What determines LNAV "circling mode"?

[Jack]

I have been trying to figure out what is happening with my Cambridge
LNAV since in several flights "speed-to-fly" bars have never appeared,
even in a long continuous glide straight at the goal indicated by the
connected Cambridge GPS. Since "speed-to-fly" bars only appear in
"cruise" mode possibly my LNAV is somehow always in "circling" mode -
but I cannot find in my very thin LNAV manual (version 4, march
1995) what determines the choice of "cruise" vs "circling" mode so am
hoping someone here has that knowledge so I can at least know how I
should _expect_ it to behave.

(FYI I do not believe I have the G-meter option, though can't say for
sure since I was not the one who purchased the LNAV new.)

[JJ Sinclair]

Jack,
If you don't have a *G* switch in your L-Nav, then you must have an external
switch to do it. On a flapped sailplane, it can be a micro-switch in the flap
drive system to put the L-Nav in climb mode when positive flaps are commanded.
In a standard class ship, the switch is usually mounted on the stick.

Having said that, I no longer feel the need to switch. With GPS constantly
driving your position in the L-Nav, the distance to your selected destination
will always be correct. I just leave it in cruse mode with the audio off and my
back-up vario (B-40) on with audio up. I get audio only when I'm going up and
no distracting speed-to-fly audio, when crusing. If I feel the need to see what
old Mr. MacCready thinks about my speed, I just look at the speed-to-fly
needle.
JJ Sinclair

[Jack Glendening]

JJ,
Thanks for the info. I know there is no switch on the stick and am
pretty sure there is no microswitch on the flaps, so perhaps I have a
G meter after all !? I have to replace the battery anyhow so will check
it out when I have the LNAV opened and will also check the back for a
hold switch connection. I also see there is a calibration for the
Gmeter and will check it if I do have a Gmeter since I certainly was at
1G in my long glide with no "speed-to-fly" bars appearing.
Jack

[Jack Glendening]

PS I can't follow your second suggestion since I have a Winter
mechanical vario with no audio - and more importantly right now it's
reading a permanent 8kts up (even on the ground) so will also be looking
at it when I have the panel cover removed.

[John Morgan]

If you don't have the G-meter option, there are two other ways the LNAV
might use to let it know if you're circling or in cruise. Your glider may
have a switch installed on the flap mechanism to sense selection of
thermalling flaps, or it could have a manual toggle or selector switch on
the panel or stick.
--
bumper >
"Dare to be different . . . circle in sink."
to reply, the last half is right to left

"Jack" > wrote in message
link.net...
> I have been trying to figure out what is happening with my Cambridge
> LNAV since in several flights "speed-to-fly" bars have never appeared,
> even in a long continuous glide straight at the goal indicated by the
> connected Cambridge GPS. Since "speed-to-fly" bars only appear in
> "cruise" mode possibly my LNAV is somehow always in "circling" mode -
> but I cannot find in my very thin LNAV manual (version 4, march
> 1995) what determines the choice of "cruise" vs "circling" mode so am
> hoping someone here has that knowledge so I can at least know how I
> should _expect_ it to behave.
>
> (FYI I do not believe I have the G-meter option, though can't say for
> sure since I was not the one who purchased the LNAV new.)
>


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[Duane Eisenbeiss]

"Jack" > wrote in message
link.net...
> I have been trying to figure out what is happening with my Cambridge
> LNAV since in several flights "speed-to-fly" bars have never appeared,
> even in a long continuous glide straight at the goal indicated by the
> connected Cambridge GPS. >

Regards the Climb/Cruise switch that you have been advised about:
If the switch is in the "Cruise" mode the Speed to Fly bars should always be
in view.
If the switch is in the "Climb" mode the Speed to Fly bars should come into
view when flying at a speed greater than about 60 kts.

If you never see the Speed to Fly bars then you may have an electronic
failure. In that case you will get to run a test as to whether Cambridge
really will still do repairs.

Duane

[Marc Ramsey]

"Duane Eisenbeiss" > wrote...
> If you never see the Speed to Fly bars then you may have an electronic
> failure. In that case you will get to run a test as to whether Cambridge
> really will still do repairs.

Which Cambridge? 8^)

[Duane Eisenbeiss]

"Marc Ramsey" > wrote in message
m...
>
> "Duane Eisenbeiss" > wrote...
> > If you never see the Speed to Fly bars then you may have an electronic
> > failure. In that case you will get to run a test as to whether
Cambridge
> > really will still do repairs.
>
> Which Cambridge? 8^)
>

The L-Nav that is being discussed.

[Duane Eisenbeiss]

"Marc Ramsey" > wrote in message
m...
>
>
> Which Cambridge? 8^)
>

I think that I maybe misunderstood your question. Did you mean which CAI
company?
If so I do not know. That is what I meant by "running a test". CAI seems
to be in a real state of flux.

Duane

[303pilot]

I'm new to my LNAV also. Are you on the screen that displays "alt req" in
the altitude field? It's a screen or two 'right' of the main one on the SW
version I have. <GPS 20 5.1 w/LNAV 5.6>
"Jack Glendening" > wrote in message
link.net...
> I checked out the LNAV on my glider today and
> (1) there is nothing plugged into the "hold" jack on the rear - so
> there can't be a hold switch anywhere
> (2) there is no mention of a Gmeter on the screen mentioned by Paul
> Remde and the Gmeter configuration setting is for "0G" - so there isn't
> a Gmeter
> (3) I can't test whether the GPS is used for the cruise/circling mode,
> which would seem to be the only option left - but as mentioned in the
> original posting in my last flight I did a straight glide to the GPS
> goal and never saw the "speed-to-fly" bars so if that is what is
> supposed to be going on then it is broken
> (4) it seems like an as yet unmentioned possbility is that Cambridge
> at some early stage had a different GPS cruise/circling criterion than
> that described in the latest manual (since nothing about such a
> criterion is mentioned in the manual I do have).
>
> For the record, I have not been flying this plane/LNAV much since
> purchase so have no idea what "normal" is! On most of my flights no
> speed-to-fly bars have _ever_ appeared (as on my last flight) but on a
> few flights they _have_ appeared and I'll be damned if I can figure out
> any difference on those flights which might explain their sudden
appearance.
>
> In any case the unit now needs to go to (whichever) Cambridge for repair
> as since my last flight I have suddenly lost three "elements" from each
> of three number characters (all in the same column!) as determined by a
> display test - I know that they were OK on my last flight since the loss
> is apparent when setting the initial field elevation, which I always do
> on startup So I will ask them to also test the cruise/circling option
> setting and see what they say. (Lately I seem to be not a pilot but an
> "airplane maintainer", since something new seems to break on every
> flight I take - my last flight was my first after a down period while I
> worked on repairing the tailwheel fork and the retraction motor
operation!)
>
> Thanks for all your help.
>

[Jack Glendening]

303pilot wrote:
> I'm new to my LNAV also. Are you on the screen that displays "alt req" in
> the altitude field? It's a screen or two 'right' of the main one on the SW
> version I have. <GPS 20 5.1 w/LNAV 5.6>

Assuming you are talking about point (2), the "Remde mentioned" screen
is 8 to the left of the "Main" screen and shows external battery
voltage. The other screen I mentioned is in the "Calibrate" section and
labelled "G meter" and should read around 611 if there is an actual
Gmeter and around 511 if not.

[Jack Glendening]

Jack Glendening wrote:
> I have been trying to figure out what is happening with my Cambridge
> LNAV since in several flights "speed-to-fly" bars have never appeared,
> even in a long continuous glide straight at the goal indicated by the
> connected Cambridge GPS. Since "speed-to-fly" bars only appear in
> "cruise" mode possibly my LNAV is somehow always in "circling" mode -
> but I cannot find in my very thin LNAV manual (version 4, march
> 1995) what determines the choice of "cruise" vs "circling" mode so am
> hoping someone here has that knowledge so I can at least know how I
> should _expect_ it to behave.

It took quite awhile for me to sort things out in this case, since in
working on it I found problems with (1) my L-NAV, (2) the L-NAV meter,
(3) my GPS-HAV, (4) the pitot tubing, and (5) the triple probe - all of
which needed to be addressed (and my manual Winter vario is also
kaput!). I've now decided that the problem lies in the L-NAV display,
since "de beeps and de boops" indicate that it switching from circling
to/from cruise mode but the "speed-to-fly" bars never appear (even
though I have heard both the "speed up" and "slow down" tones). And
there are other problems with the display as well (missing segments on
several digits), so yet another display problem would not be a surprise.
My thanks to all who volunteered suggestions.