Simviation Forums

This only seems to happen when I fly with GPS mode, IFR. It happens regardless of whether I use Direct GPS routing or waypoint (VOR or airway)

Basically, my plane keeps pulling either to the left or right, and at all times (when on NAV hold for the GPS) it is usually from 0.2 to as much as 2 nautical miles to the left or right of the flight path. It does hold close enough to the path that I don't get any vectors.

Sometimes the problem goes away temporarily, but after a while it may decide to crop back up and cause the plane to again deviate from the flightpath. Now, that in itself wouldn't be as odd, but additionally the plane is left pointing at the flight path, even though it's basically traveling parallel to it. What this means is that basically I'm pointing about one tick away from center on the HSI.

Here's some pictures to illustrate the problem:


and

(with stats)

Occasionally the plane returns to the flight path when a new waypoint is reached (on non-Direct flights)< but otherwise it stays a random distance away. When flying from Toronto to Munich (Direct-GPS), the plane actually went through some wild gyrations over the UK, where I thought there was some kind of turbulence.

Here's what I've tried so far, and hasn't worked...

Issuing the Center Ailerons and Rudder command - not sure if it has an effect because I have no confirmation

Checked and double-checked that rudder trim is centered, (I have no Aileron Trim on the A340, but this has happened on the 737 too.)

Saved and restarted the flight.

Engaging and disengaging autopilot

Ensured that de-ice and pitot heat are on.

Other than that I can't figure it out! Does anyone have any answers?

-Seth

A cockpit view with the GPS window open would be more helpful.  I assume you have:
AP on
AT on
NAV hold on
HDG hold off
NAV/GPS set to GPS.

It may be another FSUIPC setting.   Check your settings or just delete fsuipc.ini and it will re-create itself the next time you access it.

I suspect you are flying with a crosswind.  The crosswind pushes the plane off course (0-1.5 miles).  The autopilot attempts to correct by turning 0-2 degrees into the wind.  It ends up finding a steadystate position slightly off course.  The amount you are off depends on windspeed and direction.  When flying with the wind coming directly at you from front or back, error will be zero.  The wild gyrations can be caused when you transition from one waypoint to the next with the plane offcourse by more than 0.1 miles or so.  Solution is to use heading hold (whatever heading the plane is flying at) turned on about 10-15 miles from waypoint changeover.  After the waypoint changes, switch back to GPS navigation.

It's possible its a corrupt .bgl file.  Do you have Scanbgls?

I'll try all of these things! On a hunch, I also decided to switch planes for a second - the Airbus A340 has an "electronic" rudder trim which claimed to be centered. So I switched to a 747-400 with a "mechanical" rudder trim and the indicator appeared to be placed a bit left of center. So I adjusted that, and hopefully it will take. The crosswind example sounds pretty plausible too, I must say.

-Seth

Crosswind

It's crosswind.

Look at the pic:

Heading 128, speed 276.3 Knots.

Wind 201 @ 82 Knots.

It's crosswind.

Look at the pic:

Heading 128, speed 276.3 Knots.

Wind 201 @ 82 Knots.

I just need a clarification... what does the heading 201 mean in terms of wind, is that the direction the wind's coming from, or where it's going?

-Seth

i've always thought it was coming from. but i may be wrong

You're right.  It's where it's coming from.

I get this problem flying the Cessna 172. Its funny though, if I select GPS hold, it goes way off course, but if I use headding or VOR hold, it stays on course perfectly, same weather conditions. I mainly fly IFR in the 172 because it gives me a headding to use.

BTW, in my FS2002 at least, the auto pilot on aircraft, except the 172 seems to adjust itself in proportion to the wind so it stays on course.  

82 kts. That's one heck of a crosswind. People loose barns and toolsheds in that kind of wind. If your only getting blown a couple miles off course with that much wind then you're not doing bad at all.

This always happens if you have a crosswind. Then you have to set the heading you want to fly in the AP and correct for the crosswind. By the way, how did you get a 82 knot crosswind? Peace out.