Century Auto Pilot coupled to Loran swapped to GPS ?

[Indiana_Pilot]

I am in the process of buying a new plane (new to me)
It has a Century 1 Auto Pilot that is coupled to the old Loran system.

I have read/heard/was told that I can switch this over to GPS and it will work.. Has anyone ever tried this and does anyone have better details on how to do this?

How does this work ? I put in a GPS waypoint and it flys that heading ?

 

[poadeleted20]

A Century I is not "coupled" to the LORAN. A C-I has a nav tracker capability tied to the CDI needle position. As such, it can follow the CDI needle no matter what's driving it (LORAN, GPS, or VOR/LOC), so if you replace your LORAN with a GPS which drives the same CDI the LORAN was driving, it will track that needle the same way it did before (which is fairly well once established on the track, say, inside one dot deflection with the heading within 10 degrees of course) but it still won't do intercepts.

What a C-I will not do is take roll steering from a GPS, because that works through the autopilot's heading input, and a C-I doesn't have a heading input. As such, you'll have to do a good deal of manual intervention at turn points -- just as you have to do with the LORAN now. What you'll do is when the GPS tells you "TURN L/R xxx NOW", you push the C-I back into roll command mode, twist the roll knob to establish a standard rate turn, wait for it to reach the new heading, and then use the roll command to roll wings level and re-engage the track mode.

If you want to do better than that, you'll have to replace your C-I with an autopilot which has a heading or course input (the C-I having neither).

 

[Indiana_Pilot]

That makes sense.. I am fine with that and it is a step up from having nothing at all.. It will make long straight trips nice.. I need to figure out what I need to do to replace the the old replace the old LORAN device now with GPS.. I can't find any compatibility charts. Maybe I need to start with an avionics dealer..

 

[poadeleted20]

Which LORAN? The only ones which have a straight swap for a GPS are the Apollo/Flybuddy units which can be replaced with an UPSAT/Garmin AT GX-55.

However, if you just do the plug-in/plug-out and antenna replacement, it's VFR-only, not to mention GX-55 production was discontinued years ago and became a lot harder to find when they turned off LORAN in the US. A GX-55 can be upgraded to IFR with the injection of more money and installation work, but it's still only enroute/terminal with no approach capability.

So what GPS capability do you want?

 

[SCCutler]

Which LORAN? The only ones which have a straight swap for a GPS are the Apollo/Flybuddy units which can be replaced with an UPSAT/Garmin AT GX-55.

However, if you just do the plug-in/plug-out and antenna replacement, it's VFR-only, not to mention GX-55 production was discontinued years ago and became a lot harder to find when they turned off LORAN in the US. A GX-55 can be upgraded to IFR with the injection of more money and installation work, but it's still only enroute/terminal with no approach capability.

So what GPS capability do you want?

There was also the Northstar M3 to replace the M1 Loran, but why bother? The M3, while a fine and dandy unit for its day, hasn't enough room for full waypoint storage anymore. Not sure you can even get database updates at all...

 

[GMascelli]

My C1 quit tracking. Ron, know any shops that work on them ?

 

[Clark1961]

My C1 quit tracking. Ron, know any shops that work on them ?

I used Penn Avionics for some Century work and they seemed okay. They didn't gouge me when maybe they had the opportunity to do so. Basically a local shop told me a servo was bad so I replaced it and sent the old servo to Penn Avionics for repair. Penn called and said servo was fine and sent it back with a letter that I took to the local shop and got a parts & labor refund.

 

[poadeleted20]

My C1 quit tracking. Ron, know any shops that work on them ?

Not without asking around. But I'll bet the folks at Fletchair do. The biggest problem, as noted by the other response above, is troubleshooting to find the actual problem before just blindly sending the panel unit out to a repair shop. The folks at Lancaster Avionics did that for mine, and found multiple problems around the entire system that made the cost of repair greater than the cost of installing an S-Tec 20. I chose not to repair, and had the system removed.

 

[35B33]

One of the better regarded shops for repair is Autopilot Central in Tulsa. If you are interested, they will work with you helping isolate the problem before shipping anything for repair. ie does the roll trim work but not the tracker, pin voltages? Those pin connectors can be a failure point.

http://www.centuryflight.com/images/schematics/69D875-2.pdf

Like the CDI, an AP gets a left/right signal from the nav unit, either VOR or GPS and can be switched between the two. Don't know how to prebuy test tracking if, like the OP, it hard wired to a Loran.

 

[poadeleted20]

http://www.centuryflight.com/images/schematics/69D875-2.pdf

Like the CDI, an AP gets a left/right signal from the nav unit, either VOR or GPS and can be switched between the two. Don't know how to prebuy test tracking if, like the OP, it hard wired to a Loran.

That schematic is very simplistic and could create the false impression that the C-I autopilot "gets a left/right signal from the nav unit, either VOR or GPS". In fact, what it gets in the Track mode is a course deviation signal from the CDI, not from the navigation source (VOR receiver or GPS receiver). It then commands a roll proportional to the amount of course deviation. The VOR/GPS (or NAV/GPS, with NAV=VOR) switch merely determines which nav source (VOR or GPS) is feeding the CDI. Keep in mind that in combination units such as the Garmin 430, the GPS and VOR receivers are separate units with separate outputs.

The CDI gets its VOR course deviation data by comparing the VOR radial sent by the VOR receiver to the OBS setting, and moves the needle based on the difference. OTOH, the GPS unit computes the amount of deflection the CDI should be showing all by itself, and then sends that to the CDI, which needs a different input for that data -- which is why not all CDI's are compatible with GPS's.

And all this is why you can't use GPSS roll steering with the C-I, and why it cannot do intercepts. Note that if you tried to do an intercept with the C-I starting more than full CDI deflection, it might just go around in a circle all day long without the needle ever coming off the peg.

 

[asicer]

Note that if you tried to do an intercept with the C-I starting more than full CDI deflection, it might just go around in a circle all day long without the needle ever coming off the peg.

Interesting. In theory, it sounds like it's also possible for the system to inadvertently intercept a course or radial in the opposite direction than the pilot intended.

 

[poadeleted20]

Interesting. In theory, it sounds like it's also possible for the system to inadvertently intercept a course or radial in the opposite direction than the pilot intended.

If you are pointed the wrong way with the C-I, the system will command all the turns the wrong way, and you'll end up going round and round.

 

[asicer]

If you are pointed the wrong way with the C-I, the system will command all the turns the wrong way, and you'll end up going round and round.

I was thinking of the case where a pilot intends to fly to a VOR and tunes/twists to a radial but the plane happens to be on a course parallel to the radial in the general direction towards the station. The pilot turns on the C-I and it commands a 2 minute turn to the left (let's say the plane is on a parallel course to the right of the radial). 30 seconds later, the plane is on a course perpendicular to its original course and closing in on the radial. Suppose 30 seconds after that moment the plane nears the radial enough that the CDI is less than a dot off center. The plane would then be on a course opposite to what the pilot had intended and the C-I would be dutifully tracking it.

This is mainly a thought experiment intended to expand my understanding of the system and its limitations. Thank you for indulging me.

 

[poadeleted20]

I was thinking of the case where a pilot intends to fly to a VOR and tunes/twists to a radial but the plane happens to be on a course parallel to the radial in the general direction towards the station. The pilot turns on the C-I and it commands a 2 minute turn to the left (let's say the plane is on a parallel course to the right of the radial). 30 seconds later, the plane is on a course perpendicular to its original course and closing in on the radial. Suppose 30 seconds after that moment the plane nears the radial enough that the CDI is less than a dot off center. The plane would then be on a course opposite to what the pilot had intended and the C-I would be dutifully tracking it.

OK, let's say you're trying to track the 270 radial inbound (i.e., fly a 090 course to the VOR). You are located west of the VOR and north of the 270 radial, headed 090, so the needle is deflected fully south (i.e., to the right on the CDI). The system will command a standard rate turn to the right, and will continue to command that turn as long as the needle remains pegged right. If the plane is located more than 10 degrees north of the 270 radial, the needle will never come off the peg, and the airplane will just continue making 360's to the right.

If the plane is closer to the radial, so it crosses the radial in the turn, the airplane will begin rolling out of the turn as the needle centers. In the case you describe, where the plane reaches the radial on a southerly heading just as the needle centers, the plane will roll wings-level as the needle centers, but on a perpendicular heading. As the plane continues south of the radial, and the needle begins to swing north (left), it will begin turning left slowly, and increasing the turn rate as the needle deviates further to the north (left). You'll end up S-turning across the radial, possibly in an undamped oscillation which will never stabilize on the radial.

All that is why the C-I manual says not to engage the track mode until the needle is within one dot of center and the heading within 10 degrees of the desired course. If you start that close to "on course", the unit is close enough to "in sync" that it will result in damped bracketing to a good tracking solution.

 

[asicer]

OK, let's say you're trying to track the 270 radial inbound (i.e., fly a 090 course to the VOR). You are located west of the VOR and north of the 270 radial, headed 090, so the needle is deflected fully south (i.e., to the right on the CDI). The system will command a standard rate turn to the right, and will continue to command that turn as long as the needle remains pegged right. If the plane is located more than 10 degrees north of the 270 radial, the needle will never come off the peg, and the airplane will just continue making 360's to the right.

If the plane is closer to the radial, so it crosses the radial in the turn, the airplane will begin rolling out of the turn as the needle centers. In the case you describe, where the plane reaches the radial on a southerly heading just as the needle centers, the plane will roll wings-level as the needle centers, but on a perpendicular heading. As the plane continues south of the radial, and the needle begins to swing north (left), it will begin turning left slowly, and increasing the turn rate as the needle deviates further to the north (left). You'll end up S-turning across the radial, possibly in an undamped oscillation which will never stabilize on the radial.

All that is why the C-I manual says not to engage the track mode until the needle is within one dot of center and the heading within 10 degrees of the desired course. If you start that close to "on course", the unit is close enough to "in sync" that it will result in damped bracketing to a good tracking solution.

Ah, thanks. Using your 270 radial example, this is what I was envisioning could happen (attached) but it sounds like it's not possible.

And the "damped bracketing" description sounds like a recipe for seasickness. The KAP140/KLN94/G1000 combo I'm accustomed to doesn't seem to do that, but then again I haven't used it all that extensively.

 

[poadeleted20]

Ah, thanks. Using your 270 radial example, this is what I was envisioning could happen (attached) but it sounds like it's not possible.

That is correct.

And the "damped bracketing" description sounds like a recipe for seasickness.

It doesn't happen that fast (i.e., like a rolling boat) -- more like S-turns across a road.

The KAP140/KLN94/G1000 combo I'm accustomed to doesn't seem to do that, but then again I haven't used it all that extensively.

That's a completely different and far more capable system which will do intercepts just fine if you follow the manual.