CNS-80 VNAV

After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left.
The unit's computed VDI presentation is active for many GPS
approaches, as well as for all LNAV and LPV approaches.

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.

(Unexplainably, the CNS-80 annunciated LNAV/VNAV
on approaches which actually were non-precision LNAV.)

I'm guessing that the external VDI drive was withheld precisely to
prevent glideslope capture on coupled non-precision approaches.
I don't know the reason behind this, or even if that's the rule.

Of course, LNAV coupling works for all approaches, as expected.
---JRC---

It would seem that would be legal for LPV but not for VNAV minimums. VNAV
is actually Baro VNAV, which requires a lot of hookups and computational
fussing with the air data system.

"John R. Copeland" wrote:

After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left.
The unit's computed VDI presentation is active for many GPS
approaches, as well as for all LNAV and LPV approaches.

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.

(Unexplainably, the CNS-80 annunciated LNAV/VNAV
on approaches which actually were non-precision LNAV.)

I'm guessing that the external VDI drive was withheld precisely to
prevent glideslope capture on coupled non-precision approaches.
I don't know the reason behind this, or even if that's the rule.

Of course, LNAV coupling works for all approaches, as expected.
---JRC---

Do you mean that my CNX-80 is performing an illegal act when it drives
my VDI during an LNAV/VNAV approach?

The only "complicated" hookup which I have is that my altitude encoder
reading is known by my CNX-80, as well as the current baro setting,
which it prompts me to confirm from time to time.
The way the approaches fly, though, it feels like the VNAV guidance
is computed from three-dimensional GPS position.

I could test that by setting a grossly inaccurate Baro reading,
and watching the actual altitudes it steers me toward.
I think I'll try that next time I get a chance.
---JRC---

wrote in message =
...
It would seem that would be legal for LPV but not for VNAV minimums. =
VNAV
is actually Baro VNAV, which requires a lot of hookups and =
computational
fussing with the air data system.
=20
"John R. Copeland" wrote:
=20
After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left.
The unit's computed VDI presentation is active for many GPS
approaches, as well as for all LNAV and LPV approaches.

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic =
capture.

(Unexplainably, the CNS-80 annunciated LNAV/VNAV
on approaches which actually were non-precision LNAV.)

I'm guessing that the external VDI drive was withheld precisely to
prevent glideslope capture on coupled non-precision approaches.
I don't know the reason behind this, or even if that's the rule.

Of course, LNAV coupling works for all approaches, as expected.
---JRC---

John R. Copeland wrote:
Do you mean that my CNX-80 is performing an illegal act when it drives
my VDI during an LNAV/VNAV approach?

The only "complicated" hookup which I have is that my altitude encoder
reading is known by my CNX-80, as well as the current baro setting,
which it prompts me to confirm from time to time.
The way the approaches fly, though, it feels like the VNAV guidance
is computed from three-dimensional GPS position.

I could test that by setting a grossly inaccurate Baro reading,
and watching the actual altitudes it steers me toward.
I think I'll try that next time I get a chance.

I'll admit I'm guessing here, and I'd like to know exactly how the baro input is
incorporated in the navigation solution, but I suspect the baro altitude is
incorporated into the solution in a similar way to having another satellite in view.

IOW, with 5 satellites in view plus baro input, the navigation solution has 6
equations with 4 unknowns (3 dimensional space plus time). If that's the case,
the baro input participates in the lateral position determination as well as the
vertical. I suppose they could "weight" the effect of the baro input since its
precision/accuracy are different from the satellite inputs.

In TSOC129 units that use baro-aiding, there's no vertical guidance, yet the
baro input is incorporated in the navigation solution somehow.

DGB

---JRC---

wrote in message ...

It would seem that would be legal for LPV but not for VNAV minimums. VNAV
is actually Baro VNAV, which requires a lot of hookups and computational
fussing with the air data system.

"John R. Copeland" wrote:


After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left.
The unit's computed VDI presentation is active for many GPS
approaches, as well as for all LNAV and LPV approaches.

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.

(Unexplainably, the CNS-80 annunciated LNAV/VNAV
on approaches which actually were non-precision LNAV.)

I'm guessing that the external VDI drive was withheld precisely to
prevent glideslope capture on coupled non-precision approaches.
I don't know the reason behind this, or even if that's the rule.

Of course, LNAV coupling works for all approaches, as expected.
---JRC---



--
Dave Butler, software engineer 919-392-4367

If nobody else runs the test before I get a chance, I'll report back =
when I do.
I think you're too deep into speculation mode when you suggest baro =
input
might contribute to determination of lateral position.
Until I see some test results, I lean more toward thinking the CNX-80
may be relying on WAAS-augmented geometry for VNAV guidance.
It might be very ILS-like in that instance, where the baro setting
affects the DH(DA), but not the glide path.
---JRC---

"Dave Butler" wrote in message =
...
John R. Copeland wrote:
=20
I could test that by setting a grossly inaccurate Baro reading,
and watching the actual altitudes it steers me toward.
I think I'll try that next time I get a chance.
=20
I'll admit I'm guessing here, and I'd like to know exactly how the =
baro input is=20
incorporated in the navigation solution, but I suspect the baro =
altitude is=20
incorporated into the solution in a similar way to having another =
satellite in view.
=20
IOW, with 5 satellites in view plus baro input, the navigation =
solution has 6=20
equations with 4 unknowns (3 dimensional space plus time). If that's =
the case,=20
the baro input participates in the lateral position determination as =
well as the=20
vertical. I suppose they could "weight" the effect of the baro input =
since its=20
precision/accuracy are different from the satellite inputs.
=20
In TSOC129 units that use baro-aiding, there's no vertical guidance, =
yet the=20
baro input is incorporated in the navigation solution somehow.
=20
DGB

John R. Copeland wrote:
If nobody else runs the test before I get a chance, I'll report back when I do.
I think you're too deep into speculation mode when you suggest baro input
might contribute to determination of lateral position.

Certainly deep into speculation, since AFAIK all the navigation algorithms are
proprietary, ... or maybe they are published as part of some certification
process, anyway, I've never seen them.

However it seems clear that baro-aiding is incorporated into TSO C129
navigators, which offer no vertical guidance. It's hard to imagine why they'd go
to the (considerable) trouble to have a baro-aiding interface if the baro-aiding
didn't add to the navigation solution.

Until I see some test results, I lean more toward thinking the CNX-80
may be relying on WAAS-augmented geometry for VNAV guidance.
It might be very ILS-like in that instance, where the baro setting
affects the DH(DA), but not the glide path.
---JRC---

"Dave Butler" wrote in message ...

John R. Copeland wrote:

I could test that by setting a grossly inaccurate Baro reading,
and watching the actual altitudes it steers me toward.
I think I'll try that next time I get a chance.

I'll admit I'm guessing here, and I'd like to know exactly how the baro input is
incorporated in the navigation solution, but I suspect the baro altitude is
incorporated into the solution in a similar way to having another satellite in view.

IOW, with 5 satellites in view plus baro input, the navigation solution has 6
equations with 4 unknowns (3 dimensional space plus time). If that's the case,
the baro input participates in the lateral position determination as well as the
vertical. I suppose they could "weight" the effect of the baro input since its
precision/accuracy are different from the satellite inputs.

In TSOC129 units that use baro-aiding, there's no vertical guidance, yet the
baro input is incorporated in the navigation solution somehow.

"Ron Rosenfeld" wrote in message =
...
On Sun, 24 Oct 2004 23:57:17 GMT, "John R. Copeland"
wrote:
=20
However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.
=20
John,
=20
I won't be upgrading my CNX80 until the end of December, but in =
Newsletter
5 from Garmin there is a description indicating that Glideslope =
guidance
should be available on your HSI for LNAV/VNAV, LPV *and* =
"non-precision
approaches with defined vertical paths" so long as the HPL or VPL =
minimums
for the approach are not exceeded.
=20
Perhaps something has changed since that document was written; or =
perhaps
there's a glitch in your wiring.
=20
--ron

Ron, I no longer have newsletter 5, but I can't fault the newsletter =
announcing=20
availability of the version 2 upgrade, because the new NAV display in =
the
CNX-80 performs as the attachment to that newsletter described.
For LNAV approaches, I have not found where that attachment promised
glide-path guidance on an external indicator, only on the internal NAV =
display.

The undocumented part concerns the drive to the external VDI in my =
flight director.
Wiring can't be faulty, because it works flawlessly on ILS and VNAV =
approaches.
My avionics installer promised today to inquire about that with his =
factory rep.
If I get clarification, I'll share it here.
---JRC---

"John R. Copeland" wrote in message =
...
"Ron Rosenfeld" wrote in message =
...
=20
=20
Perhaps something has changed since that document was written; or =
perhaps
there's a glitch in your wiring.
=20
--ron

Ron, I no longer have newsletter 5, but I can't fault the newsletter =
announcing=20
availability of the version 2 upgrade, because the new NAV display in =
the
CNX-80 performs as the attachment to that newsletter described.
For LNAV approaches, I have not found where that attachment promised
glide-path guidance on an external indicator, only on the internal NAV =
display.

The undocumented part concerns the drive to the external VDI in my =
flight director.
Wiring can't be faulty, because it works flawlessly on ILS and VNAV =
approaches.
My avionics installer promised today to inquire about that with his =
factory rep.
If I get clarification, I'll share it here.
---JRC---

So here's the clarification as it's been relayed to me.
As I speculated earlier, the external VDI is purposely not driven unless =
VNAV
minima are published for the approach, and the reason is exactly to =
prevent
coupling the autopilot to the computed glide slope on LNAV-only =
approaches.

This affects only the external VDI.
The NAV display page in the CNX-80 remains useful for stabilized =
descent,
but only under manual control, not by the autopilot.

In addition, there has been at least one malfunction report of an =
external
VDI presentation anomaly at the published DH in a coupled approach.
I do not yet know any details about what happened at DH in that report,
so be careful out there, y'all hear?
---JRC---

On Mon, 25 Oct 2004 18:52:13 GMT, "John R. Copeland"
wrote:

"Ron Rosenfeld" wrote in message ...
On Sun, 24 Oct 2004 23:57:17 GMT, "John R. Copeland"
wrote:

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.

John,

I won't be upgrading my CNX80 until the end of December, but in Newsletter
5 from Garmin there is a description indicating that Glideslope guidance
should be available on your HSI for LNAV/VNAV, LPV *and* "non-precision
approaches with defined vertical paths" so long as the HPL or VPL minimums
for the approach are not exceeded.

Perhaps something has changed since that document was written; or perhaps
there's a glitch in your wiring.

--ron

Ron, I no longer have newsletter 5, but I can't fault the newsletter announcing
availability of the version 2 upgrade, because the new NAV display in the
CNX-80 performs as the attachment to that newsletter described.
For LNAV approaches, I have not found where that attachment promised
glide-path guidance on an external indicator, only on the internal NAV display.

The undocumented part concerns the drive to the external VDI in my flight director.
Wiring can't be faulty, because it works flawlessly on ILS and VNAV approaches.
My avionics installer promised today to inquire about that with his factory rep.
If I get clarification, I'll share it here.
---JRC---

Perhaps I misunderstood your original post.

I didn't see anything in either promising external VDI signals for an LNAV
approach unless it fell into the category of being a non-precision approach
with associated vertical path data. (And which could therefore include
other types of non-precision approaches).

There is an extra explanatory paragraph in newsletter 5.

-----------------------
Attachment:

Glideslope guidance for LNAV/VNAV (Gamma 2) and LPV (Gamma 3) approaches,
and advisory vertical guidance for other non-precision approaches with
vertical path data associated with them.

-------------------------------------

Newsletter 5:

Glideslope guidance for LNAV/VNAV (Gamma 2) and LPV (Gamma 3) approaches,
and advisory vertical guidance for other non-precision approaches with
vertical path data associated with them.

We’ve discussed this feature in the newsletter before and here are some
nuts and bolts. All LNAV/VNAV and LPV approaches as well as non-precision
approaches with defined vertical paths will have vertical guidance
displayed on your HSI or CDI. This will drive the CDI and VDI and they will
behave as they would during an ILS approach.

--------------------------------------


--ron

"Ron Rosenfeld" wrote in message =
...
On Mon, 25 Oct 2004 18:52:13 GMT, "John R. Copeland"
wrote:
=20
"Ron Rosenfeld" wrote in message =
...
On Sun, 24 Oct 2004 23:57:17 GMT, "John R. Copeland"
wrote:
=20
However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic =
capture.
=20
John,
=20
I won't be upgrading my CNX80 until the end of December, but in =
Newsletter
5 from Garmin there is a description indicating that Glideslope =
guidance
should be available on your HSI for LNAV/VNAV, LPV *and* =
"non-precision
approaches with defined vertical paths" so long as the HPL or VPL =
minimums
for the approach are not exceeded.
=20
Perhaps something has changed since that document was written; or =
perhaps
there's a glitch in your wiring.
=20
--ron

Ron, I no longer have newsletter 5, but I can't fault the newsletter =
announcing=20
availability of the version 2 upgrade, because the new NAV display in =
the
CNX-80 performs as the attachment to that newsletter described.
For LNAV approaches, I have not found where that attachment promised
glide-path guidance on an external indicator, only on the internal NAV =
display.

The undocumented part concerns the drive to the external VDI in my =
flight director.
Wiring can't be faulty, because it works flawlessly on ILS and VNAV =
approaches.
My avionics installer promised today to inquire about that with his =
factory rep.
If I get clarification, I'll share it here.
---JRC---
=20
Perhaps I misunderstood your original post.
=20
I didn't see anything in either promising external VDI signals for an =
LNAV
approach unless it fell into the category of being a non-precision =
approach
with associated vertical path data. (And which could therefore =
include
other types of non-precision approaches).
=20
There is an extra explanatory paragraph in newsletter 5.
=20
-----------------------
Attachment:
=20
Glideslope guidance for LNAV/VNAV (Gamma 2) and LPV (Gamma 3) =
approaches,
and advisory vertical guidance for other non-precision approaches with
vertical path data associated with them.
=20
-------------------------------------
=20
Newsletter 5:
=20
Glideslope guidance for LNAV/VNAV (Gamma 2) and LPV (Gamma 3) =
approaches,
and advisory vertical guidance for other non-precision approaches with
vertical path data associated with them.=20
=20
We've discussed this feature in the newsletter before and here are =
some
nuts and bolts. All LNAV/VNAV and LPV approaches as well as =
non-precision
approaches with defined vertical paths will have vertical guidance
displayed on your HSI or CDI. This will drive the CDI and VDI and they =
will
behave as they would during an ILS approach.
=20
--------------------------------------
=20
=20
--ron

Thanks, Ron, for excerpting newsletter 5, which I've lost.
Yes, there's been a change since newsletter 5, which seemed to promise
that an external VDI would display vertical guidance on non-precision =
approaches.
For VNAV precision approaches, the external VDI indeed does behave
as it would during an ILS approach, and the autopilot responds the same,
too, going through the modes of GS-ARM and GS-CAPTURE.

Now, though, non-precision LNAV approaches with defined vertical paths =
will
display vertical guidance only on the new NAV display page of the =
CNX-80,
but the external VDI instrument will not display that same vertical =
guidance.
In LNAV, my glideslope flag is retracted, but the glideslope pointer is =
driven
beyond the end of the scale, hiding it from view throughout the =
approach.

In the remaining case of LNAV approaches without defined vertical path,
the VDI portion of the CNX-80's NAV page is entirely absent.

As I mentioned in a follow-up post, later than the one you replied to,
the intentional suppression of drive to external VDIs effectively =
prevents
a 3-axis autopilot from locking onto the vertical guidance information
during a non-precision approach. The pilot must manage the descent.

I do not know whether that choice was at Garmin's volition,
or whether it was an FAA requirement.
---JRC---