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Autopilot problem in Twin Otter


nuke

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Hi Everyone,

Thanks for the replies :-)

The main problem I am having is that the Autopilot is not doing what I tell it to.

Before takeoff I turn on the Autopilot, and set the Altitude on the Autopilot to say, 5000 Feet ready to engage in flight. I then turn off the autopilot.

Next I input my destination airports ILS Frequency say, 109.9 in the NAV 1 standby and switch to make it Active. At the same time I set the Runway Heading Course of the destination airport runway 260 degrees.

I then program a direct GPS route between the two airports, in this case Cardiff to Exeter in Wales/England.

Okay so I have prepared and I am now ready, I takeoff and shortly afterwards I engage Main Autopilot and engage ALTITUDE. The Otter climbs to assigned 5000 FEET. I then Switch NAV/GPS button to GPS and select NAV on the Autopilot. The Otter tracks and follows the direct GPS route Automatically at 5000 Feet as I have instructed.

As I get in range of my destination airport I hear the Morse Code signal from the ILS frequency, that I input prior to takeoff. I then switch on APP (Approach) on the Autopilot, and Set my Altitude to 2000 Feet for the ILS Approach. The NAV has automatically disengaged on the AP as I think it should.

Now what I believe should happen, and this has happened but only once, is that the Otter deviates from the GPS track and heads towards the Start of the ILS beam turning etc to align with the runway, it even descends at a rate therefore taking over the altitude hold, and basically lands for me with the exception that I control the Thrust and Flaps. It has done this once.

My other attempts it has just flown off in some random direction and I have become frustrated, disengaged the Autopilot and crashed it onto the runway manually lol

What I am looking for is an answer to what I might be missing that I must have done when it worked to get the Autopilot to Fly along my GPS route, then when in range fly the NAV APP and basically land itself, or I can take over just before touch down.

I would like this for flying in very poor weather conditions.

I hope you can help me out. Thanks again for the help :-)

By the way I don't have this problem with other FSX aircraft with AP.

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Yes, you do. As it is the default Maule autopilot it will do the same thing with the Maule. And as the core coding is the same as with the Bendix-King autopilots it WILL do the same thing with the others too...

There are two DIFFERENT functions in the autopilot Nav control. When under GPS guidance the autopilot takes its cue from the GPS, which CANNOT fly your approach, only set you up on a line that coincides with that approach - the extended runway centre line, or the pre-programmed approach course. While you are switching to APProach mode,. you are forgetting to switch the navigation source selector switch from GPS back to NAV, so causing the autopilot to set you up on a circular holding pattern around the GPS destination as it does not know you now wish to land. It is still using GPS as the primary source.

You can set the A/P to fly the Lateral Navigation (LNAV) section of the Approach under GPS guidance where such an approach exists in the GPS database and the airport/runway concerned, but no default GPS system (and no GPS system in FS, other than the Reality XP WAAS-capable addon) will fly the vertical portion for you. And not every runway with an ILS has a GPS approach listed...

This is the basic sequence, and the element you have missed.

Now the point at which you switch from primary GPS to NAV guidance is down to you, and a lot will depend on your initial approach altitude. You must enter the glideslope from beneath for the ILS to engage, so in addition to your lateral navigation via the GPS, you also need to ensure you are at the correct height for interception by the time you switch GPS/Nav back to NAV, and engage APP mode. Typically about 2,500 ft above runway altitude will do it, but local rules may vary.

You can work out your TOD (Top Of Descent) point - the point at which you should start descending - either by reference to ATC if under IFR, or by using the `3x` rule where you multiply your total intended vertical descent by three to get a convenient point at which to begin the descent:

e.g. Cruising at 12,000 feet you wish to descend to 2,000 feet to intercept the ILS and glideslope of a runway at sea level so you have 10,000 ft to lose, so you start your descent at 10 x 3 = 30 miles out. That is thirty miles from your GPS destination, and NOT 30 miles from the airport, so if you have set the GPS to fly Direct to the airport, you may need to add another ten miles or so to allow for a reasonable rate of descent.

So you manually set up the descent on the A/P while still flying under GPS lateral guidance: The next stage is to do what you have done, prepare the nav 1 radio with the appropriate frequency for the ILS. But no matter what distance you are from the ILS, the NAV 1 instrument at this point continues to indicate ONLY deviation from the GPS path - it is NOT functioning as an ILS. When you are within range of the ILS - or as the aircraft straightens up after turning via the GPS to enter the runway extended centreline, at that point you switch the Nav/GPS switch back to NAV and engage the APP button on the A/P. You will then find that the ILS will `come alive` on the NAV 1 gauge, the ILS will be acquired, followed by the glideslope in due course.

This lengthy procedure is detailed extensively in FSX Learning Center, and I would urge you to take those tutorials and fly those lessons as any mistake or forgetfulness will have unfortunate consequences. Personally, I use GPS guidance only until TOD, and use the cue to switch altitude to also switch to NAV guidance, usually by synching HDG mode to the current GPS course. This gives me time to manually intercept the glideslope at a reasonable angle, and alows me to see the ILS needle swing and KNOW it the glideslope, and it is active.

Hope this helps

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Hi,

Ah damn I left that little bit of vital info out sorry. I do select the NAV on the NAV/GPS switch at the point I get a Signal from the ILS. The aircraft should then deviate from the GPS track and align with the runway.

Taken from my last post : As I get in range of my destination airport I hear the Morse Code signal from the ILS frequency, that I input prior to takeoff. I select NAV on the NAV/GPS switch and then I switch on APP (Approach) on the Autopilot, and Set my Altitude to 2000 Feet for the ILS Approach (to be under the ILS beam. The NAV has automatically disengaged on the AP as I think it should.

So basically I am doing what you say and it is still not working out. I use GPS switch and NAV button on AP to follow the GPS route and then Switch to NAV switch and APP to align with runway. Another thing I have noticed is that on a trip from Swansea to Cardiff, the GPS takes my straight into runway 12. The ILS freq is the same for both approaches (12 & 30) but the course is obviously different. Anyway I selected the 301 course to see if it would direct the plane and align with runway 30 on the opposite side instead of flying straight in. Anyway it decided to align and descend onto runway 12 landing about 5 meters to the left of it and then turning across the runway to the right after landing :-S

Edit: I notice the ILS morse code sound no longer works. I do not hear the beeping of the ILS. Is that normal on this A/C?

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It sounds like you were actually using a Back Course Localiser approach before, for which no vertical guidance exists, either under GPS or NAV guidance. There are many ILS approaches that use the same frequency for either end of the runway. As for the GPS, this will take you where you tell it to take you - so if you have specified Rwy 12, that's what it will line you up for. As all other traffic will be sharing airspace with you at the runway, and you could be flying right across the circuit/pattern it's never a good idea to choose the destination runway as your GPS destination - it's nearly always preferable to choose a navaid some way from the airport unless you are also selecting the GPS approach - with all that entails as far as sequencing and button selection.

It's easy to check on the default FS map. Open the map in flight, zoom to your destination airport and simply look at the nature of the green ILS Feathers denoting LOC/ILS approaches. The click on the airport and up pops the information for the airport, including frequencies for the ILS, if one exsists. All this information is also available in flight from the GPS. Again, the Learning Center tells you what you need to know.

Finally, the outside chance: These problems can also come about when you update some of the navaid database in FS. Do you use Navigraph updates for example?

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Hey,

Hmmm you know I have only had this problem since installing PMDG's 4100. If I remember rightly that had a Navaid installation with it or something along those lines. I`ll uninstall FSX and reinstall fresh leaving that add on off and hope that was the problem.

Thanks a lot. I will let you know if this solves it so anyone else experiencing this can sort it out.

Cheers for now :-)

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Yes, it is possible that PMDG may have installed something unknowingly, but not likely. Check it out and if not, come back with a detailed .flt involving the problem and we can see if the problem can be repeated.

FWIW, I've landed the Twotter at many airports using ILS approach (did so just this evening at LFBL using 110.10 after a GPS route to the local VOR 114.50, then manual HDG and NAV commands to enter the traffic pattern, collect localiser, ILS engaged at 2000 and disconnect at 500 AGL to land. And I do have the J41 installed. So it might be a local problem to your computer, your addons or something else.

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Which airport? Still Cardiff? default or addon?

Cardiff (EGFF) all standard FSX freshly installed with Acceleration, no other Add On other than the Otter. I fly to Exeter (EGTE) on direct GPS. Exeter ILS freq is 109.9, course 260 for Runway 26.

My Otter flies about 20 degrees to the right of the GPS track on autopilot and when I disengage and fly free hand to the airport there is no ILS morse code noise.

I chose this route because the runway is at 90 degrees to the GPS track and when it has worked correctly for me the plane will deviate from the GPS a few miles out and line up on the ILS taking me straight into the runway and even landing it if I decide not to take over. Note: I make changes to the autopilot like switching to APP, GPS to NAV and descending to 2000 Feet on receiving the ILS morse code signal. (which I do not experience anymore for some unknown reason.

By the way on an AC like the 737 I would highlight a NAV1 button that if I do not highlight then will not receive a morse code tone, but when it is highlighted I recieve the morse code tone. It is the activation button I think. I don't see one in the Otter, I just input the frequency in the NAV radio, maybe this is the reason?

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Just flew the same route with no problems whatsoever.

Used GPS to pick the route from EGFF to EGTE via flight plan. Set switch to GPS, checked aircraft bang-on red line.

At 4,000 ft en route used GPS to set Procedure EX approach while still on GPS nav guidance. Aircraft turns to new course. Set Both Nav1 and Nav2 to 109.90 (purely to crosscheck that there is a signal showing from Exeter 26 while Nav 1 is slaved to GPS). Set Heading bug and ILS needle to 260, checked for morse identifier on Nav 2 - present. Set descent to 2000 ft. at 800ft/min, Retard power levers slightly.

Crossed Ex at 2000ft, well within range of APR and ILS. Switched to Nav control, HDG mode activated to finish turn to 260. APR selected on the A/p. Aircraft automatically acquires Localiser and shortly thereafter the G/S. Aircraft flies perfect ILS approach right down the beam. Disconnect A/P at 100 ft AGL, landed.

Seems straightforward here, so you either have some finger trouble or system problem. Are you SURE you have 109.90 selected and displaying on Nav 1? The active frequency is in the left pane of the display window. You are aware of the small inner Nav Frequency Select Knob has an `in` and `out` position, which affects the ability of the Frequency Transfer button to actually swap frequencies? You did remember to switch the frequency to active? As for the morse identifier, it is activated by the radio control itself - the `pull ident` knob. 109.90 brings the correct Morse code signal in my Twotter.

Sorry can't be more specific, but I think at this stage reinstalling the Twotter couldn't hurt, in case something got hosed, and also a quick re-read of the Nav section of the manual - p34 and on.

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  • 3 weeks later...

Interesting discussion here and I don't mean to hijack this thread, but I have a couple of questions regarding things mentioned here:

1) Where is the GPS/NAV button? I have a key mapped to that function, but I'm not seeing an actual button toggle anywhere.

2) Where do I turn on/off audio tones (Morse code) for NAV aids? I'm not seeing that either.

Thanks!

Stacey

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Interesting discussion here and I don't mean to hijack this thread, but I have a couple of questions regarding things mentioned here:

1) Where is the GPS/NAV button? I have a key mapped to that function, but I'm not seeing an actual button toggle anywhere.

2) Where do I turn on/off audio tones (Morse code) for NAV aids? I'm not seeing that either.

Thanks!

Stacey

1. The NAV/GPS button is not visible in the "modern" cockpit Twotter. You have to use a keyboard key assignment to toggle it (found in controls)

2. The morse code is activated when you click on the knob on the left of the NAV Freq box. Lol hope this making sense, it's the nob left of the box that displays the NAV freq.

Hope this info helps. If not I`ll try and get some pics with more detailed information. :-)

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  • 2 months later...

I'll have this checked out next week guys, indeed something weird going on.

Hi, Mathijs,

Did you find a solution to the strange behaviour of the Twotterload in Windows 7 64-bit?

Rgds,

Knut

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Hello Knut,

maybe running both FSX and the Twotterload s Administrator might help, right clicking on there start icons and selecting, "Run as Administrator".

Hi, Shaun,

Unfortunately, that didn't work. The load station's positions are still getting overwritten by the station directly above, e.g. station_load.4 inherits station_load.3's position (15.0, 0.0, -4.0), when using the Twotterload. After using Twotterload for some time, almost all of the stations will have a position of 15.0, 0.0, -4.0, which completely ruins the CG of the aircraft.

Regards,

Knut

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