Why does default A/P overshoot?

[ryanbatc 12,820]

Every addon plane I have which uses default A/P logic overshoots the localizer, or final approach course. Speed is a factor, but even at approach speeds it still misses.

 

Am I the only one with this issue?

[mgh 89]

Probably because the FSX autopilot is a simplified representation of real-world one. This is a sub-$50 game.

[henri5 16]

It should not do that. Perhaps you have runway altitude too low (on most runways this should not be an issue) or have changed the barometer setting and the plane altitude is wrong. To adjust the barometer, press the B key. Although I don't think that this is a factor, make sure the plane is not too heavily loaded (most airliners can't land full of fuel). On complex planes like the PMDG 737NGX you have to set the runway altitude.

 

Although most FSX planes do not have autoland, I can usually land them without crashing just by slowing enough before touchdown.So try reducing speed until the plane stalls to make sure you are not going too fast, then take a landing speed 5-10 knots faster. Speed is the most frequent cause of overshooting the runway.

 

Henri

[Old_As_Dirt 160]

Ryan,

 

By the sheer number of posts that you have, I feel ill-equipped to try and answer your question, but feel that two things need to be asked.

 

1. Is it only add-on aircraft? Do others (default) exhibit the same traits?

 

2. If the answer to #1 is that the defaults have the same problem, have you changed your terrain to make the elevations a problem?

 

These questions would give us more information to think about.

 

John

[geoffco 0]

Overshooting the localiser is a common problem of the default FS autopilot. You can improve things by tweaking the NAV and ILS parameters of the autopilot section in the aircraft.cfg file but the system is just too simplified to deal nicely with all situations. After extensive tweaking I reckon mine is pretty good as long as the speed isn't too high and the LOC capture angle is 50 degrees or less...

Of course you have to tweak each aircraft individually and most of the time the settings that already exist are a reasoned compromise by the developers to get it working in most situations. If you're really keen you can write your own gauges and use more complex algorithms than the default ones.

Tweaking info here: http://msdn.microsoft.com/en-us/library/cc526949.aspx

 

Geoff

[ryanbatc 12,820]

Ryan,

 

By the sheer number of posts that you have, I feel ill-equipped to try and answer your question, but feel that two things need to be asked.

 

1. Is it only add-on aircraft? Do others (default) exhibit the same traits?

 

2. If the answer to #1 is that the defaults have the same problem, have you changed your terrain to make the elevations a problem?

 

They don't mean much...

 

1. No it's every plane that uses default A/P logic (the only ones that don't in my hangar are PMDG planes and my planes equipped with RXP, unless I use the default navigation)

 

2. I have added 3rd party mesh but why would that affect localizer intercepts? That's a lateral signal.

 

I figured by now since we've smashed G3D.dll errors, runway friction problems etc we'd have fixed this too

 

By the way, at least in the US, when being provided with vectors to final the angle (course) should not exceed 30 degrees off final approach course... so I always do that and sometimes I'll use 20 degrees if close to the FAF.

[Old_As_Dirt 160]

Ryan,

 

I have found that some third party meshes actually relocate items within FSX and, if they did, even by a few yards, it could cause alignment problems with runways. I only use UT and GEX (which are both old programs) and find some things completely out of place. Supposedly, they are more inline with the actual locations, but some missions (for instance) are realigned to the point that you cannot reach the goal. You are right - elevation would not be a factor - I was thinking glide slope. With my limited add-ons, I find very little overshoot, even when approaching at 90 degrees. In x-plane, that same approach will swing way out and there is some re-alignment to intercept the path. Both default and add-on aircraft seem to exhibit the same traits. Speed seems to be more critical (very critical) with intercepting glide slope than the path.

 

John

[Bob Scott 7,716]

The FSX autopilot logic is actually fairly sophisticated--it uses a decent implementation of a PID feedback control algorithm. The problem, as I see it, is that the PID parameters supplied with the default aircraft are seriously up a duck's butt (they are located in the autopilot section of the aircraft.cfg--the nav_* and gs_* parameters in particular).

 

The feedback loop does appear to be somewhat more sluggish in FSX than it was in FS9 (my theory is that's due to a lower update frequency), so getting the PID params right may entail using different values than what might have worked in FS9.

 

Regards

[ryanbatc 12,820]

That was an interesting technical response. What exactly can one tweak in the autopilot section of the aircraf.cfg?

 

I've noticed even at slow speed (120 kias) the a/p will miss the localizer and go through about 1/4 mile then correct. I like to fly approaches faster with my quick props, sometimes 140-180 kias... then the a/p goes about 1/2 scale deflection before correcting..

[brucek 38]

 

 

Ryan, the approach to landing is the best part of the flight for manual flight- why are you using the AP?

 

Bruce.

[ryanbatc 12,820]

I fly on A/P often.... even if what you say is correct (haha) I still want the A/P to intercept correctly.

[geoffco 0]

That was an interesting technical response. What exactly can one tweak in the autopilot section of the aircraf.cfg?

 

Have a look in the link I posted earlier - that is a complete listing of all the possible aircraft.cfg parameters...

 

Geoff

[scott967 62]

Look up any text on control theory on how PID control law works.

 

scott s.

.

[Guest]

Maybe you are trying to intercept the loc too close to the airport? The closer you are the smaller and more sensitive the er... cone is, if you know what I mean. (Don't really know how to say this in English ^_^ ). Just like VOR: the closer you are the harder it is to keep the CDI centered. I think I've noticed lately that doing the intercept further out helps to prevent overshooting.

[Simicro 247]

Thanks for your feedback for guys who will have the issue in the future. It's rare to see someone who contributes to the community knowledge in his 1st post 🙂