Is the autopilot flying downwind of the localiser beam in a crosswind realistic?

  • To make the FS2 plane fly on the localiser centreline in autopilot APP mode the opposite drift angle has to be dialed on the instrument's OBS. San Jose 12R 126 degrees ILS with 10 degrees of drift to the right. 116 on the HSI instrument's OBS flies the plane on the localiser centreline.

    Without this OBS alteration the plane will track about one degree (2.5 degrees X 2 dots/5 dots) downwind of the localiser centreline. 126 selected on the HSI ILS instrument.

    Is this seen on any real life autopilots?

  • How much wind did you select?

    The basic autopilot does not fly the track, it just flies the heading and if the heading difference it zero you are pushed away from the localizers beam. That means in crosswind it should show a slight deviation. But depending on the autopilot model this is compensated. I'm pretty sure the one we model does not compensate it.

  • The wind was 90 degrees to the localiser with the wind slider giving 10 degrees of drift. For 100 knots on the approach that works out at 17.4 knots.

    Wind blows from heading to track, 100 knots airspeed on 116 degrees heading, wind 17.4 knots on 036 degrees gives groundspeed 98.5 knots on track 126 degrees. The wind and track form a right angled triangle in this case.

    That makes sense Jan. With that OBS set on the localiser heading the autopilot would be permanently adjusting to the track error. Flying an ILS manually to 1 degree would be a very good performance.

    Most of my light aircraft flying was in a Cherokee 150-180/Warrior-Challanger/Archer Is and they had a wing leveller and a heading hold only Piper autopilot in the better ones. I wasn't allowed to use a twin’s autopilot in training and don’t remember them. I do remember having a very nice 172 autopilot shown to me but it was maybe 30 years ago.

    The Aerofly 172 has a wonder autopilot. I’m sure new ones are brilliant.

  • Looking at the AP section of a rather basic 172, there shouldn't be any deviation at all in any crosswind.

    If the AP can track a VOR radial, it can track a localizer as well.

    That's from a 172 POH

    Approach (APR) mode - locks on-course with greater sensitivity than NAV mode.

  • The advanced autopilot in the airliners uses the track and thus can easily account for crosswind conditions. The a320, b777 and b747 should be able to autoland even in crosswind.

    The issue in the basic autopilot is that it does not have any source of the aircraft track. Just the heading, so it has to compensate differently.

  • Not sure about that. That's a quote from the rather basic KAP 140 autopilot manual:

    Notice that correction for wind drift - in this case, a 080° heading on a 088°course - is completely automatic.

    The associated picture also shows a perfectly centered CDI.

  • Not sure if this has been resolved yet, but I think there's a basic misunderstanding here.

    When the autopilot is in APR (approach mode) it will track the localiser, heading has nothing to do with it when in the approach mode other than the AP turning the aircraft to react to the wind trying to blow the aircraft off the localiser course.

    Adjusting the course setting on the RMI or HSI will have no effect on the autopilots ability to track the localiser as the localiser beams are fixed to the approach track. I think some are getting confused with using the course settings for tracking a specific radial for a VOR (which radiates 360 different radials).

    The CDI will remain centred on the localiser when captured by the AP in APR mode, though the heading indication will deviate as the AP adjusts for the wind to stay on the localiser beam.

    If you fly the localiser manually, you are making the heading corrections to keep the localiser CDI centred.