Stall Warning Buzzer

[Joachim Gruber]

Hello fellow Comanche owners and pilots.
I'm a relatively new owner of a 260B. As many of you probably know, the stall warning system in this plane consists of ONLY a flashing light on the panel. There's no audible alarm to warn you. According to the FAA, 25% of all single engine fatal accidents are caused by stalls. I assume that most of these occurred when the pilot was distracted, and probably in a high stress situation. I believe that an audible alarm, in addition to the flashing light, would make our type of aircraft much safer. Therefore, I've designed an electronic circuit that can simply replace the original stall warning circuit board (same size and connections) and add a loud audible alarm in addition to flashing the original bulb. The design is solid state (no moving parts such as relays). This also makes a mechanical relay failure impossible. The buzzer will sound even if the bulb is burned out or shorted, and the light will continue to flash if the buzzer burns out. I've tested the device on a breadboard and it works well. Once I have an actual prototype in hand (PCB has been ordered) I'd like to get together with a DER and hopefully apply for an STC. I'm throwing this out here to see if there's an interest (other than my own) in our Comanche community for such a device? Does anyone here know of a DER who'd probably be willing to work with me on this?
Thanks in advance.

[LeWayne Garrison]

I might be interested if approved and price is reasonable.

[Malcolm Dickinson]

I have a 260B, and am of the same opinion - the lack of an audible stall warning is a significant hazard, because at the critical moment during takeoff, approach, or landing, the pilot's attention is directed outside, not at the small yellow light on the panel.

I feel strongly enough about this that I purchased a used stall warning horn (yellow tagged unit that came from an Aztec) and asked my local mechanic to install it. He got back to me saying it was not possible because there was not enough voltage in the circuit to power it.

I spoke to another mechanic this weekend who said that a small piezo horn (a small black cylindrical buzzer found on many car backup beepers) is used in many other aircraft types, and due to its size draws very little current. He feels that would be a better and simpler solution.

Here are two examples of piezo horn:
http://thesensorconnection.com/egt-prob ... ezo-buzzer

https://www.alliedelec.com/rs-pro-7800709/70652796/

I would like to see as many Comanches as possible be outfitted with an audible stall warning. Let me know how I can support your project!

Malcolm Dickinson, CFI
N9284P - Connecticut

[Joachim Gruber]

Thanks for your response, LeWayne
Do you know if any single engine Comanches have audible stall warning systems? I think that they don't.
I'll post updates as I make progress (if any).

[Joachim Gruber]

Malcolm, thanks for your support.
I've emailed two DERs and haven't had a response yet. If you know of a DER who'd be willing to examine the circuit and prototype, please put me in touch with him/her.
The problem is that these buzzers are not FAA approved for the Comanche. My circuit also uses a piezo buzzer. It is very loud and emits a warbling sound easily distinguishable from any other cockpit sound/noise. It also sounds very different from the gear up buzzer. My plan is to have the FAA give its nod of approval via an STC. Then the entire thing can be done legally with the blessing of the FAA. The two links in your post are for single frequency buzzers. My circuit produces a variable frequency sound.

[Matt Bogard]

Minor alteration and owner-produced part, right? Install it in your plane for testing and publish the steps for anyone else who wants to "fabricate" a similar setup?

[Joachim Gruber]

Matt,
I'm not familiar with all the legalities regarding minor alteration and owner produced parts. Maybe someone familiar with the rules can help me out here. Today I built the prototype and recorded a small movie of it in "action". I'm trying to figure out how to upload the movie (or a link to the movie) here. Bottom line, it works well. I measured 110 Decibels at about one foot from the device (my ears are still ringing ) and it causes a regular GE 330 bulb to flash effortlessly.

[Joachim Gruber]

I think this may work as a link to the video I made. It's an MP4 file.

[Joachim Gruber]

After having read an AOPA article regarding major vs. minor alterations, I think that this invention could be considered a minor alteration because:

1) It doesn't affect weight and balance
2) It doesn't affect structural strength
3) It doesn't change flight characteristics
4) It doesn't affect operational characteristics

I would, however, need to have the design approved by the FAA, for it to qualify as an owner produced part.

Hopefully someone more familiar with the "law" (14 CFR 21.93 and 14 CFR Part 1.1) can chime in here and clarify.

[Matt Bogard]

[Joachim Gruber]

After having recently had a conversation with a DER I've been made aware that this would not be considered a minor alteration. This is mainly the case because I'd be replacing the existing stall warning PCB with a new product. Even though it may have the same function, it's still different. To get the FAA's blessing (STC) it would cost something between $15,000 to $40,000, he estimates.
Obviously, this price tag isn't going to make this a viable project, if we want this item to cost less than $200/piece. We could start a "fund me" campaign .

Now I'm looking into possibly only adding an audible siren, without replacing anything original. I believe that this could be legally and truthfully considered a minor alteration as long as it doesn't affect the airworthiness of the aircraft. The airworthiness claim can probably be made by isolating the device using a low amp (1A or less) fuse and installing it as a parallel circuit. This way any type of failure (short circuit or electric discontinuity) would not affect the existing electrical system. The siren draws less than 200mA. This is clearly an acceptable additional electrical load, in my opinion.

[Matt Bogard]

I'm still not sure what you're trying to accomplish. If you set this up in your plane as an owner-produced part and it's a minor alteration, all you need to do is make an entry in the logbooks (the A&P doing the work). The FAA, DER, etc etc etc don't need to know anything about it.

If you want to then sell this new "thing" as "FAA Approved," you need to apply for the STC and spend the bucks and have the headaches getting approval.

BUT, I don't believe anything keeps you from writing up a nice little "how-to" article for the Comanche Flyer about this new owner-produced part you put together and had installed in your Comanche as a minor alteration by your mechanic. You could even list the exact parts a person would need to build the same thing on their own.

I reference Matt Kurke's website: . He not only has detailed instructions on installing certain upgrades but he sells the kits of parts to make it simpler for owners and their mechanics. No STC needed. It's a minor alteration. Matt references a 337 he has but I don't believe even that is necessary because it doesn't affect any of the four items you mentioned above.

I'm no lawyer and I could be completely wrong on this, but I think a lot of mechanics and DERs and FSDOs have gone overboard in determining when something needs a 337 or STC or is a major alteration, and I think it's mainly a game of "cover your ass."