APPR Behavior During a CDA

Background

During a basic approach, the plane will level off at around 2500 ft AFL to intercept the glide slope, but this level-off costs fuel and can be noisy. In a continuous descent approach, or CDA, the plane will not level off except to slow down before 10,000 ft. The glide slope can be intercepted from a 3 degree descent in line with it, or from a shallower descent below it.

The Problem

I have attempted to fly a CDA in Infinite Flight a few times, and have not been able to do so with APPR. As soon as you hit the APPR button, the VS is set to 0 and the autopilot yanks the nose up. Instead, you must let LNAV line you up (which might not be possible for all ILS procedures due to a lack of waypoints past the intercept…), and then use VS or fly manually to control pitch.

Why It’s Important

Many of the altitudes in the approaches and STAR’s set you up for a shallower descent to intercept the glide slope from below, and if you look at the charts, this is confirmed by a diagram. Unless you want to “eyeball it” with VS (like I used to) or calculate your own altitudes (as I do now), I would fly the already-implemented CDA.

Simple Solution

Instead of trying to be smart and leveling off automatically, APPR should continue flying at the current heading and VS until intercepting. This would make flying realistic approaches much simpler and allow us to take full advantage of the altitudes already in procedures, and make VNAV a more viable system.

Note

Unfortunately, I am not at a high enough trust level to create a feature request, so I will move this topic sometime in the future, unless someone else puts in a request.

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I think that could be useful, it certainly would help with my struggles with appr sometimes, I end up way above the glide slope by the time appr intercepts the localiser so this could help that aspect too

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Just as a rule of thumb for a regular approach, level off at 2500 ft AFL before intercepting the LOC and don’t hit APPR until you’re level. That way you’ll always be set up for an intercept.

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I completely understand your point here, that alot of attentions should be pointed to that feature

I also mentioned for a division of that same system in question here if you wish to have a look, if you want to point out the issue encoutered with and if you can vote on it to support the importance needed to that particular feature in the Sim.

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Freed up a vote for it! On another note, the speed limit of 180kts is a little low. Correct me if I’m wrong, but during a low drag or delayed flaps approach, a 737 will intercept the LOC at flaps 1 at a speed of about 190kts. And given that we’re on the topic of accommodations to alternative approaches… You know what I mean.

Or would you just use LOC and G/S until slowing down? Wasn’t quite sure how to interpret the proposed system…

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I kinda struggle with levelling off tho tbh because I just follow VNAV all the way down to the final altitude or until loc intercept and then just follow the glide slope. I’m just unsure how far out to be/how to judge my position to be at 2500ft. If that makes sense. Still learning tho

Oh I see. In my experience, VNAV isn’t much help at higher altitudes unless you calculate your own altitudes. Unless you’re doing a sharp turn or U turn to land, I’d be level the waypoint before or at the localizer intercept, or about 0-5 miles.

There are plenty of ways to ensure you’re at the right altitude at the right distance, you just gotta learn ‘em. I suggest watching Infinite Flight tutorials and searching around aviation forums.

I’d explain how I descend but it’s a complicated combination of many methods and has contingencies.

Maybe one day I’ll create a topic.

EDIT: or a YouTube video… hard to explain in writing and without live examples😂

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Oh, and just to be clear, the altitudes don’t always work out to an intercept. Infinite Flight procedures can be… questionable; so take a look before punching VNAV.

Someone correct me if I’m wrong, but I believe irl there are two AP modes to consider for intercepting the GS: altitude capture vs glideslope capture.

Altitude capture is where you’re level, so zero VS, as you wait for the GS to descend to you.

Glideslope capture is where you’re descending while you capture the GS.

Both are routine irl, but glideslope capture presents more challenges, for having the aircraft configured properly and descending at an appropriate angle so that the GS intercept will occur at an acceptable altitude and distance from the runway.

The IF tutorial, if I remember correctly, only discusses constant level GS capture (so, altitude capture mode).

And every sudden pitch up transition I have been able to reproduce comes from me being in a descent when I engage approach.

So for smooth GS transitions, don’t be descending on VNAV, but rather be flying with VS set to zero.

It’s safe to say that APPR in IF is “altitude capture mode” only?

And perhaps this is due to additional complications imposed by including a GS capture mode?

It’s very smooth, as long as you approach GS at constant alt.

Yeah that’s what I meant. I’ve seen the IF tutorial… I don’t think it would be too complicated, but I have no idea how the devs coded the autopilot.

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Of course, it is possible to code specifically for a smooth CDA intercept (that is VS<0), but imagining myself in the position of the developers, I was wondering about other issues they may or may not have had to confront.

For example, ATC giving clearance for an ILS approach. Does it cause complications in IF, considering the wide range of user experience levels, if a descending course to the ILS intercept is allowed? (perhaps expanding the complexity of managing challenges to violations)

Such questions of mine are just speculative, but I wondered if there was a need to limit the complexity of certain scenarios somewhat, to prevent causing complications elsewhere in the sim.

I was previously engaged in a separate topic about intercepting the localizer while turning (anticipating an LOC intercept from as much as 90 degrees from the start of the turn).

While it can be done, it will also fail if you don’t do it carefully, and with some prior experience of what mistakes to avoid. So it does reduce complications to just say “don’t exceed a 30 degree intercept angle,” which always works.

So, I wondered if not prioritizing a smooth CDA transition, might be due to some such type of complication making descending intercepts a challenge to currently integrate into IF flying procedures.

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Ah. I see. Haven’t thought about it that way. So you’re saying that the actual change wouldn’t be to hard but integration would…

I could see there being something like:

Airline 123, expect the CDA ILS runway 1L.

Probably not how it is IRL but it would work. Perhaps something only on the expert server, and maybe disabled when there’s crazy traffic.

If this became a widely used approach, then ATC might have to start enforcing certain STAR crossing restrictions to deal with CDA planes in a way that keeps it all organized.

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???

this needs to be at least 10 characters

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Not sure if anyone has mentioned this already but I’m pretty sure the issue is that the glideslopes aren’t matching the indicator on the HUD. I did some test flights a while back at KLAX on 25L and found that if you activated APPR when the glideslope indicator was a certain distance from center then it would smoothly transition to being locked. Then, as you got closer to the runway, you could watch the ball slowly go up the indicator, passing the on-glideslope line, and moving well into the too low section way before you get close to the runway.

The real problem here is that the autopilot doesn’t lock until the indicator shows you are lined up, not when you actually are. In other works, APPR is not waiting for you to intercept the glideslope. It’s waiting for the glideslope indicator to show that you’re lined up. That’s why you get that sudden jerk every time. You actually pass the glideslope first, then a second or two later the indicator hits the line, then the AP corrects because you’re below the glideslope.

I have no idea how the glideslopes are misaligned with the indicators, but I’m pretty sure that’s the issue. Worst part about this is that there’s no way around it unless the misalignment is exactly the same for all approaches. I haven’t checked to see if it is.

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I’m now rethinking some of what I said. I previously demonstrated to myself that sudden pitch up occurs when VS is set with a descent value (negative number), but the GS is above me.

But is it true or false that “glideslope capture” mode is typically desired when the GS is below you? This seems a smoother transition when the GS is captured as you descend into it at a steeper angle the GS itself. I manually dial in the steeper VS I believe will allow me to intercept the GS from above.

To be honest, I don’t really understand what you’re trying to explain. Also, I was just doing some approaches to KLAX 25L and I didn’t really have much problem, whether hitting APPR as early or as late as possible. The only problem I could get repeated, as before, was hitting APPR with GS above and VS negative.

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This fixes the issue with APPR being indicator dependent…

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These are the quality of life improvements I’m looking for in updates rather than being able to see an aircraft push back with a tug visible or a new aircraft. Guess everyone just has different “needs/wants”.

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About the whole indicator-not-being-correct thing:

Many approach plates of standard approaches (excuse my poor terminology) and CDAs specifically say “VGSI (vertical glide slope indicator) and glide path angle not coincident”

So it’s not supposed to be used the way Infinite Flight APPR does. In something like a 737, the flight directors tell you where to go, and the indicators may be off a bit.

I agree… but to be honest, I think most people don’t even know what a CDA is, or care that APPR uses the indicator (probably makes since to a lot of people that APPR follows what you see anyways)

Exactly what we’re saying. Also, you would never intercept the G/S from above because of false glide slopes. (has caused incidents before)