On Thursday, July 28, 2022 at 7:35:56 AM UTC-5, Mark Mocho wrote:
On Thursday, July 28, 2022 at 5:29:58 AM UTC-6, krasw wrote:
I have struggled with magnetic switches for years. Attaching them to gear/airbrake pushrods seems easy, but the tolerance between magnet and switch is very small making installation easily unreliable. Last time I did gear warning system I used
microswitches and everything works 100%. I would not bother with magnet switches anymore.
I too detest magnetic switches. Unless they are mechanically fixed in place with screws or clamping devices, the tape will not hold in a hot environment. If the switch moves out of the intended position even a small amount, the alarm either triggers or
worse, is disabled and you don't know it. Use microswitches and clamp them firmly in place. Test the circuit on the ground repeatedly. After reinstalling the seat pan or any other panel that covers the switches and actuating rod/lug/tab, test again.
Finally, test it in flight. Repeat the inflight test occasionally to make sure everything is operating normally.
Finally, consider something other than a warning horn. Many of these inexpensive (cheap) buzzers have a tone that is difficult to hear in the cockpit while in flight. Additionally, the pitch sometimes coincides with hearing loss in a particular
frequency range. (Fairly common in people who spent time around loud airplanes, firearms and other noisy environments. Many of our flight instruments like audio variometers, radios and anything else that makes noise might be enough to mask the buzzer. A
pilot in a stressful situation like an outlanding may just ignore another beep, honk or tweet. I changed my gear warning to a set of red/blue flashing LED lights in the panel. They hook up just like a buzzer would and only need a couple of 3/16 holes in
the panel. Since I don't have ANY other lights in the cockpit, there is no mistaking what is going on. Oh, and I installed a test switch to check whether they are working. And, yes, I did have a gear up landing once. But not with this system.
Mark,
I have to respectfully disagree with your opinion of magnetic (reed) switches versus mechanical micro-switches. I started installing gear warning systems in gliders by using micro-switches and quickly converted to the use of magnetic switches.
A few comments.
- I agree that the check buzzers are trash and should not be used. If you want a robust warning horn that will wake up the dead use the Sonalert brand (SC series). W&W's has that in their kit.
- I have seen too many examples (in gliders and otherwise) of lever activated micro-switches being bent, broken and/or destroyed. If a standard magnet isn't strong enough, use a rare earth one instead.
- Micro-switches requires a particular mounting arrangement where the activation "arm" must push on the lever from a specific direction. A magnetic switch allows for activation in a variety of orientations.
- I have often see micro-switches with elongated holes for the screws. Elongated holes are a recipe for movement caused by vibration.
- I suggested in my presentation of mounting the magnetic switch and magnet with tape but only during the trial fitting. I suggest, and use, a good brand of epoxy to permanently mount the magnetic switch and the magnet. West Systems is very good.
- Relying solely on warning lights to tell you of an impending gear up landing just doesn't seem right and can easily be not noticed, especially in a brightly lit cockpit. Airliners don't rely only on a warning light if important things aren't correct -
why should we?
I do need to add another slide talking about adding a test switch. But I prefer testing after rigging by having the gear up and the brakes out!
The latest version of the presentation can be found at
http://aviation.derosaweb.net/presentations/#gearwarning.
Thanks, John (OHM)
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