F

Flying Scotsman

Well Known Member
I'm starting to wire up all the switches and whatnot on the panel, and doing my schematics (btw, ExpressSCH seems to work well for a free tool :) ).

I'm using the Aveo Rockrack switches which are internally lit by LEDs. Nice!

So here's my question...I have a keyswitch which must be ON in order for the mags to be ungrounded. No key in the switch, can't turn the system on, no way to get a hot mag. I also figured out how to wire a pushbutton starter switch (also internally lit with an LED :) ) so that during start, the non-impulse-coupled mag is grounded during starting.

So...I can have the L and R mag switches "light up" only when the keyswitch is on and the mag is in the ON position, or I can have them light up *whenever* the mag switch is in the ON position, as a sort of visual "HEY! YOUR MAGS ARE ON!" warning (even though they shouldn't be ungrounded due to the keyswitch safety interlock).

Hot mags scare me...I think I'd like the idea of a RED light on a mag switch whenever it's in the ON position, but maybe that's confusing?

Which do you think is the better option?
 
Flying Scotsman said:
....So here's my question...I have a keyswitch which must be ON in order for the mags to be ungrounded. No key in the switch, can't turn the system on, no way to get a hot mag....
Click to expand...

Have you created a single point failure that would affect both mags at the keyswitch?
 
As with Gil, the whole concept of what you are doing frightens me :eek:

Yes - a hot mag on the ground is not desirable, but discipline / training should keep it safe (a standard Cessna style switch is as good as any, and ensure the keys are "out" when cockpuit unoccupied). No mags in the air is definitely even less desirable / far more hazardous...

It's difficult to do at this time, but think resale value as well. Keep it (relatively) conventional.

Andy
 
My opinion is to do away with the keyswitch entirely. I got rid of mine because I hated having a key fob flopping around on the panel while flying. And personally I would want Green lighting for anything that is "on," regardless of whether that means the mag is "ungrounded."

If you want an external indicator that the mags are "off," wire up a simple LED flasher that pulses once every 5 seconds and hook it up to come on when the mags are "off." Then put the LED on the glare shield where you can see it during your walk around.
 
I've been harping at guys for years: When you stop, pull the keys and throw them on the glareshield. When someone approaches the plane to hook up a tug or towbar etc. they look at the glareshield. Keys= warm-fuzzy. No keys, hair stands up on your neck.
 
A single point failure...you mean like if the keyswitch suddenly mysteriously went from the "On" position to the "Off" position? Why would that make it any different than this part:

11-03169s.jpg


Basically, I'm doing *exactly the same thing*, but just with a keyed switch and two toggles, instead of everything all in one.

So let me take these in order, and then maybe someone will address the original question?

Failure: Yes, but it's no more of a single-point failure than the switch above. I'm not worried about that, as the worst failure other than the key turning itself in flight (which it is not wont to do, as this is a pretty sturdy keyswitch with very positive detents) would be a broken lead or contact, which would fail the mag *open*, not grounded.

Cessna style...correct me if I'm wrong, but all the Cessnas I've flown had the switch style shown above, which as I've said, would have the same potential for failure.

Getting rid of it...not an option, in my mind. A simple toggle switch which makes the mags hot is not safe, IMHO. A quick search here will find all sorts of horror stories of people at airshows and fly-ins dicking around with peoples' cockpits, even so far as to let their kids sit in the planes and diddle the switches. All it takes is ONE idiot like that to set up a potentially deadly situation. No thanks, I'll take something a bit more positive when it comes to safety.

Lastly, whatever color light it is is irrelevant, and I don't want to goober up the cockpit with blinky lights and whatnot. A flashing LED for hot mags is most certainly NOT any kind of standard.

I agree with aerhed, but not everyone around an aircraft is an experienced ground crew.

So...let me try this again. There will be a keyswitch, sorry if that worries anyone, but that's my decision. There won't be additional flashing lights and buzzers and stuff...the lighted toggles are already capable of being lit, why add more lights. The wiring I have is safe, and failure modes are such that in-flight loss of mags is no more of a probability than with the "standard" ignition switch.

Back to the original question, and maybe I won't get an answer, and I'll just make a decision...mag indicator lights (in the switch) on regardless, or only on when the interlock is "armed"?
 
I designed my panel so that a light means "Something may be wrong here, check it out.". It's less error prone than a system where sometimes a light means good, sometimes a light means bad as well as quicker to scan.
 
are the LEDs in those switches wired to the battery bus..in other words they are independent of your master switch otherwise the master switch will need to be on as well. You can have a hot mag with the master off and not see those lights if they are not connected directly to the battery - right?

I think the lights are useless to anyone careless enough to pull a prop around without checking the status of the mag switches since they will not be seen from the prop arc.

It would be better to wire up a really loud bullhorn that yells, "Mags are hot, mags are hot" :) be sure to buy lots of spare batteries though...
 
Steve,
I have just the part you need! Master ignition switch and individual magneto controls!

IMG_0114.jpg

Just my .02, but as one previous poster has already stated, keep it simple and develop a habit pattern of when the key switch goes off, the keys come out and go to the same place each time (in my case, hanging from the DG knob).

However, as one member's signature states...."Build the plane you want...."
Good luck!
 
Flying Scotsman said:
......Back to the original question, and maybe I won't get an answer, and I'll just make a decision...mag indicator lights (in the switch) on regardless, or only on when the interlock is "armed"?
Click to expand...

Here's the quick answer....NO. Why? Because all you manage to do is have an indicator that tells you the position of the switch, which you obviously can tell visually just by looking at it. It does little to tell you if the mag is hot or not. Also, if it's a light that you intend to only have on when the engine is not running (who wants to continuously look at a warning light during the normal phases of flight) how do you intend to tell the light to only illuminate when the engine is stopped or you are on the ground?

Also just a quick comment on switches (and mind you this is not personal at all, only my opinion)...just because it's a "key switch" looks like a key switch doesn't mean the internals of all switches are created equal and one shouldn't assume as much. Aircraft mag switches are actually fairly robust (look at their size) and aren't exactly identical to other simple "on/off" type switches. Perhaps you found one that is similar, but I don't know for sure as I've not seen it's guts so I very well could be speaking out of school on that.

I'm not trying to argue, but remember we should always design stuff primarily for the 99th percentile, not the 1th percentile. In some cases when you endeavor to increase safety you actually have a net negative effect on reliability. I'm not saying this is 100% the case, but some of what you're doing and the stated reasoning behind it does give me pause.

Just my blunt 2 cents as usual.

Cheers,
Stein
 
BE CAREFUL.....

aerhed said:
I've been harping at guys for years: When you stop, pull the keys and throw them on the glareshield. When someone approaches the plane to hook up a tug or towbar etc. they look at the glareshield. Keys= warm-fuzzy. No keys, hair stands up on your neck.
Click to expand...

using this method only. I have seen worn ignition switches that would allow the key to be removed in the "left" position.
I witnessed a Swift one time start with a touch of the prop.
The key was OUT of the switch. The aircraft had been shut down with the mixture and been sitting for several days.

You just never know!
 
That had to be a bad switch Mel. I've been droning along in old planes and had the key just fall out on the floor. Thats not supposed to happen either. I think best defense is to not have a heavy keychain and maybe change the switch each half century.
 
Everyone is concentrating on the reliability of the switch, but I bet in reality that the reliability of the wiring is more in question. How many folks have seen poor wiring on P-leads?

As the wiring scheme gets more complicated, any failure in the P-leads will usually create a hot mag.

Keep it simple with only 4 wire "joints" per magneto and follow standard practice.
 
It's like a gun!

Everyone should treat a prop like it is a loaded gun. If you move it, it could fire!

A switch in the "UP" position is a good indicator and the one I use on my aircraft.

Also, this is one advantage of having dual P-mags. If the master is off OR the ignition switch is off, the chances of having an ignition fire is almost astronomical. For them to fire, the switch has to be on (and I have a three position switch) AND the master has to be on.

Still, I treat the prop with great care.
 
Still overlooked, but still applicable are Bendix AD 76-07-12 and Gerdes/ACS AD 93-05-06 for switches with start function. Everyone should do the Bendix check periodically to ensure consistent mag grounding. Quite a few switches will go past off & leave a mag hot. Others will have inconsistent point contact (You wiggle the key in the off position and get ignition). Like Mel was saying, if they get worn the key can pull when beyond or not quite off.
 
SteinAir said:
Here's the quick answer....NO. Why? Because all you manage to do is have an indicator that tells you the position of the switch, which you obviously can tell visually just by looking at it. It does little to tell you if the mag is hot or not. Also, if it's a light that you intend to only have on when the engine is not running (who wants to continuously look at a warning light during the normal phases of flight) how do you intend to tell the light to only illuminate when the engine is stopped or you are on the ground?

Also just a quick comment on switches (and mind you this is not personal at all, only my opinion)...just because it's a "key switch" looks like a key switch doesn't mean the internals of all switches are created equal and one shouldn't assume as much. Aircraft mag switches are actually fairly robust (look at their size) and aren't exactly identical to other simple "on/off" type switches. Perhaps you found one that is similar, but I don't know for sure as I've not seen it's guts so I very well could be speaking out of school on that.

I'm not trying to argue, but remember we should always design stuff primarily for the 99th percentile, not the 1th percentile. In some cases when you endeavor to increase safety you actually have a net negative effect on reliability. I'm not saying this is 100% the case, but some of what you're doing and the stated reasoning behind it does give me pause.

Just my blunt 2 cents as usual.

Cheers,
Stein
Click to expand...

Stein...Thanks for an actual answer to the original question :). After thinking about it, I believe you are correct in your analysis. RV8R999 also made a good point about the necessity of wiring the LED to the hot buss. So...decision is: Mag switch lights ON iff Keyswitch is On and Mag is On (i.e., the only condition under which the Mag should normally be "hot").

Second, it's good, solid industrial grade keyswitch, rated appropriately, so I'm not worried about it failing. I'm pretty sure it will be at least as reliable as the ignition switches I've seen on a lot of Cessnas and Pipers (yes, I've seen ones where you could take the key out in positions other than Off, or that any key from any plane could be used to turn it, etc.).

Third, the p-leads themselves have only one additional wire on one lead (the R, to ground it during starting regardless of position).

Lastly, for the gazillion concerns about keys falling out and whatnot...IF the whole keyswitch blew apart in mid-flight and all the connections were lost...guess what. My right mag (the non-impulse-coupled one) would just keep right on working, never having been connected to anything other than its own Mag switch. For that matter, so would the left one, because that would be a fail "On" scenario.

I'd post the schematic, but I get the feeling the discussion would go even further afield. :)
 
Mel said:
using this method only. I have seen worn ignition switches that would allow the key to be removed in the "left" position.
I witnessed a Swift one time start with a touch of the prop.
The key was OUT of the switch. The aircraft had been shut down with the mixture and been sitting for several days.

You just never know!
Click to expand...

Yes, this happened to me with my 1976 Piper Warrior two years before the AD came out on the switches.

The key was turned to the left to remove. But the switch could catch all the way to the left and not spring back....... that would make the mag HOT and the key was removed.

Aircraft key switches are a joke. They came along with the yoke to make the airplane more like a car so the public could relate. Most can be operated with a tool box key. And the wire connections and contacts are no better.

If you want to be safe with your mag switches, mount them in a recessed hole and have a locking cover that you can install when you require increased security. The cover could only be installed if the switches were in the off position. A simple hex wrench latch would work fine. Or you could use a cabinet lock. The switches would be out of sight and locked down.

I think I would mount the starter button in there too.

Then if you really wanted a key switch, use one for your master.
 
aerhed said:
Still overlooked, but still applicable are Bendix AD 76-07-12 and Gerdes/ACS AD 93-05-06 for switches with start function. Everyone should do the Bendix check periodically to ensure consistent mag grounding. Quite a few switches will go past off & leave a mag hot. Others will have inconsistent point contact (You wiggle the key in the off position and get ignition). Like Mel was saying, if they get worn the key can pull when beyond or not quite off.
Click to expand...

Forgive the "thread drift", but if your mag switch fails the test specified by AD 76-07-12, the entire switch does not need to be replaced. Bendix makes a kit to repair it for a lot less money. It is a very simple installation.
 
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