autopilot question

[Pappy44]

I was flying the shamu sw 737 last night (great skin btw) from St Louis (my hometown) to Ohare (where i grew up btw...well, not at the airport...um, nevermind).  I have the flight planned IFR and in the planner.  Before I took off, I set altitude at 7000 (initial alt given by tower) and pushed the alt hold button.  Set the speed at 250 and pushed the spd button.  Set the heading at the runway heading and hit heading, set the gps/nav switch to NAV.  Left the master switch off of course.  FD and AT off at this point.

Take off, positive climb, 1000 ft, gear up, FD on, AT on, AP master switch on, hands off and away we go.  I correct course as given by the tower, and when told to resume own navigation, I set the gps/nav switch to GPS so the AP will follow the course to ORD.  A few minutes later, I look at our course, and we haven't made the left turn to follow the gps track, we have flown right through it.  Tower tells me to turn to so and so to intercept course, and I do so and I look like i am on track again.  Few minutes later, at the CAP waypoint, the track makes a slight turn to the right.  the plane doesn't follow.  I had to go a little later so I saved the game, so I don't know what the deal is.  There is a left turn coming up, so is it possible that the AP is just "cutting the corner" and staying on the left side of the track knowing that a left turn is coming up?  I didn't have this problem on my flight down to Princess Juliana in the 747, so I am concerend that the AP might not be working correctly on this plane.

Thoughts?  Did I do something wrong in setting up the AP?

[Brett_Henderson]

Switching the NAV/GPS switch to GPS, is the first step... but then you gotta hit the NAV button on the autopilot (which turns off the heading hold, if it's engaged) ..

[Pappy44]

Switching the NAV/GPS switch to GPS, is the first step... but then you gotta hit the NAV button on the autopilot (which turns off the heading hold, if it's engaged) ..

Ah...duh, I knew that.  I am not sure why I didn't do that.  I am sure that I did on the 747 flight!  Last night was a REALLY busy night (trying to get house ready for company...fiance` stressing out...etc), so I was not thinking clearly.  Thanks for clearing that up.  I knew it was something really easy.

Ok, so let me ask this...I turn off the nav button.  If it is set to gps it follows the gps.  If it is set to Nav, does it follow nav1 on the radio stack?  Makes sense that it would, since I think that is what you do if you are trying to intercept the ILS on landing, correct?

[Brett_Henderson]

To intercept and hold an ILS by autopilot, you need to use the approach button (there are three functions that the autopilot uses for heading input... HDG, NAV and APP (or is it APR ?))

If you're using the NAV function and the NAV/GPS switch is on NAV.. it will try to track the radial selected. On an HSI, that radial is the dialed in course. On a regular VOR it's what the OBS is set to.

You must be rusty, hu ?

[Pappy44]

[quote]To intercept and hold an ILS by autopilot, you need to use the approach button (there are three functions that the autopilot uses for heading input... HDG, NAV and APP (or is it APR ?))

If you're using the NAV function and the NAV/GPS switch is on NAV.. it will try to track the radial selected. On an HSI, that radial is the dialed in course. On a regular VOR it's what the OBS is set to.

You must be rusty, hu ?

[Brett_Henderson]

LOL

[Pappy44]

Switching the NAV/GPS switch to GPS, is the first step... but then you gotta hit the NAV button on the autopilot (which turns off the heading hold, if it's engaged) ..

Ok, I think this is where I am getting confused...on the 737 panel, there is a VOR LOC button...when the mouse is held over it, it says NAV HOLD SWITCH....To follow the gps, that needs to be on, and the HDG light then goes off, that right?

I think  that fixed it...and I get what you are saying...makes more sense now...and now that i can see my flight deck, makes more sense...thanks!

[Pappy44]

ok, i followed the guide here...

http://www.simviation.com/cgi-bin/yabb2 ... 1111322151

and i could get lined up with the glide slope, but the AP wouldn't follow the glide slope in...i have no idea what i am doing wrong...as soon as it made its turn in, it would turn off the alt hold, and either go right over the GS or stay under it the whole way....I am so freakin lost right now...I made sure everything was set correctly...I am lost...going to eat before I get too frustrated...

[Brett_Henderson]

That's an excellent tutorial, but save it until you're nailing instrument flights/approaches in a 172 (and then a Baron, and then a KingAir, and then a Lear). We're just doing hit-n-miss stuff here. You're giving bits and pieces of info, and I'm only touching on applicable points.

Don't get frustrated. Have fun with this. You're learning the exact same skills you'd need to navigate a real airplane, and it can get a little overwhellming when you try to run before walking.

I'm gonna say this first, almost as a disclaimer, like I always do in these threads. You need to master this stuff a little at a time. We should be learning this stuff in a simple 172. You need to understand each little phase of an instrument flight completely; and know how/why each Nav-aid is used; and what role each piece of equipment plays. Like the NAV/GPS switch, for example. It's easy to get the big picture confused when you don't understand each little thing, completely.

OK... Quick guess at this latest dilema. If you are flying within the localizer, and reasonably close to runway heading, and from under the glidescope... AND you have the ILS frequency tuned into NAV1.. AND you're at or near approach speed.. AND the NAV/GPS switch is on NAV.... AND as the needles get near center you've activated the approach (APP) (I think that's the step you missed)(I always turn of the ALT and HDG holds first, just out of habit.. but 9 times out of 10 I'm flying manually anyway.. even the approach), the autopilot will latch onto, and hold, both the localizer and glidescope. Since you're flying right through it, you're missing something from this list.

Here's what I'd do.....

Fly around in the 172 tracking VOR radials finding VOR intersections using two VORs... Obvioulsy the NAV/GPS switch will be on NAV at that point. Then, when you're confident in your ability to navigate by VOR manually... go ahead and start experimenting with the autopilot. Set the OBS to the radial you want to track... activate the Nav function on the autopilot experiment with letting the autopilot intercept and track radials (switch the NAV/GPS switch to GPS and see how it affects the VOR and how it renders tracking by VOR useless (swittch it back to NAV, because we won't even be looking at a GPS until we get this stuff mastered).

Then.. when you know you can find your way to an airport by VOR navigation in 1 mile visibility (no GPS or sneaking a peek at the map page !).. We'll move on to instrument approaches; both precision and non-precision. (if you think it's a handful "winging" it.. wait until you try it following a published approach, exactly

[Pappy44]

ok, the noob is back...so i tired something different...instead of having the heading hold on and the app on, i used the nav hold on, then when i got on the localizer, i hit the app, and it is taking me in...so is that then the right approach?  The guide has me confused...

[Brett_Henderson]

It doesn't matter.. you can fly it it by hand if you want.. so long as the APP function is the only heading specific button activated on the autopilot.