5.37 Why does the autopilot seem to "hunt"?
From FlightSim
WHY DOES THE AUTOPILOT SEEM TO "HUNT" AND CAUSE THE AIRCRAFT TO TAKE A SERIES OF "S" TURNS WHEN TRYING TO LINE UP WITH A VOR RADIAL?
What does a "real" autopilot do if you leave it locked on a VOR as you pass over it? I would think it would try to turn back to the beacon. I'm usually careful to disengage the NAV HOLD before I get there. Incidentally, if you tune to the next VOR, the NAV HOLD will disengage anyway. When you get the radial set up you turn it back on.
Manually locating a radial and lining up with it has always been a "cut and fit" operation. Sort of like learning to turn a corner when learning to drive a car. You have to anticipate the alignment and start your turn before you get there. Some back and forth "chasing" is usually needed unless you're really skilled and comfortable with the airplane's characteristics. Aligning with a pre-set radial with the autopilot is sort of tricky. First it turns to a 30ΓΈ or more angle to intercept the radial and then overflys it and has to turn back. Just like a rookie. I seldom try to follow a set radial, but adjust the radial to align with my airplane position and then activate the NAV HOLD function. The plane turns directly to the VOR direction and then, if you really want to be on a set radial value, adjust it slowly and the plane will find it smoothly.
ILS beacons are something else since their reception area is in a rather narrow arc and the OBS has no control, since there is only one radial to intercept. You just have to maneuver to as shallow an angle as possible to intercept and then allow enough distance so that the wiggle waggles will have subsided before you get to the runway. I'm not sure I'd want it any more "automatic". There wouldn't be any skill needed. Sort of like a GPS.
Bob Fiedler
A real autopilot coupled to a VOR will try to track the needle near station
passage. I never let one stay coupled long enough to see what kind of
gyrations might ensue, because I know how wild that needle can get. Your
described technique above is what I would also use in real a/c. Actually, if I
wanted to stay on autopilot, I'd revert to heading hold and gently adjust the
"bug" to slightly cut the corner around the VOR; in a left turn to the next
radial, I'd wind up flying slightly to the left of the VOR station. Then once
I was past the station enough to have the needle stabilized and had a gentle
intercept angle, I'd reengage NAV mode. Of course hand flying can be preferred
in many instances.
One thing that affects real life more than a sim is ATC. And I don't mean just talking to them. There are a lot of rules and procedures for IFR flying. The actual flying of the aircraft on the gauges is the simplest part of IFR. Traffic, unexpected clearance changes, weather you don't dare penetrate (and knowing when that is), planning ahead, holding procedures, are just a few of the things that make IFR training one of the most difficult ratings to get.
Larry N.
A procedure I use in real world seems to work relatively well, and keeps the
hunt to a minimum.
1. Continue on inbound course to .5 DME from the station
2. Keep HDG bug on the inbound course, engage hdg hold
3. At .5 DME, rotate hdg bug to outbound hdg, aircraft initiates standard rate turn (20deg bank)
4. leave hdg hold engaged till 2-3 DME FROM the station, or until wings level on bugged/outbound hdg
5. During turn, rotate OBS to required outbound hdg/course
6. Engage NAV mode
7. The aircraft will generally fly right into the radial, but make minor adjustments with hdg bug as required
NOTE: In the case of non vortac stations (no DME avail) initiate turn to outbound hdg at half needle deflection
Joe W. Clemmons,jr
