Airspeed Switch Issues

[PT20J]

2 hours ago, Mike A said:

The manufacturer won’t work on anything over like 10 years old.

I wonder if they will sell parts. Here’ an article with a picture of the switch disassembled. http://www.knr-inc.com/shoptalk-articles/25-shoptalk/93-201901-electric-gear-safety-system

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[Mark89114]

Now if we could only find the microswitch that goes in our FIKi stall warning system.  That system makes this look cheap.

[thinwing]

On 1/26/2018 at 4:29 PM, xcrmckenna said:

Can I get their info? I still have my old one and would like to get it rebuilt.


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Sure looks repairable...parts like the diaphragm can’t be that expensive

[thinwing]

1 hour ago, PT20J said:

On my ‘78 J that switch was connected to the Davtron clock for recording flight time. Probably difficult to calibrate accurately. 
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The way I would calibrate the switch is to use an asi in line with switch and slowly pressurize with a rolled up surgical tubing hose till desired airspeed and measure switch closing with a multi- meter...should get it close.The large screw on face of airspeed switch is for calibrating I assume.Turn In to reduce airspeed activation

[PT20J]

4 minutes ago, thinwing said:

The way I would calibrate the switch is to use an asi in line with switch and slowly pressurize with a rolled up surgical tubing hose till desired airspeed and measure switch closing with a multi- meter...should get it close.The large screw on face of airspeed switch is for calibrating I assume.Turn In to reduce airspeed activation

Agree that is the way to adjust the air switch as it's the test method described in the service manual. I was referring to one reason why the little flapper switch used for clocks is probably not a good choice for the gear safety switch.

[thinwing]

Both micro switches in the airspeed switch both go low (ground)when activated at 60 or so.Ive seen it used on AdsB installs requiring air/ground status

[larryb]

I have had a horrible time with this part in my aircraft. 

It started in my first annual. I have TKS, and part of that system is a heated stall switch. The stall switch heater is so powerful it will burn out if left on while on the ground, so they use the airspeed switch to keep it on "low" on the ground and "high" in the air. There are two switch circuits on the airspeed switch, one is generally un-used unless you have TKS. My stall switch heater would not go to "high" mode.

After the shop replaced it, things worked but half the time I would have to press the override button to get the gear up. The calibration was off and un-stable, and could not be suitably adjusted. My new switch went back for repair. Twice.

At the next annual, my shop informs me the switch has failed again. But this time the opposite way, the gear was not "safe." If I had moved the gear switch on the ground, it would have gone up. Another new switch.

Fortunately I only had to buy the first one at $1000. All the rest of them were under warranty.

Next time I'll be temped to design a circuit to decode RS-232 airspeed data from my Aspen and activate a relay. Could be done for less than $100 in parts and would be much more reliable. Too bad something like that could not be "FAA certified."

Larry