What autopilot mode to use with GPS signal....

[TangoWhiskey]

This thread from another forum (Beech Owners) was interesting. Thought it might generate some discussion here about the subleties of using your Autopilot on approaches:

You always use *approach* mode when coupling to the GPS. Goes for all the autopilots. KFC types, too.

Reason: The *approach* dynamics are much more suited to GPS type needle indications than NAV mode.

NAV mode has all sorts of patches to deal with VOR coupling, such as limiting rate of turns to give a decent ride when dealing with VOR scalloping and needles flopping around when going over the VOR stations. These patches interfere with crisp coupling to GPS needles when changing to a new heading.

If you couple to a VOR with Approach mode, you will quickly see why the NAV mode was invented. The ride is pretty bad.

The GPS needle is so solid that approach mode can be used with no problem. And since the needle sensitivity remains relatively constant, the autopilot coupling is really effective.

Try it. Approach mode. Even without a Sandel, you will find that in the worst case of a T - shape GPS approach that you can manually feed the new headings to the HSI as cued by the GPS and the coupling will be flawless. You do not need to decouple the approach mode as you go from one segement to the next--just dial in the new heading.

So if you use approach mode, with the sandel, it WILL be as good as a real gpss!! Or almost.

Another thought expressed in this thread was the advantage of using the Approach mode is that (most? all?) autopilots will not capture the Glideslope (if they are capable of doing so in the first place!) unless it is in approach mode; and, since the 430 will autocycle to VLOC if it sees a localizer frequency, then if you're already in "Approach mode" on the autopilot, you're all set up to capture and fly the full approach... no need to remember to switch from NAV mode to APPROACH mode on your A/P.

Troy

 

[grattonja]

The Cessna Pilot center material teaches using the autopilot in nav mode until close to the approach. They do not seem to differentiate between GPS and VOR en-route. The autopilot is supposed to be a bit smoother en-route in nav mode, less likely to have a deviation during en-route make the ride "jumpy" due to excessive correction. I would think, too, that the turn anticipation of the GPS for airways would be geared to the nav type handling of the autopilot.

The new hawks generally have a King autopilot, either single or dual axis, and a KLN 89B or KLN 94 gps.

Jim G

 

[poadeleted3]

GPS for airways would be geared to the nav type handling of the autopilot.

The new hawks generally have a King autopilot, either single or dual axis, and a KLN 89B or KLN 94 gps.

Jim G

Or Garmin G1000s.

 

[wsuffa]

Trimble recommended 1 mile or less course sensitivity on the enroute GPS box. That to get reasonable sensitivity to the autopilot.

I highly recommend the S-Tec GPSS with their autopilots. On the money.

 

[gismo]

This thread from another forum (Beech Owners) was interesting. Thought it might generate some discussion here about the subleties of using your Autopilot on approaches:



Another thought expressed in this thread was the advantage of using the Approach mode is that (most? all?) autopilots will not capture the Glideslope (if they are capable of doing so in the first place!) unless it is in approach mode; and, since the 430 will autocycle to VLOC if it sees a localizer frequency, then if you're already in "Approach mode" on the autopilot, you're all set up to capture and fly the full approach... no need to remember to switch from NAV mode to APPROACH mode on your A/P.

Troy

On the Century III in my airplane, the radio coupler has four coupled modes for course guidance. The least sensitive and most heavily filtered is called "NAV" mode and it's intended to be used on a heavily scalloped VOR signal. Another setting is called "OMNI" which is where I fly it most of the time on GPS or VOR. This allows the CDI to remain about one dot off the centerline in a worst case crosswind situation. The most sensitive settings called "LOC" and "LOC REV". These will keep the CDI pretty darn well centered albeit with a fair amount of wing rocking and needle chasing.

I normally use the OMNI selection for all GPS operations. When the GPS is used for an approach it changes the scale gradually from 5 NM full scale, to 1 NM full scale as you get into the terminal environment and then further reduces it to 0.3 NM on the final inbound course. I have found that the sensitivity in the OMNI mode is sufficient for any approach I've flown with the GPS and that the combination of LOC sensitivity and the GPS in 0.3 NM full scale makes the autopilot over correct sometimes. Also my GPS (KLN-94) has no LOC capability and I am in the habit of switching the autopilot from OMNI to LOC when I select an ILS on the NavCom.

I do believe that selecting a higher gain on the autopilot will have nearly the same effect as decreasing the scale setting on the GPS, but I don't see the value of either unless your autopilot isn't staying as close to the course centerline as you would like. I also expect that different autopilots will behave differently in this regard and so it would be wise to try various combinations of autopilot sensitivity and GPS scale to see what works best.