Cambridge L-Nav problem -- runaway vario

I have a nagging, recurring problem with my trusty L-Nav. Occasionally, when I power the unit on, the lift tone sounds constantly, and the vario display is pegged at maximum lift (10 kts). It's clear that the problem is with the L-Nav, and not the electric vario display, as the tone continues to sound once the electric vario display is unplugged, and the problem continues once the vario is plugged back in.

Cycling power once or twice will sometimes, but not always, make the problem go away -- until the next time. Last Friday, the problem persisted for two hours, finally disappearing as mysteriously as it came. Once the L-Nav powers on correctly, it has always stayed operational for my entire flight.

Has anyone else experienced this, and found a solution?

Thanks,

Jordan

I had the same problem with my (similar) SNAV.

Turned out that the external temperature sensor cable, which went into the air duct and was flexed every time the canopy opened, had worn and shorted a couple of leads out.

Unplugged it from the back of the SNAV and the problem went away. Fixed the cable, and it stayed away.

Paul

At 17:02 07 July 2014, jherzog wrote:
I have a nagging, recurring problem with my trusty L-Nav. Occasionally
when I power the unit on, the lift tone sounds constantly, and the vario
display is pegged at maximum lift (10 kts).

Long time ago but I had a similar problem. Mine tended to stick at a random
figure, say 2kts up and would often occur after I'd done a pull up. The
LNav would then continue to work but around the new datum - very difficult
to fly when you have 2kts of audio as your datum! On the ground when the
problem occurred, even taking all the pipes off made no difference and I
seem to remember the fault lay in a sticking airspeed transducer. Don't
know if you can get these replaced any more.

Originally Posted by Paul Ruskin[_2_]

I had the same problem with my (similar) SNAV.

Turned out that the external temperature sensor cable, which went into the air duct and was flexed every time the canopy opened, had worn and shorted a couple of leads out.

Unplugged it from the back of the SNAV and the problem went away. Fixed the cable, and it stayed away.

Paul

Thanks so much Paul. I will check my OAT probe wiring this weekend.

Jordan

At 17:02 07 July 2014, jherzog wrote:

I have a nagging, recurring problem with my trusty L-Nav. Occasionally
when I power the unit on, the lift tone sounds constantly, and the vari
display is pegged at maximum lift (10 kts). It's clear that the proble
is with the L-Nav, and not the electric vario display, as the ton
continues to sound once the electric vario display is unplugged, and th
problem continues once the vario is plugged back in.

Cycling power once or twice will sometimes, but not always, make th
problem go away -- until the next time. Last Friday, the proble
persisted for two hours, finally disappearing as mysteriously as i
came. Once the L-Nav powers on correctly, it has always staye
operational for my entire flight.

Has anyone else experienced this, and found a solution?

Thanks,



I have exactly the same problem with my L Nav but over the season have
concluded that the problem is more likely to occur if there has been
dampness in the trailer. Over last winter it stopped when I put a
dehumidifier in the trailer. Over this summer it only happens after a few
wet days. but thats in UK type weather conditions. The summer has been
fairly dry by our standards and as you say cycling the power usually makes
it work fine.


Mike

At 17:02 07 July 2014, jherzog wrote:

I have a nagging, recurring problem with my trusty L-Nav. Occasionally
when I power the unit on, the lift tone sounds constantly, and the vari
display is pegged at maximum lift (10 kts). It's clear that the proble
is with the L-Nav, and not the electric vario display, as the ton
continues to sound once the electric vario display is unplugged, and th
problem continues once the vario is plugged back in.

Cycling power once or twice will sometimes, but not always, make th
problem go away -- until the next time. Last Friday, the proble
persisted for two hours, finally disappearing as mysteriously as i
came. Once the L-Nav powers on correctly, it has always staye
operational for my entire flight.

Has anyone else experienced this, and found a solution?

Thanks,


I have exactly the same issue with my L-Nav and have concluded it is a damp
issue. The problem stopped when I put a dehumidifier in the trailer over
the winter but has recurred over this summer after a damp period. As you
say normally recycling the power stops the problem but I too have had
occassions when the unit simply screams climb for the whole flight.

Exactly similar symptoms in my old L-Nav.
I had to relight at a contest once because of it. In frustration I banged the side of the instrument panel and the L-Nav became normal. So, I took off and flew the flight without further problems.

Occasionally, the problem would recur....and banging it always cured it!
Never did anything more involved than that.

Maybe a bad internal ground? Loose circuit board? Who knows?

J4

As many times as we've all used it, percussive maintenance is not the correct answer. Send it to ClearNav.
Jim

The frustrating thing is that a fault amenable to percussive maintenance can be darned hard to isolate and fix permanently in the repair shop.

-John, Q3


On Sunday, July 13, 2014 8:14:13 AM UTC-4, JS wrote:
As many times as we've all used it, percussive maintenance is not the correct answer. Send it to ClearNav.

Jim