Gear/stall warning alarm

Timcote1960 · #1 08-27-23, 08:05 PM

I have periodically had trouble with this errant alarm that sounds both as a stall warning and as a gear up alert. Years ago, the damn thing went off constantly whenever I backed off to half throttle, around MP= 18”. My AP fixed it with a simple adjustment on the console.

Later, I found it going off whenever I flew into precipitation. Frankly, the blaring horn doesn’t do much for my scan in IMC, but I manage.

Today was my least enjoyable experience with this wonderful gizmo—just after takeoff it decided to sound and wouldn’t stop. I entered a cloud layer on an IFR plan and, to complicate matters, my trusty STEC autopilot suddenly decided it wasn’t in the mood to keep on the heading bug’s heading. So yes, I had to hand fly through IMC for about an hour with a blaring horn until I could finally glance over to include the breaker panel in my scan and find the proper circuit to pull.

Did I mention that my wife did not enjoy this flight one bit? While I knew and told her we were totally safe, the insistent alarm had the compelling argument.

I’m thinking this time maybe it’s the stall warning switch due to the recent precipitation-induced alarm and my preflight inspection of the rusty vane tha, while it t showed free movement, looks every bit of its 47 years age.

What think?

Tim

Timcote1960 · #2 08-27-23, 08:41 PM

I see from prior threads that RTV102 applies to throttle body switches is the lost likely fix. I see many others have had this and there was even a service bulletin about it (though I cannot find said bulletin, ME77-24). Just ordered rtv102; if anybody has pics of exactly where this ought be applied to prevent precipitation induced horn blaring, I sure would appreciate it.

Kim Geyer · #3 08-28-23, 11:14 PM

I may have one of the switches that Cessna provided with the service kit for that SB. I’ll see if I can find it in my stuff. They used epoxy instead of RTV. I’ll post a pic when I find it

Kim Geyer · #4 08-29-23, 04:14 PM

I just listed the switch I had on eBay P/N 1570412-1
It’s got the epoxy

n86121 · #5 09-03-23, 03:16 PM

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On top of the throttle for each engine is a little microswitch.

As I recall, the gear horn goes off if either throttle goes back near idle and ger NOT down.

I believe they both have to be closed.

Annoying little buggers.

Easy to check w cover off and a meter.

They are a common microswitch.

Aka 'digikey' common.

Any shop on any field should be easy to fix / adjust or replace

B2C2 · #6 09-25-23, 01:36 PM

The problem is that the circuit that looks at the microswitch has a pretty high resistance threshold, like 400 ohms or so to detect closure of the microswitch. Moisture on the switch body between the two contacts can cause the circuit to leak to ground and think the switch is closed. It doesn't help that the screws holding the switch to the throttle body are grounded and quite close to the terminals. Closed is the same as idle, so when you raise the gear with a leaky switch the horn goes off since it thinks you are at idle with gear up. You can test for this on the ground by advancing the throttles with the engines off and seeing if the horn test that you get by pressing the gear up indicator light stops buzzing once you advance the throttles. If it doesn't you have a leaky path in the circuit. I have this happen periodically. Usually flying for a bit dries out the switch and fixes the problem, but the RTV is a good idea. Ill try that.

B2C2 · #7 10-04-23, 12:24 PM

One other thing, once you pull the breaker that will shut off the noise, you cannot raise the gear. I'm sure its a safety feature Cessna put in there to prevent gear up landings if the breaker is pulled or popped. So if this happens you have to raise the gear, endure the noise for a few seconds then pull the breaker. On my last flight when this happened I did this, then about 1/2 hour later I reset the breaker and no noise as the switch had dried out.