the Dynon AP does it wrong.

The clearance to join the final approach course or "cleared for the approach" is not really relevant to joining the final. The only difference is being cleared for the vertical component. Just how far from the final approach course are you seeing these deviations? 3 miles 2 miles 1/2 mile or less? How many degrees are you seeing it turn? I have seen minor turns just before intercept, usually within 1/2 mile of the final. Those minor turns that close are irrelevant. You are cleared to join the final? Then you can adjust heading as necessary to join the final. Minor deviations in heading to join are perfectly acceptable. Just how many degrees deviation are you seeing? Does the change in heading help or hinder finishing the turn right on the center of the course?

I am not saying that you are wrong because if this is going on 3 miles or from the final approach path then I would be concerned about it. If the turns are less than 10 degrees deviation from the heading assigned and less than a mile from the approach path then they are really irrelevant. For me that is about 30 seconds from completing the turn down the final. If you are doing a GPS approach, the box is going to try to intercept without over shooting. With a tail wind on the vector it may make a shallower intercept to avoid overshooting. With a strong head wind on the vector it may make the intercept angle a little steeper so that you are established on the final before the FAF. If I was flying manually I would be doing the same thing. So this comes back to, how much is the autopilot deviating from the assigned heading, and how close to the final are you when this starts happening?

 

[BradThePilot]

The clearance to join the final approach course or "cleared for the approach" is not really relevant to joining the final. The only difference is being cleared for the vertical component. Just how far from the final approach course are you seeing these deviations? 3 miles 2 miles 1/2 mile or less? How many degrees are you seeing it turn? I have seen minor turns just before intercept, usually within 1/2 mile of the final. Those minor turns that close are irrelevant. You are cleared to join the final? Then you can adjust heading as necessary to join the final. Minor deviations in heading to join are perfectly acceptable. Just how many degrees deviation are you seeing? Does the change in heading help or hinder finishing the turn right on the center of the course?

I am not saying that you are wrong because if this is going on 3 miles or from the final approach path then I would be concerned about it. If the turns are less than 10 degrees deviation from the heading assigned and less than a mile from the approach path then they are really irrelevant. For me that is about 30 seconds from completing the turn down the final. If you are doing a GPS approach, the box is going to try to intercept without over shooting. With a tail wind on the vector it may make a shallower intercept to avoid overshooting. With a strong head wind on the vector it may make the intercept angle a little steeper so that you are established on the final before the FAF. If I was flying manually I would be doing the same thing. So this comes back to, how much is the autopilot deviating from the assigned heading, and how close to the final are you when this starts happening?

I agree that if the clearance is "Fly heading 270º to join the ILS 30R final approach course", that is not a clearance for an approach nor is it a clearance to start descending. It is also not a clearance to fly a heading of 255º from about two miles away, which the Dynon wants to do. I though it might be AP tuning parameters, but it's not - Dynon really wants their own intercept angle and will make that turn every time regardless of what the heading might be.

Using the above example, if I'm flying inbound at 270º to intercept the course, I would expect that having heading mode active at 270º and having nav mode armed would result in the 270º heading being maintained until the turn to track the inbound course needed to be made. Actual behavior seems to be hold 270º until within some non-trivial distance, then turn to 255º or thereabouts, then turn to track the inbound course at some point later on.