Cessna 172 autopilot

[jc6828 0]

Hi, I am new to XPlane 10 but have the full package installed after having used the demo for a few weeks.

 

When flying the 172 it does not seem to 'obey' the autopilot. I have it turned on and heading and altitude selected to be held but it ignores the heading bug and the flight levels I set and goes round in circles.

 

I have come from FSX where the autopilot worked correctly and have set it using the same procedures.

 

Is there something I should know, I have read the manual and think I am doing everything correctly but it still doesn't hold course or height properly.

 

Any advice would be welcome.

 

Thanks

[PWJT8D 446]

I think the Flight Director has to be toggled to auto

[jc6828 0]

Hi, thanks for that you are right - so may I ask what the 'on' position is actually for then and also another question, I can control heading with the heading bug and change it during flight but how do I change the altitude that I have set during flight?

[PWJT8D 446]

I think the "ON" position is more of a simulation of the flight director being armed but not in use. As the default Cessna 172 autopilot does not have a vertical speed function am I correct? I believe you have to disconnect your ALT hold, fly to your desired altitude and then at that point, click the ALT button to hold that altitude.

[jc6828 0]

Thanks for that I will try it later this morning.

 

Do you kow the significance of the border colour round the alt button, it has amber and green whereas the heading button just has amber. Also what is the button by the altimeter for which changes the height but doesn't seem to do much else.

[PWJT8D 446]

Usually a amber color is armed and green color is captured. The button by the altimeter is your barometric setting.

[wb5okj 26]

The button by the altimeter sets your barometric pressure. If you know the barometric pressure in inches of mercury, set it at ground level the altimeter tells you your altitude. If you already know what your ground height is set that with the button, and the little kolsman window, (zoom way in with the "+" key), will tell you what your current barometric pressure is. The correct way is get the barometric pressure from a weather station and set your altimeter to that setting.

[MdMax 233]

You'll find more about the autopilot here:

http://wiki.x-plane.com/Chapter_6:_Navigation,_Autopilots,_and_Flying_on_Instruments#Turning_It_On

[kickin_chicken 3]

The "On" for the flight director is sort of like "arming" the autopilot - it shows the needed corrections on your attitude indicatior (the yellow chevron) so if you follow the chevron, you're following the "autopilot's" directions - but you're actually controlling the aircraft. When set to "Auto" the autopilot is actually flying.

 

Regarding the "ALT" button: After smashing my aircraft (and my head on the keyboard) a few times, I figured it out. It's not "armable" and doesn't actually level off at the pre-set altimeter setting - it flies the current altitude. Sorta. Let me explain:

The ALT button "reads" the current altitude and "sets" the autopilot altitude hold to that altitude - despite what you might have set the pre-set altimeter setting to. I found this out by setting the pre-set altitude to say 3500 ft, and climbing though 2500 feet, I hit the "ALT" button, which should have armed it - but instead put my aircraft into a dive to its original altitude setting of 300 or whatever. Sooo..

 

This is how I use the ALT hold on the Cessna 172: I fly by hand and put myself on the right course, and hit HDG (after dialing my desired heading). I fly to my desired heading and altitude, then HIT ALT button - this sets my current desired altitude to the autopilot. Note that the flight director is still on "ON" not "auto" --- this is important.

 

After hitting the ALT button at my desired altitude, I then hit "AUTO" on the flight director and then the plane holds my altitude and heading. It does not "fly" to an altitude despite what the manual says. Maybe in more advanced aircraft it does in fact do this - not in the Cessna 172. Not in my experience with it at least.

 

Hope this helps.

[Oracle427 45]

After setting your desired altitude, you need to select a vertical speed to climb or descend to the altitude. The autopilot will hold the current altitude and ignore the desired altitude until you also select a VS to get there.

 

Be careful because if you set a VS and do not arm a target alt, the autopilot will climb or descend indefinitely.

[kickin_chicken 3]

Oracle427, is there a verticle speed setting for the Cessna 172 default airplane in X-Plane? Maybe I missed that.. I don't recall seeing a button or dial for it. You are correct however, but I believe the Cessna doesn't have a V/S setting for the autopilot - other aircraft do and do function as you described.

[Oracle427 45]

Let me check, I'm basing this on a RW KAP 140... You know, I never used the A/P on the XP default 172. I see it lacks the buttons and this function. Doh!

[kickin_chicken 3]

Cool, I'm glad I wasn't missing something then. In my (very short) experience with the C172 in X-Plane, the autopilot operates in the manner I described, at least for me. I'm sure other aircraft have better autopilots but I haven't even flown them yet. I'm using Air Hauler as my introduction to X-Plane. So the only plane I'm flying is the C172 for now.

[Oracle427 45]

This XP 172 (which I use) does have the VS settings. It also flies a little better than the default. I recommend taking the few seconds it takes to install this one as a very quick and free upgrade.

 

http://cbpowell.word...el-for-x-plane/

 

Edit: One caveat I'd like to add. IMO the default 172 climbs a little too well vs reality. This one climbs even better and it isn't difficult to hold a 1200FPM climb at 74kts, though it should really be around 800FPM on a standard day. If you are willing to overlook this it is otherwise decent. You could also opt to learn to use PlaneMaker and tweak it a little as I have.

 

This link also has some basic details on using a typical 2-axis autopilot on a 172.

 

http://www.google.co...KLuxBKSYSQ8Ptuw