Nav Mode / HSI

Guest Discussion
1

Maybe I’m crazy. In dealing with the people at S-Tec and my local avionics tech, as we try to get to the bottom of why my autopilot is suddenly off track, a conflict of opinion has arisen.

My contention is that in the nav mode on the S-Tec 55x (would be the same as the 55), non GPSS mode and not in the OBS mode on the Garmin, the yellow heading selector direction does not influence the CDI and therefore the autopilot. In other words, the course is set by the GPS inputs, not the heading selector, Am I correct?

2

Maybe I’m crazy. In dealing with the people at S-Tec and my local avionics tech, as we try to get to the bottom of why my autopilot is suddenly off track, a conflict of opinion has arisen.

My contention is that in the nav mode on the S-Tec 55x (would be the same as the 55), non GPSS mode and not in the OBS mode on the Garmin, the yellow heading selector direction does not influence the CDI and therefore the autopilot. In other words, the course is set by the GPS inputs, not the heading selector, Am I correct?

Be careful of your terminology, its the source of a lot of confusion in the avionics world. When you say off track, do you mean that the autopilot won’t follow the CDI when only Nav mode is engaged ? You are correct, when in Nav mode the autopilot only cares about the HSI course pointer, needle, and flag and will ignore the heading bug.

3

Maybe I’m crazy. In dealing with the people at S-Tec and my local avionics tech, as we try to get to the bottom of why my autopilot is suddenly off track, a conflict of opinion has arisen.

My contention is that in the nav mode on the S-Tec 55x (would be the same as the 55), non GPSS mode and not in the OBS mode on the Garmin, the yellow heading selector direction does not influence the CDI and therefore the autopilot. In other words, the course is set by the GPS inputs, not the heading selector, Am I correct?

Andy

If while in the NAV mode you have VLOC selected in the Garmin, the HSI acts off the VOR freq or ILS/LOC freq you have selected without any input from the GPS. If you’re tracking a VOR just keep adjusting the CDI needle to keep the bar from being deflected(crab for the wind etc). The autopilot flies the HSI.

If you have GPS mode selected in the Garmin, the HSI will treat the course laid out by the GPS as if were direct to a waypoint, however you have to manually turn the CDI heading to the appropriate Direct Track (DTK) heading. Once the CDI needle is on the DTK you don’t need to adjust the HSI again as it will track the direct course laid out by the GPS much like it tracks the LOC in VLOC mode.

So I think the answer is no. When you want to track the GPS course on the S-Tec 55 in the NAV mode, you have to set the CDI needle to the DTK you want to fly as indicated by the Garmin, it will tell you when to turn the needle. Once you set it, leave it alone until you reach the next waypoint.

This is my understanding of how the S-tec 55 works with the autopilot. If someone sees a mistake here, please correct me asap.

Brig

4

My contention is that in the nav mode on the S-Tec 55x (would be the same as the 55), non GPSS mode and not in the OBS mode on the Garmin, the yellow heading selector direction does not influence the CDI and therefore the autopilot. In other words, the course is set by the GPS inputs, not the heading selector, Am I correct?

In NAV and APR mode, the 55 and 55X essentially fly the heading defined by the course pointer, and then make wind correction adjustments based on the needle deflection. So when you hit a waypoint and the next leg is on a different course, you have to twist the course pointer to the new course (except if you have a Sandel; it will automatically twist the course pinter for you to the desired track of the next leg, as the GPS has told it so.)

This is also how the 55/55x know how to do course intercepts–it will fly a 45 degree intercept to the selected course until the needle begins to center, and then turns to the selected course.

Contrast this with the 20 and 30, which do not have any course input; when you hit a waypoint and need to change course, you have to manually turn the plane (hand-flying or using HDG mode) until you’re on the desired heading and the needle is more or less centered, and then you can reenter NAV mode.

With GPSS, all of this is bypassed; the GPS just tells the autopilot when to turn and how much (it will do variable turn rates as well, including faster-than-standard rate, which will get your attention!)

Oh, and a subtle difference between the 55 and 55x; when you turn to the next course with the 55 and it’s more than about 10 degrees, you need to re-arm NAV mode (I hit APR and then NAV to accomplish this.) This causes the 55 to more aggressively turn to the course and center the needle (after which it mellows out some to smooth the ride.) The 55x does this automatically (which it ought to do since it can see you twist the course pointer.)

Understanding this came in handy a couple of nights ago; I departed OAK on runway 27R and wanted to make a right turnout to the southeast, so I got partway through the turn and hit NAV mode. The plane turned and kept turning and kept turning…the HSI gyro had failed, and the autopilot was trying hard to turn the plane until the course pointer was pointing straight up, except that the course pointer kept pointing to the right…just what I needed at 2:30 am.

5

Maybe I’m crazy. In dealing with the people at S-Tec and my local avionics tech, as we try to get to the bottom of why my autopilot is suddenly off track, a conflict of opinion has arisen.

My contention is that in the nav mode on the S-Tec 55x (would be the same as the 55), non GPSS mode and not in the OBS mode on the Garmin, the yellow heading selector direction does not influence the CDI and therefore the autopilot. In other words, the course is set by the GPS inputs, not the heading selector, Am I correct?

Andy

If while in the NAV mode you have VLOC selected in the Garmin, the HSI acts off the VOR freq or ILS/LOC freq you have selected without any input from the GPS. If you’re tracking a VOR just keep adjusting the CDI needle to keep the bar from being deflected(crab for the wind etc).

You don’t adjust the CDI needle. Do you mean you fly the airplane to keep the needle centered ?

The autopilot flies the HSI.
No, it follows the needle from the GPS or VLOC receiver. You just happen to see a needle on the HSI.

If you have GPS mode selected in the Garmin, the HSI will treat the course laid out by the GPS as if were direct to a waypoint,

The HSI is too dumb to treat anything. It’s just a display, Sandel or steam gauge.

however you have to manually turn the CDI heading to the appropriate Direct Track (DTK) heading.

Desired Track. They’re not headings, they are courses / tracks, which is an important distinction when discussing autopilots. Courses are ahead of you and track is behind you in spite of the “DTK” nomenclature, but that’s being nitpicky

Once the CDI needle is on the DTK you don’t need to adjust the HSI again as it will track the direct course laid out by the GPS much like it tracks the LOC in VLOC mode.

So I think the answer is no.

??? We’re not even sure what the original poster is asking. Does his term “heading selector” mean heading bug or course pointer ?

When you want to track the GPS course on the S-Tec 55 in the NAV mode, you have to set the CDI needle to the DTK you want to fly as indicated by the Garmin, it will tell you when to turn the needle. Once you set it, leave it alone until you reach the next waypoint.

This is my understanding of how the S-tec 55 works with the autopilot.

??? these are the same

If someone sees a mistake here, please correct me asap.

Brig

Like I said, the terminology will get you. You don’t “set a CDI needle” or “manually turn the CDI heading”, but we understand what you’re trying to say even if its explained poorly.

You set a course pointer on an HSI. If that course pointer setting is sent to a VLOC receiver, it can change the needle deviation seen on the HSI and thus seen by the autopilot. If that course pointer setting is sent to a GPS receiver in auto leg mode, it doesn’t change the needle deviation. If that course pointer setting is sent to a GPS receiver in OBS mode, it can change the needle deviation. The autopilot follows the needle when the autopilot is in nav mode and it uses the HSI course pointer to help it do course intercept and wind correction, which is why you should follow the Garmin’s prompt to set the course pointer to the GPS desired course (DTK).

If you’re off course while navigating with GPS (auto leg mode) and you decide to return to course by engaging nav mode on the autopilot, you need to set that HSI course pointer to the GPS desired course (desired track or DTK) to help the autopilot set up a course intercept.