RNAV approach using LNAV/VNAV

[tango4]

Hi everyone,

This is my first post here, so let me just start by saying that even though this product is still clearly not perfect, I am absolutely amazed at what you have achieved. This aircraft alone made me decide to make the move to XPlane and so far, I am extremely pleased with what I am seeing. 

So really, first of all congratulations to the dev team for an amazing job, and keep up the good work. 

 

Here is my question: I read in this forum that SPD intervention was not modelled because it was not very common on the 737 Classic.

So I assume it is only possible to fly an RNAV approach using LNAV minima only, or am I missing something and it is somehow possible to fly it in LNAV/VNAV with the associated minima ?

I just wanted a clarification here.

 

All the best.

 

Charles MOULIN

[Eddie]

To my knowledge (although I almost never use it this way), if left in VNAV, the plane will gradually slow itself down to Vapp as you deploy more flaps. You can then fly the approach at normal speed.

I agree that having no manual control feels a little awkward, but that method's SOP on the MD-11 for every landing and the 737 will make sure you're within all speed margins throughout the approach.

[tango4]

I do think that it behaves that way, but it looks more like a "default" behavior rather than a certified use.

Because that would mean that once established on the approach, the pilot can not precisely maintain a speed. 

I had the PMDG MD11 many years ago so I don't remember precisely but I think it was more like the Airbus managed speed (meaning the pilots controls the airspeed and at some point during the final approach will tell the aircraft to reduce to its approach speed. I know it is not the full story, I am willingly oversimplifying for the purpose of this post). 

The problem I mention is especially true for long final segment where you are established on the final approach, but are still under assigned speed by ATC. (I do work as an ATC  at LFPG where we just validated new RNAV approaches as a backup for the ILSs and I can guarantee you that if an aircraft starts slowing to final app speed at 5000 more than 12 miles away, he is going to hear from me !)

[tango4]

By the way, I think that this capability to reduce speed "automatically " when extending the flaps is a 737 specificity due to their Smith FMS as opposed to the big Boeing which are equipped by Honeywell and don't behave that way. (or at least I think but I might be wrong)   

So to sum up the question would be :

Without SPD intervention option, am I legally allowed to fly an RNAV approach with LNAV/VNAV minima (and using these autopilots modes), with the limitation that you mentioned (the pilot not being able to control his speed precisely). Or am I restricted to flying them with LNAV minima and using V/S for vertical guidance.

 

And thanks for your help Eddie !

 

Charles

[Litjan]

12 hours ago, tango4 said:

By the way, I think that this capability to reduce speed "automatically " when extending the flaps is a 737 specificity due to their Smith FMS as opposed to the big Boeing which are equipped by Honeywell and don't behave that way. (or at least I think but I might be wrong)   

So to sum up the question would be :

Without SPD intervention option, am I legally allowed to fly an RNAV approach with LNAV/VNAV minima (and using these autopilots modes), with the limitation that you mentioned (the pilot not being able to control his speed precisely). Or am I restricted to flying them with LNAV minima and using V/S for vertical guidance.

 

And thanks for your help Eddie !

 

Charles

Hi Charles,

First, thanks for the nice words in your initial post. And yes, there is still lots to do on this aircraft, and we are well aware and working on it ;-)

you are correct - the 737-300 is not certified for LNAV/VNAV approaches (to my knowledge) - you have to fly the RNAV in LNAV and V/S, using the deviation bar like a "glideslope". At least that´s how we did it when I still flew the 737.

Cheers, Jan

 

[tango4]

Many thanks Jan !

This is precisely the answer I was looking for.

And regarding the quality of your simulation, here is what i like the most.

It is not perfect yet of course, and some features are missing altogeher. But your team is 100% honest about it, stating in a list what it DOES NOT simulate. It is one of the first times I saw this and was hugely appreciated on my part. You can't imagine how many hours I lost on competing products trying to understand a behavior, just to figure out after many research that this was not simulated... Very frustrating ! And I told myself: okay, if these guys are crazy enough to publish such a list, they must have some confidence in their product !

Regarding the bugs, after browsing the forum and reading a few posts, I really once again appreciated your constructive attitude, and the fact that you are not denying anythig and working hard to improve it. By the way, I think I have an issue where the autopilot does not follow correctly the LLZ signal on autoland (initial capture is perfect, and when closing to the runway, it slowly drifts and had me land outside the runway). But I'll have to run some more tests when I have time and will make another post if needed.

And regarding the product itself, although i am not an airline pilot, I do love harcdore simulations. I have all PMDG products, and made the move to Aerowinx PSX a bit less than 2 years ago. What seems to make your product stand apart in quality is the feeling of "being there". Your virtual cockpit is beautiful, the sounds are the best i've ever heard, and those small "vibrations and shakes" are the first time i see such an effect to be convincing and not just annoying. Flight model "seems" (I insist on that as I can only judge by my feeling, and readings, and so on) really good, and general smothness is much better than with PREPAR3D (even with the same FPS).

For the moment, apart from the choice of aircraft (I would like to see the same quality on more "modern" liners), I don't miss PREPAR3D. The only product which is really in a league of its own is Aerowinx PSX (honestly, I'd say without a doubt that this is the best ailiner sim avaible to the general public), but you don't have this "immersion" factor that many simmers appreciate and that you got just right as it is a standalone sim with 2D panels.

After some more flying I'll try to post a small review on X-Aviation, but i really wanted to let you know that you can count one more happy customer !

 

Keep up the good work and see you on the forums for some more questions !

 

Charles

 

[Litjan]

Hi Charles,

again, thanks for the feedback, both positive and constructive ;-)

We are very aware of our shortcomings - and although it has been a bit more "quiet" on the update front in the last few weeks, we are at work on the the first big comprehensive update (no ETA yet). I expect to scratch quite a few items off the "not in 1.0" list for that, and you may have seen a teaser shot of the new TCAS, for example.

The problem with the LOC signal not leading you down to the centerline can probably be attributed to (95%) bad X-Plane navigational data - usually the runway is misplaced by a bit. Go to your local map and zoom in on the runway (with display of ILS´s selected), then you can see how well the LOC lines up with the runway. The remaining 5% are attributed to deliberately offset LOC approaches (like KFLG, for example).

Happy landings, Jan

 

[tango4]

Very good news, thanks !

Regarding the approach, I'm not sure this is the explanation. (But I'm not sure it is not either...)

I remember it was an ILS at LFPG but can't remember which one. I just checked on the local map as you suggested and could not spot any visible misalignment (I'm using default scenery at LFPG).

What I do remember for sure was that initial LOC capture was spot on. And when closing to the runway (from 4/3 Nm onwards I would say), I saw that my plane was slightly drifting to the left, AND the LOC signal was showing just this ! What I mean is that what I saw in the outside view through the windshield matched the LOC signal I was receiving. It's just that the airplane let itself slowly drift to the left. (This was a coupled A/P approach, very stable, slowed to approach speed before glide intercept).

Anyway, I'll try and see if I can reproduce it or not. If it was a one time accident, my bad, perhaps I did something wrong. And if I am able to reproduce it consistently, I'll open a new topic in the support section if that is OK.

 

Charles

[Litjan]

51 minutes ago, tango4 said:

What I do remember for sure was that initial LOC capture was spot on. And when closing to the runway (from 4/3 Nm onwards I would say), I saw that my plane was slightly drifting to the left, AND the LOC signal was showing just this ! What I mean is that what I saw in the outside view through the windshield matched the LOC signal I was receiving. It's just that the airplane let itself slowly drift to the left. (This was a coupled A/P approach, very stable, slowed to approach speed before glide intercept).

Ok, this certainly does not sound like the cases I described - they would all have the LOC deviation pointer centered. I could also imagine a strong shift in winds - the gain (how hard the AP tries to track the LOC) is reduced when closer to the ground - otherwise the AP would get too twitchy and hectic. This is analog to the real aircraft, and a reason why automatic approaches are limited to 10kts of crosswind. There is also the need for the Pilot Monitoring to watch the deviation scales and call for a go-around if deviation exceeds 1/4 dot LOC or 1/2 dot GS (iirc), because this exact same think can happen with shifting winds in real life.

If it happens to you again, try to capture a screenshot (with the FMA showing), or possibly even a short movie (I am using ShadowPlay so I can capture the "last two minutes" with a press of a button).

Thanks again, Jan

 

 

[tango4]

I think you might have nailed it !

I am rather new to the X-Plane world and still fiddling with things. I'm using SkymaxxPro, RealWXConnector and XPNOAA weather and I think I forgot to check the wind on that specific test where I had the issue. And as an added factor, I am more used to the 737NG/747/777 screen layout and so I might not have paid attention to it on the ND.

I just ran another quick and dirty test with almost no wind this time, and did not see the issue !

But once on the runway, the aircraft did not properly track centerline. But I did notice that there was no "rollout" mode on the FMA.

Sorry to bother you again (promise, I'll start working on my FCOM !) but on the 737 classic, when performing an autoland, are you supposed to disengage the AP as soon as you are or the ground ? On the 747 for example, where only ailerons and elevators are controlled by AP in normal modes, when going to LAND3, rudder becomes active too, and on the runway, aircraft does follow the centerline. But once again it is not the same aircraft so this might be a 737 classic specificity.

Many thanks for your very kind support.

 

Charles

[Tchou]

737 classic doesn't follow centerline, it's one of the quirks making it not certified for Full instrument landing, you have to see the runway...

[tango4]

Thanks a lot for Chiming in Tchou !

Charles