Hi,

 

Turn prediction needs tweaking. At the moment it overshoots turns, and if it has a tailwind, the error increases.

 

Best regards,

Robin.

 

 

Turn prediction needs tweaking. At the moment it overshoots turns, and if it has a tailwind, the error increases.

 

If you believe it's truly an error, you should know by now that the official avenue to report it is via the support portal.  Chances of things being seen here are slim, especially with the team constantly working on finishing the SP up...

 

I will say, though, that no aircraft is perfect, and I'm not quite sure this is actually a bug.

Good point. I'll raise a support ticket so they're aware.

 

Regarding the turn prediction - flying S turns is not what the real aircraft would be allowed to do.

 

Example: turn is a 60 degree right turn. Aircraft will turn right, overshoot, so it has to sustain the turn back towards the track, then it will have a small overshoot during the left turn back on course, then it will turn right to correct and then be on course.

 

I also had such a 60 degree right turn on the LAM3A to RNAV RW27R at Heathrow. Even at 137 kts it overshot the turn and had to turn to correct. This is in no way how the real thing would fly.

 

Try a flight from Heathrow to Schipol. DVR5F REFA1A. Make sure the aircraft has a tailwind of 50 kts. For me it overshoots such turns, and spends a rather long time turning to correct itself, when all it had to do was start the turn a mile earlier.

 

Best regards,

Robin.

 

 

Example: turn is a 60 degree right turn. Aircraft will turn right, overshoot, so it has to sustain the turn back towards the track, then it will have a small overshoot during the left turn back on course, then it will turn right to correct and then be on course.

 

It's definitely worth a look, but there are tolerances to everything.  There's a reason controllers are allowed a limit of 30 degrees (20 in certain cases) for a final vector for the LOC.  Part of it is so that the aircraft/pilot can establish themselves in a stable state within a reasonable time before landing.

 

With angles like 60 degrees, I'm not at all surprised it would cross the course.  You may be right (so definitely get that ticket in), but I wouldn't be surprised if this is accurate to the aircraft's behavior.

Good point. I'll raise a support ticket so they're aware.

 

Regarding the turn prediction - flying S turns is not what the real aircraft would be allowed to do.

 

Example: turn is a 60 degree right turn. Aircraft will turn right, overshoot, so it has to sustain the turn back towards the track, then it will have a small overshoot during the left turn back on course, then it will turn right to correct and then be on course.

 

I also had such a 60 degree right turn on the LAM3A to RNAV RW27R at Heathrow. Even at 137 kts it overshot the turn and had to turn to correct. This is in no way how the real thing would fly.

 

Try a flight from Heathrow to Schipol. DVR5F REFA1A. Make sure the aircraft has a tailwind of 50 kts. For me it overshoots such turns, and spends a rather long time turning to correct itself, when all it had to do was start the turn a mile earlier.

 

Best regards,

Robin.

I noticed that for a long time, and the overshoot will get worse if the next waypoint is very close ( under 20 nm). Perhaps a 777 pilot can comment on whether this is the case in the real world.

 

Francis Leung

Hi,

 

I raised a ticket, and the response back was that they have numerous videos of the Level D simulator showing over- and under-shoots in similar situations.

 

Best regards,

Robin.

In the interests of "as real as it gets", does the 777 overshoots in the real world then? If not, then I would say it is desirable if PMDG can improve that in the next updates. I won't call that a bug though, not that serious.

 

Francis Leung

I must say that if it over-shoots the way the (FS) sim does, then I'm definitely raising an eyebrow at it. I thought it would be (much) cleaner in turns than it is.

 

A340-300 is a similar vintage, and my friend recently jumpseated on it (actual aircraft) and watched it flawlessly capture the ILS, with a 90 degree intercept at 250 kts, with no overshoot, and roll out on course. Good luck doing that in the sim...

 

Best regards,

Robin.

I might be wrong, but 60 or 90 degrees turns to intercept the localizer is quite extreme. Intercept angles of 20 to 40° are more realisitc and as Kyle said, most ATC will give your angles of 30°. When a 60 to 90° turn is needed to intercept, I usually do it via the MCP heading selector and when established, might go back to LNAV or APP or manual if visibility is good. I'm not sure if a RW pilot trusts LNAV for a 60° intercept angle during approach.

The PMDG 777 can do a 90 degree intercept.

 

If that 90 degree intercept is part of an RNAV STAR, such as LIZZI7A Arrival to Runway 16 at YMML Melbourne.

 

http://www.airservicesaustralia.com/aip/current/dap/MMLSR05-135.pdf

http://www.airservicesaustralia.com/aip/current/dap/MMLII01-137.pdf

 

For this, the LNAV/VNAV would be engaged and the APP mode armed after the 352°radial is passed.

 

I don't see a 90° intercept working off "HDG SEL" mode with APP mode armed.

 

 

I don't see a 90° intercept working off "HDG SEL" mode with APP mode armed

 

No, I manually intercept using HDG SEL if a 90° turn is required. When on the LOC, I arm APP. Not saying this is the correct procedure, but for steep intercept turns, it puts me on the glide nicer and smoother than LNAV/VNAV. Just my experience.

When visibility is fine, I might just turn the AP off before the turn and continue hand flying.

I might be wrong, but 60 or 90 degrees turns to intercept the localizer is quite extreme.

 

Let me quote what I wrote in post #8:

 

 

 

my friend recently jumpseated on it (actual aircraft) and watched it flawlessly capture the ILS, with a 90 degree intercept at 250 kts, with no overshoot, and roll out on course.

 

You may *think* it is extreme, but it isn't. Aircraft are more than capable of flying these intercepts. Concorde could make a 179 degree intercept! It is written in the flight manuals.

 

People saying these things are "sim limitations" really need to get a clue about what actual systems can do and how they work. Where it would be a real sim limitation is when using the FSX localizer signal to determine when to turn, if it is incorrectly modelled.

 

In short: the greater the turn angle, the earlier the turn needs to be made. If you'd turn at Standrd Rate 0.5 nm before intercept for a 20 degree change of heading, then you'd turn 2.5 nm before for a 90 degree intercept. It is all down to turn radius, and is easy to calculate for a given ground speed.

 

http://en.wikipedia.org/wiki/Standard_rate_turn

 

 

Best regards,

Robin.

 

 

Concorde could make a 179 degree intercept! It is written in the flight manuals.

 

"Can" and "flawlessly" are two very different terms.

 

Knowing the type of navigation Concorde was using, I can nearly guarantee a few S-Turns as it tried to capture whatever signal it was trying to follow.  Of course, closer to the source of this signal, it would be tougher.

 

 

 

People saying these things are "sim limitations" really need to get a clue about what actual systems can do and how they work. Where it would be a real sim limitation is when using the FSX localizer signal to determine when to turn, if it is incorrectly modelled.

 

You're assuming "correctly modeled" is a perfect system, though.  The GS antenna of an ILS creates false glideslopes.  This, of course, is not "perfect."  FSX's depiction of ILS in the sim is not perfect because it's perfect in the sim. GS is available and perfect no matter where you are on approach.  False glideslopes don't exist.

 

So, modeling something to be perfect isn't necessarily modeling it to be correct in all cases.  Sure, I bet someone could easily write code for turn prediction to be more accurate, but is that going to be accurate to the real thing?  I doubt it.  There's a reason we still have RNP 0.3 as a max tolerance for "very precise" approaches.  Technology isn't always perfect, and there's still error in it, even when it's "more perfect than not."

There's a reason we still have RNP 0.3 as a max tolerance for "very precise" approaches

 

As you should know, that is related to the tolerance of position of where the aircraft is vs. where it thinks it is, and nothing to do with how tight it can turn or anything else. The aircraft can have an ANP of 0.29 and still fly an RNP 0.3 approach, though it wll put it right at the edge of the protected corridor.

 

http://www.smartcockpit.com/download.php?path=docs/&file=Getting_To_Grips_With_RNP-AR.pdf

 

Best regards,

Robin.

 

 

As you should know, that is related to the tolerance of position of where the aircraft is vs. where it thinks it is, and nothing to do with how tight it can turn or anything else. The aircraft can have an ANP of 0.29 and still fly an RNP 0.3 approach, though it wll put it right at the edge of the protected corridor.

 

Yes, I should (and do) know.  I helped design them...

 

You're correct in that it's a tolerance of calculated position versus actual position.  My point wasn't that it is the same technology.

 

RNP limits are set because we know technology is imperfect.  That's why we have ANP to begin with - it evaluates how much imperfection is in our assumption of position.  Hit POS on the 777 or the NGX and you'll see how out of sync the different sources of nav data are (especially as flight gets longer).  If technology was perfect, we wouldn't need ANP to measure how imperfect it truly is.

 

Using a different example: TERPS requirements (FAA, of course - though I'm sure ICAO members use similar standards) are really there because of the error present in the systems.  There is error in an ILS (both in LOC and GS systems) signal.  There is also potential error in how the aircraft tracks that signal, even if that signal were perfect.  Both of those limitations are accommodated in the tolerances set forth in the TERPS requirements.  Again, if all systems were perfect, those wouldn't be neccessary.

 

Aircraft can't even be 100% sure of their actual location.

Systems that guide aircraft cannot provide a 100% accurate signal.

 

So how is it that, given all of this error in those other technologies, aircraft tracking would somehow be 100% "on rails?"