VOR/DME approach into AVX
Printed From: Avidyne
Category: Avidyne General
Forum Name: IFD 5 Series & IFD 4 Series Touch Screen GPS/NAV/COM
Forum Description: Topics on Avidyne's IFD 5 Series and IFD 4 Series Touch Screen GPS/NAV/COM
URL: http://forums.avidyne.com/forum_posts.asp?TID=536
Printed Date: 21 May 2025 at 5:40am
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com
Topic: VOR/DME approach into AVX
Posted By: brou0040
Subject: VOR/DME approach into AVX
Date Posted: 05 Jan 2015 at 12:21am
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There was a post on the AOPA forums asking about how to turn outbound on the approach if inbound from the northeast, see here http://forums.aopa.org/showthread.php?t=94098" rel="nofollow - http://forums.aopa.org/showthread.php?t=94098 . There is a discussion of how a pilot is supposed to, but not legally required to lead the turn and a screen shot example of how the GTN750 sim leads the turn as you would expect. I tried the same example with the 540 PC sim and it does not lead the turn. It flies directly to SXC then overshoots SXC and then turns right to intercept the proper heading. The first way I tried this, I entered LGB SLI V21 SXC VORDME-B AVX with the SXC transition. I was thinking that perhaps it was somehow due to having the airway end at SXC and the procedure begin at SXC that it somehow forced it to fly over that point, although that is not what I expected it to do. The second time I entered LGB SLI VORDME-B AVX with the SXC transition without using SXC as a waypoint or the victor airway. In this case, it created a gap between SLI and SXC. When I cleared the gap, the behaviour was the same as above. Any thoughts as to why the 540 does not lead the turn in this example?
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Replies:
Posted By: AviJake
Date Posted: 05 Jan 2015 at 10:31am
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Any time the course change at a waypoint is more than 135
degrees, the IFD540 considers the waypoint a flyover. In the subject case,
as stated in the original AOPA post, the course change at SXC is 150 degrees. I have added additional verbiage in the Rel 10.1 version of the Pilot Guide that describes this design. ------------- Steve Jacobson sjacobson@avidyne.com |
Posted By: brou0040
Date Posted: 05 Jan 2015 at 2:05pm
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Thank you for identifying what was causing this behavior. Can you expand on the reason the 540 considers it a flyover? I'm assuming this was a tradeoff of some sorts so I'm trying to understand the side of the trade.
If it's not a tradeoff and was just an assumption, is there any reason this couldn't be removed or modified so that it would lead the turn?
I don't think it's a procedures thing because the AIM recommends leading the turn and the 750 doesn't treat it like a flyover.
Is there a way the user can currently remove the flyover assumption similar to closing a gap in the flight plan?
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Posted By: AviJake
Date Posted: 06 Jan 2015 at 3:05pm
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I've spoken at length with the lead FMS designer here. Based on his explanation below, we don't have any plans on changing this behavior. I'm curious to see if you agree after reading this: 135 degrees is a number that our lead FMS designer air transport FMS days. All of our limits were based on Cirrus type speeds and we ended up using 180 knots as the max. We also defined a max turn initiation distance of 2.5nm. At 180 knots and a 24 degree bank angle, to accomplish a 135 degree flyby turn you would have to start turning 2.5 nautical miles before the waypoint. The 135 is an arbitrary kind of number, but you can see what would happen as you increase that number by going to the extreme. If you have to start 2.5 miles for a 135 degree turn, then you would have to start 4 miles before the waypoint for a 150 degree turn. For 170 degrees, it's 12 miles. Of course, at 180 degrees, it's impossible. So, at some point it's nonsensical to fly by a waypoint because you're not even actually close to the waypoint when you fly by it. All of these limits are based on the 180 knot speed and the max 24 degree bank angle, though. Since the turn radius is proportional to the square of the speed, you can make much tighter turns closer to the waypoint if the groundspeed is slower. Of course, we try not to command 24 degrees of bank either. The higher you go from 135, the more likely it is that someone will define a flight plan that is not flyable at high groundspeeds. Actually, the GTN pictures in the AOPA posts illustrate the problem pretty well. With the 150 degree course change, the turn starts early enough that you're still a couple miles from the next waypoint RIGLI when you roll out. But with just a few knots increase in groundspeed, you would easily get into a situation where you would start the turn so early that you wouldn't roll out in time to hit RIGLI. In neither of these cases are you actually close to the SXC VOR when you "fly by" it. Bottom line is that we could increase it and this case would work out. But what about the next guy who wants to lead a 160 degree turn? I had to increase the limit to 151 in order to get the picture below. We're going to stick with what we have.
------------- Steve Jacobson sjacobson@avidyne.com |
Posted By: brou0040
Date Posted: 06 Jan 2015 at 4:37pm
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Thanks for the detailed reply, I really appreciate that.
I'd have to study the TERPS to determine if it is true, but I doubt it's permissible to make an approach that you would miss a downstream waypoint if you lead the turn (at the proper airspeed), especially considering that it is written into the AIM to lead the turn - understanding the difference between guidance and regulation. I would think that the TERPS are written such that the approach designer would be forced to state approach NA for flights arriving on V21 from the NE if it weren't possible.
Sure someone could define an enroute flight plan that would break the fly by logic, but I think the 540 should be capable of leading all turns for valid approaches. This approach is only valid for category A and B aircraft so it limits the speed on the approach to less than 121 knots. So basing the fly by on 180 knots is not appropriate for this case.
Since I'm not sure you have easy access to the categories valid for each approach via the nav data, I'd suggest that you allow two additional user inputs. The first being the typical cruise airspeed. That way you can calculate the fly by radius to suit the cruise airspeed for each installation. You can calculate the max degree turn if you assume the 2.5 max turn initiation and bank angle. Second, I'd suggest adding a user defined approach speed. Use that number for the fly by radius when flying or beginning an approach.
I haven't been through your code, but I wouldn't think this is a significant code change. It simply calculates some existing parameters based on user inputs rather than using constants based on a Cirrus. All of the rest of the coding is already there.
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