safe flight stall horn (mis)wiring?

GeorgeC

GeorgeC

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GeorgeC
My plane has a Safe Flight electric stall horn.

At last my annual, I squawked that the stall horn was inop. Shop said it worked for them. Harrumph.

Fast forward six months or so, I'm out practicing some stalls, and sure enough, the horn sounds. Hooray!

After I shut down, I'm curious, so I flip the master back on and go diddle the switch on the wing, and there's no horn.

Hmm, undo the last thing you did before shutdown... turn the alternator field switch back on, and the horn works.

Presumably the alt field circuit runs from the regulator (plane power), through the field breaker, and back out to the alternator.

I assume that whomever installed the horn just powered it off the field circuit because it was convenient.

Is that bad?
 
GeorgeC said:
I assume that whomever installed the horn just powered it off the field circuit because it was convenient.
Click to expand...
Or that was how the installation instructions showed how to wire it? If you don't have those available, I believe Safe Flight is still around so perhaps reach out to them for input?
 
So if you have a charging system fault your stall horn quits by design? I wonder what DER would put a -9 on such a design.
 
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MauleSkinner said:
but I’m sure the same principal applies.
Click to expand...
Maybe. But would need to see how the original drawing showed the install to figure out what it would take to alter the wiring. Cant see how the alt field circuit would be any more convenient than the main buss.

The link you have is for an AoA system and is newer than the stall system. Regardless Safe Flight should be able to give George what he nedds to confirm the install.
 
So, just thinking here... the main(battery) bus might be divided from the alternator output with a diode. Maybe the stall horn got hooked up to the alternator output side of the circuit and turning on the field either allows power to backfeed or is energizing a solenoid that bypasses the diode somehow?

I can't imagine why anyone would connect the stall horn to the field... accidentally maybe but even that seems like a stretch to me.
 
I had a similar screw up with the safeflight device used for my Navion gear horn.
 
Alt/Gen field is typically an electrical control input from voltage regulator to the Alt. I think it's very unlikely it was anyone's informed intention to put an electrical load on it.

It's probably it's a mistake.

There are what ever drawings you have for the aircraft electrical system, there is how the plane is actually wired and there are the modification/rework instructions. When a modification is accomplished you want finished installation that is functionally identical to the intentions of the mod package.

I've seen mod packages that tell people where to make a connection that don't make it clear what the entire circuit should look like or exactly which wire to connect to. IMO a good mod package should always include post installation power off continuity checks and power on power/ground checks to make sure you only see power on wires you want it on, before connecting a device.

You can probably get away with minimal mod instruction that assume a lot while an aircraft is still on the assembly line under tight configuration control. On old airplanes you really need to make sure to check all assumptions.

I recommend you get someone that knows what they are doing to figure it all out. My personal rule for my airplane is that if I took the time to slither down in there to figure out the mess, I sketch it out and make a cad DWG for my notes.
 
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cowman said:
So, just thinking here... the main(battery) bus might be divided from the alternator output with a diode.
Click to expand...
I've never encounered any diode between the alternator and bus. The alternator already has diodes in it that not only rectify the AC generated in it, but also prevent any reverse flow through the alternator.

Someone just made a mistake. Like any other electric stall warning system, it needs to be connected to the bus through a breaker. Here's how Cessna does it:

1728666666403.png
 
Just tossing out notional requirements off the top of my head:
- you want it continuously powered day/night/VFR/IFR
- wire size will determine breaker rating that is appropriate (see 43-131B ch 11)
- continuous total load on selected breaker shouldn't be more than 80% circuit breaker rating.
I don't think POH schematics contain enough detail to really understand the wiring for any particular circuit. Chances are, OEM provided more wiring detail, by system, in the airplane maint manual or wiring diagrams and if it's an old airplane it's probably not 100% accurate.
 
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kontiki said:
I don't think POH schematics contain enough detail to really understand the wiring for any particular circuit. Chances are, OEM provided more wiring detail, by system, in the airplane maint manual or wiring diagrams and if it's an old airplane it's probably not 100% accurate.
Click to expand...
I posted that diagram because the free service manuals that used to be on the 'net until recently have been taken down. I suppose Cessna exerted their copyrights. In any case, the schematic from the service manual would be the same. The manual just breaks down the system into individual circuits rather than showing the overview as the POH does. Note that the POH doesn't show any symbols for the loads such as radios or lights or pitot heater, so the grounds for those loads don't show either. Nor does the POH diagram give the specifics for wire size and connectors and so on, all stuff the manual gives.
 
Mine has a light but not a sound, I was wondering if I could hook a speaker up to the light. What’s the point of the light without the sound?
 
GeorgeC said:
My plane has a Safe Flight electric stall horn.

At last my annual, I squawked that the stall horn was inop. Shop said it worked for them. Harrumph.

Fast forward six months or so, I'm out practicing some stalls, and sure enough, the horn sounds. Hooray!

After I shut down, I'm curious, so I flip the master back on and go diddle the switch on the wing, and there's no horn.

Hmm, undo the last thing you did before shutdown... turn the alternator field switch back on, and the horn works.

Presumably the alt field circuit runs from the regulator (plane power), through the field breaker, and back out to the alternator.

I assume that whomever installed the horn just powered it off the field circuit because it was convenient.

Is that bad?
Click to expand...
Well, the stall horn should function without being dependent on the alternator field power. I would change that when convenient. Main DC bus is appropriate, very low power draw on the stall horn
 
mandm said:
Mine has a light but not a sound, I was wondering if I could hook a speaker up to the light. What’s the point of the light without the sound?
Click to expand...
It took them a while to figure out you're not really staring at the panel during VFR maneuvers.

Yes, you can get a horn in parallel with the light.
You can buy the expensive one "approved" for aviation use, or you can work with a good A&P that understands a minor alteration and is willing to sign off a $5 piezo buzzer wired in parallel with the light. Add a resistor in series with the buzzer, thst way if the buzzer fails it won't impact the existing light.
That satisfies the "or other factors affecting airworthiness" part of the rule.
 
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