LNAV problems

[Hellman]

As I posted a few months ago, my LNAV was having problems (crazy g-
meter and averager readings) and Cambridge could find nothing wrong
with it, plus after their first try they stopped servicing LNAV's. I
then purchased a used LNAV from someone who contacted me from my post
and for a while that worked fine, but on a recent flight it too was
acting crazy, but in a different way.

It read 1200 feet high when the altimeter was properly set, the
altitude required was independent of the distance to the goal (and of
course therefore wrong), and the McCready setting jumped from 0 to 3.7
to 7.4 to 9.5 and then (when decreased) jumped down in 3.7 increments
to 5.8, 2.1 and 0. (It seems to saturate at 9.5 and 0.) It also was
telling me to push the whole flight even though the vario was reading
correctly. On my next flight, this erratic behavior was not present.

All of this leads to two questions:

1. While I could have two LNAV failures in close proximity in time, a
more likely hypothesis is that there is a wiring fault or electrical
noise or something else causing both units to either fail or seem to
fail. Does anyone have any experience here?

2. When I posted about my original LNAV's problems in March, Paul
Remde indicated that Cambridge was in the process of solving it's
problems and I might be able to get LNAV service there again. Any news
on that?

Thanks very much.

Martin

[jcarlyle]

Hi, Martin,

I can't break a confidence, but I can assure you that great service on
the L-Nav and Gps-Nav units is going to be available soon. I know this
because I have been trying to arrange repairs on my Cambridge Gps
Model 25. I'm thinking that service will again be available by mid-
July 2008.

As to your problems, they may not be coming from inside the L-Nav. I
was experiencing a problem whereby my L-Nav would initially show a
distance to the selected way point that was off by 18.5 nm. Choosing
the next waypoint in the task would cause the distance error to
disappear. I finally traced the cause of this problem to my Gps Model
25. I'm temporarily using a friend's Model 20 and distance measurement
is always correct now. Whether this has any relevance at all to the
difficulties you're seeing, I don't know. But it may be worthwhile
trying a different Gps-Nav, just to see what happens...

Good luck!

-John

On Jun 17, 2:56 am, Hellman > wrote:
> As I posted a few months ago, my LNAV was having problems (crazy g-
> meter and averager readings) and Cambridge could find nothing wrong
> with it, plus after their first try they stopped servicing LNAV's. I
> then purchased a used LNAV from someone who contacted me from my post
> and for a while that worked fine, but on a recent flight it too was
> acting crazy, but in a different way.
>
> It read 1200 feet high when the altimeter was properly set, the
> altitude required was independent of the distance to the goal (and of
> course therefore wrong), and the McCready setting jumped from 0 to 3.7
> to 7.4 to 9.5 and then (when decreased) jumped down in 3.7 increments
> to 5.8, 2.1 and 0. (It seems to saturate at 9.5 and 0.) It also was
> telling me to push the whole flight even though the vario was reading
> correctly. On my next flight, this erratic behavior was not present.
>
> All of this leads to two questions:
>
> 1. While I could have two LNAV failures in close proximity in time, a
> more likely hypothesis is that there is a wiring fault or electrical
> noise or something else causing both units to either fail or seem to
> fail. Does anyone have any experience here?
>
> 2. When I posted about my original LNAV's problems in March, Paul
> Remde indicated that Cambridge was in the process of solving it's
> problems and I might be able to get LNAV service there again. Any news
> on that?
>
> Thanks very much.
>
> Martin

[[email protected]]

I doubt it's applicable to your situation but twice in the past few
years I've had problems caused by the "fingers" in the RJ connectors
on the back of the LNAV that connect the meter and the GPS-NAV. They
get tarnished or lose their spring or some combination. Doug Jacobs
mentioned it to me so although it may not be widespread, it's
apparently not confined just to my instrument. The easy fix is to take
the cover of the LNAV and gently (with great care) push the contacts
on those connectors from the inside of the instrument to add a little
more tension.

Chip Bearden
ASW 24 "JB"
USA