Cambridge 302 Gear/Airbrake Warning - How to Test?

I am in the midst of a project to install a gear/airbrake warning
system on my glider based on the built-in capability of my Cambridge
302. This system is better than the simple "airbrakes open, gear up"
warning system (http://www.wingsandwheels.com/pdf/gear%20warning.pdf)
as it also alerts the pilot to "airbrakes open on tow".

My "sensors" will be strategically placed reed switches and associated
magnets (and tefzel wire of course). My primary question revolves
around how to do a pre-flight test of the system.

From the manual...

"Warnings depend on presence or absence of airspeed as well as the
switch contacts. Here
is the logic: Unlocking the airbrakes connects pin 7 to ground. When
airspeed rises above 25 knots, the
alarm sounds. The alarm is cancelled when the airbrakes are locked.
During flight with the
landing gear retracted, unlocking the airbrakes connects pin 7 to
ground and sounds the
alarm. Lowering the landing gear opens the connection between pin 8
and ground and
cancels the alarm."

Reading from the above it would seem that the alarm system is totally
inactive until the GPS indicates I am moving 25+ knots. If so, how
can I test this system on the ground? Anyone else install this system
and have any experience? This weekend I will experiment to see if
the 302 programming is smart enough to allow testing while I am
standing still.

Thanks, John DeRosa

ContestID67 wrote:
I am in the midst of a project to install a gear/airbrake warning
system on my glider based on the built-in capability of my Cambridge
302. This system is better than the simple "airbrakes open, gear up"
warning system (http://www.wingsandwheels.com/pdf/gear%20warning.pdf)
as it also alerts the pilot to "airbrakes open on tow".

My "sensors" will be strategically placed reed switches and associated
magnets (and tefzel wire of course). My primary question revolves
around how to do a pre-flight test of the system.

From the manual...

"Warnings depend on presence or absence of airspeed as well as the
switch contacts. Here
is the logic: Unlocking the airbrakes connects pin 7 to ground. When
airspeed rises above 25 knots, the
alarm sounds. The alarm is cancelled when the airbrakes are locked.
During flight with the
landing gear retracted, unlocking the airbrakes connects pin 7 to
ground and sounds the
alarm. Lowering the landing gear opens the connection between pin 8
and ground and
cancels the alarm."

Reading from the above it would seem that the alarm system is totally
inactive until the GPS indicates I am moving 25+ knots. If so, how
can I test this system on the ground? Anyone else install this system
and have any experience? This weekend I will experiment to see if
the 302 programming is smart enough to allow testing while I am
standing still.

Thanks, John DeRosa


Connect a large syringe (ideally with a ballast volume) to the pitot
tube system and use this to carefully increase pressure.

This feature of the C302 is very nice.

Darryl

If your airbrakes "suck open" after you are going faster than 25 knots
it will not warn you, just FYI...

It would work a tad better if the "spoiler open on tow" worked until
the first "gear up" condition before it reverted to the "spoiler
open / gear not down" mode.

Darryl,

Thanks for the response. Yes, this does seem like a nice system.
Alerts no matter what the volume is set to, brakes on tow warning,
etc.

Questions/comments...

1) While the syringe approach to mimic moving at 25+ knots would
logically work, it isn't optimal as something easily done during each
day's pre-flight.
2) What is a "ballast volume"?
3) Do you know how the 302 determines the difference between a warning
for "brakes deployed while gear down and on tow" and the warning for
"brakes deployed while gear up and landing"? The two states seem a
contradiction. My only thought is that it determines the glider is
descending versus assending.

Thanks again.

On 9/8/2011 1:03 PM, Mark wrote:
If your airbrakes "suck open" after you are going faster than 25 knots
it will not warn you, just FYI...

It would work a tad better if the "spoiler open on tow" worked until
the first "gear up" condition before it reverted to the "spoiler
open / gear not down" mode.

If you install the airbrake switch correctly, the Cambridge 302 will
warn you even before the airbrakes "suck open".

The Cambridge system is designed to work with a switch that detects when
the spoilers are _unlocked_, not after they are extended. It's an
important difference, but it's usually easy to install a switch that
does that. On the gliders I've had, there is at least 0.5" of movement
of the spoiler handle and connecting linkage to unlock the spoilers.

So, it's more accurate to refer to it as "spoiler's unlocked on tow",
instead of "spoilers open on tow".

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

On 9/8/11 1:29 PM, ContestID67 wrote:
1) While the syringe approach to mimic moving at 25+ knots would
logically work, it isn't optimal as something easily done during each
day's pre-flight.

How about using the spoilers to help aileron control during takeoff and
leave them open until 30 KIAS? ;-)

Actually, mine stopped working a few years ago... Tests just fine on
the bench, and the syringe on the pitot works too. But leaving the
spoilers unlocked to well over 30 KIAS doesn't set off the alarm. Had
Gary, when he was at CAI, check it out during the last calibration and
it worked just fine. Very odd indeed.

-Tom

On 9/8/11 1:29 PM, ContestID67 wrote:
Darryl,

Thanks for the response. Yes, this does seem like a nice system.
Alerts no matter what the volume is set to, brakes on tow warning,
etc.

Questions/comments...

1) While the syringe approach to mimic moving at 25+ knots would
logically work, it isn't optimal as something easily done during each
day's pre-flight.

It is possible to worry too much - You don't pre-flight test your ASI
and that is likely more critical than these warning buzzers. And you can
certainly test behavior of say the gear warning while in the air.

2) What is a "ballast volume"?

A bottle/flask connected in series with the syringe so that moving the
syringe plunger has less of an effect on the pressure than if used
directly. You really don't want to blow out instruments connected to the
system. You can disconnect the instruments but then the futzing with
connections itself may introduce problems. And it may just be easier to
just slide a tube over the pitot for these kind of tests.

And remember the manual says to connect the ASI and static ports on the
C302 to the ships pitot and static not that of the TE/triple probe - one
reason is to have these alarms etc. to work properly if the TE/triple
probe is left off

3) Do you know how the 302 determines the difference between a warning
for "brakes deployed while gear down and on tow" and the warning for
"brakes deployed while gear up and landing"? The two states seem a
contradiction. My only thought is that it determines the glider is
descending versus assending.

I believe it is simply state based. If the gear has been raised and the
airspeed is high then it knows its "gone flying" and is ready to detect
subsequent spoiler deployed before the gears is locked down. For example
I've pulled gear up while on tow and later cracked spoilers to handle a
little bit 'o rotor while on tow and had the undercarriage alarm go off
as expected. There is no way altitude or climb rate affects this. The
developers are too smart to do that.

Darryl

On 9/8/11 9:29 PM, Tom Serkowski wrote:
On 9/8/11 1:29 PM, ContestID67 wrote:
1) While the syringe approach to mimic moving at 25+ knots would
logically work, it isn't optimal as something easily done during each
day's pre-flight.

How about using the spoilers to help aileron control during takeoff and
leave them open until 30 KIAS? ;-)

Actually, mine stopped working a few years ago... Tests just fine on the
bench, and the syringe on the pitot works too. But leaving the spoilers
unlocked to well over 30 KIAS doesn't set off the alarm. Had Gary, when
he was at CAI, check it out during the last calibration and it worked
just fine. Very odd indeed.

-Tom

Tom well then obviously you need to order a new ClearNav vario... I
expect/hope they are U/C and spoiler switch pinout compatible with the C302.

Darryl

On Sep 8, 12:09*pm, ContestID67 wrote:
*If so, how
can I test this system on the ground? *Anyone else install this system
and have any experience? * This weekend I will experiment to see if
the 302 programming is smart enough to allow testing while I am
standing still.

Thanks, John DeRosa

To test the warning just turn on the 302 with the brakes unlocked and
the gear up. Mine sounds most times I rig the glider. To cancel it
press the 302 knob, or lock the brakes, or lower the gear.

The gear warning does not depend on either airspeed or ground speed,
only the warning for unlocked brakes with gear down does.

Andy

On Sep 8, 12:09*pm, ContestID67 wrote:

Reading from the above it would seem that the alarm system is totally
inactive until the GPS indicates I am moving 25+ knots. *If so, how
can I test this system on the ground? *Anyone else install this system
and have any experience? * This weekend I will experiment to see if
the 302 programming is smart enough to allow testing while I am
standing still.

The 302 gear warning is fully functional on ground at zero airspeed
and ground speed. It will also sound when the 302 is powered up with
brakes unlocked even if the gear is down and locked. To cancel it
press the knob or lock the brakes.

My gear warning sounds nearly every time I apply power after rigging
the glider as the brakes have to be unlocked for assembly.

For standard class glider using airbrakes for roll control on takeoff
the warning may sound briefly before it becomes time to lock the
brakes.

As has been mention by others, the brake switch should detect the
difference between "closed and locked" and "closed". Unless the gear
won't stay down if not locked, the gear switch should detect the
difference between "down" and "down and locked". (28 gear floats
halfway if not locked up or down so detecting full forward is
sufficient)


Andy