Autopilot Fail on Approach

[nicnacnic]

Baciclly, when I approach an airport (with NAV hold on and ILS frequency set) the approach hold button just makes me stall and I can't recover from it. Also, the altitude hold and vertical speed buttons don't work on approach. I have checked that I set up my approach with the GPS and ILS frequency several times but it keeps happening. I have already tried to reinstall FSX on my computer but it did not solve the problem.

[SteveFSX/P3D]

So the aircraft autopilot is slaved to the GPS on your journey to the airfield. When flying the ILS are you changing that and slaving the Autopilot to the ILS radio Navigation frequency? .... in the Cessna for example there is a selector toggle for GPS/NAV radio. The autopilot needs to be told what input to follow - and ILS and GPS are two different sources. The GPS might display the ILS feathers and approach, but it can't usually make the approach on the glideslope ( there are exceptions with some modern units)

On the likes of the G1000 glass cockpit it's the CDI button on the screen bottom. That changes the colour of the tracking display to magenta for GPS to Green for Radio Navigation. Sometimes it's a lighted button on the dash to change it.

I don't know if this is your problem but it does sound like a wrong mode is selected

[nicnacnic]

I use the GPS for the approach/lineup and then switch to NAV and approach hold for landing. That is where the autopilot fails. I'm not sure if its the mode but i don't think that is the problem.

[SteveFSX/P3D]

Originally posted by nicnacnic:

I use the GPS for the approach/lineup and then switch to NAV and approach hold for landing. That is where the autopilot fails. I'm not sure if its the mode but i don't think that is the problem.

Ok just to be clear ... you are selecting NAV mode on the autopilot? ...or changing the NAV/ GPS selection that slaves the autopilot?

[nicnacnic]

I use NAV mode for approach hold when I get there. I think the approach hold is causing the problem because the autopilot works fine until I use approach mode with NAV...that is where things start to fail

[TextRich]

I don't think altitude hold and V/S are supposed to work in approach mode. When I fly the default 737, for example, as soon as the glide slope is captured, the altitude indicator turns off by itself. In approach the autopilot is programmed to stay centered on the glide slope and ignore altitude and descent rate. The key is to make sure you capture the glide slope first. Like Steve said make sure you have Nav selected in your aircraft

[SteveFSX/P3D]

Originally posted by BF Bullpup:

I don't think altitude hold and V/S are supposed to work in approach mode. When I fly the default 737, for example, as soon as the glide slope is captured, the altitude indicator turns off by itself. In approach the autopilot is programmed to stay centered on the glide slope and ignore altitude and descent rate. The key is to make sure you capture the glide slope first. Like Steve said make sure you have Nav selected in your aircraft

Yes ..... same for me BF. The altitude hold and the likes of heading mode are extinguished on ILS capture. Heading with localizer first, then Alt with glideslope.

[SteveFSX/P3D]

Originally posted by nicnacnic:

I use NAV mode for approach hold when I get there. I think the approach hold is causing the problem because the autopilot works fine until I use approach mode with NAV...that is where things start to fail

If I have the GPS flying toward the airport before ILS capture I am usually setup as follows:

I will have the GPS/NAV frequency autopilot toggle selected on GPS - autopilot on NAV mode - and typically Alt hold on. The autopilot is now slaved and following the GPS course

When reaching a point where I am intercepting the ILS:

Altitude hold is still on

I turn my heading bug to match the GPS heading or my desired interception heading for lining up with the ILS

I switch the GPS / NAV Radio toggle to NAV. The autopilot is now ready to accept the ILS frequency from the NAV Radio

I turn on heading mode on the autopilot

I arm the approach button on the autopilot - and watch for the heading mode light on the autopilot to go off - to signal localizer capture. If quite close to the ILS this will happen as soon as you select approach.

I then watch for the Alt hold light to go off to signal glideslope capture. This usually happens later as a result of the fact that localizer and glideslope are two different components/signals - and are acquired separately by the autopilot. One controlling lateral movement, the other controlling vertical - both combining to make an ILS.


Things that can go wrong (and I do them regularly):

1. Being in GPS mode
2. Being in Navigation mode on the autopilot ( as you will capture the localizer but not the glideslope)
3. Trying to capture too late when you already have flown through the glideslope. The approach chart / procedure or ATC control ensures correct capture height for ILS.

When I am flying straight in manually, at say 3000ft - I think I can capture the localiser at around 20ish miles - and intercept the glideslope at around 10. I know 3000ft was recommended to me in some lessons as a good straight in ILS interception height for GA aircraft.

Interception height determined by altitude. If only at 1500ft you will be intercepting at 5 miles. The rough rule of thumb is that the glideslope goes up 300ft in height per mile from the airfield. So at ten miles the interception height for the GS will be at 3000ft 10x300 = 3000.... at five miles it will be at 1500ft 5x300 =1500.

[nicnacnic]

That is basicly what I usually do but when I put on NAV mode and turn on approach mode the autopilot does not capture the glide slope and I crash away from the airport. And if I then turn off approach mode and use altitude hold and vertifcal speed, the plane just stalls even when I turn the throttle to full.

[TextRich]

Which aircraft and airport, nicnacnic?

[nicnacnic]

Its the Airbus A321 at KMCO. I have to test another airport before I can tell you if its only that airport or other airports as well. It might just be that airport.

[nicnacnic]

I have just flown to other airports and I did not have this problem. The Approach hold and autopilot worked fine and I was able to make a smooth landing. I guess it does not work at KMCO.

[TextRich]

I just landed on Runway 35L at KMCO using ILS (but turned it off when I reached the threshold). I had to teach myself how to fly the A321 first LOL. See the screen shot - the only thing not on that I usually have on is the autothrottle (I just forgot about it).

http://steamcommunity.com/sharedfiles/filedetails/?id=957398927