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italianjon
Joined: 05 Aug 2015
Posts: 68
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 Posted: Wed Jul 20, 2016 7:32 am Post subject: New Panel - Master/Ignition Switches |
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Hi All,
I am well into designing a new panel, that I will probably ultimately install over the winter months.
My thoughts are to place the Master Switch on a keyed 12V switch (One that does not allow the key to be removed in the "live" position); and then put the Left/Right Mags and "Alternator" Relay (i.e. Voltage Regulator disconnection relay) on to standard pole switches.
The starter circuit would go on a protected push button switch.
Through this process I am wondering why aircraft are wired the way they are. Is there any reason that the Master Switch is free to be switched on and the ignition is keyed?
. I can see the safety aspect of starting the engine, but in my proposal this would be covered, by 1) a keyed master switch, and 2) a protected push button switch.
I guess I am asking you all to sanity check the idea, because I am not sure of/can't see the negatives myself.
Regards
Jon
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zwakie
Joined: 03 Aug 2009
Posts: 157
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| Posted: Wed Jul 20, 2016 9:28 am Post subject: Re: New Panel - Master/Ignition Switches |
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Hi Jon,
I am currently going the same process as you, and have decided to go with two toggle switches for the ignition circuits (with toggle guards) and a push button for the starter circuit. Exactly what you are looking into, except for that I am not putting in a keyed switch (I can see the safety aspect of having one, but in my case that isn't much of an issue so I left it out).
I have done quite some research before deciding to go this way, and I could not find a single argument against this approach. I could find one benefit however: you will be able to crank the engine without firing the engine by leaving both ignition switches to the OFF position.
Also I figured that by using 3 switches, I will have not a single point of failure for the 3 functions involved with this aspect (this might carry little -if any- scientific significance, but at the very minimum gives me a little extra piece of mind...)
FYI: the basis of my new wiring schema is the Z16 figure from the AeroElectric Connection and the AeroElectric list here on Matronics. Tons of information can be found there, if you haven't already, please go and loose "some" hours absorbing all the knowledge available there.
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Marcel Zwakenberg
Europa XS TG || 912ULS || PH-SBR |
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Thu Jul 21, 2016 7:00 am Post subject: Re: New Panel - Master/Ignition Switches |
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Hi Jon,
Here are my thoughts regarding standard power and ignition switches.
First, recognize that you are talking about four separate circuits here:
1) Master Buss Contactor power
2) Alternator Field power
3) Magneto Grounds
4) Starter Contactor
In commercial light aircraft, these are usually grouped into two physical switch units. The standard keyed, rotary ignition (plus starter) switch is extremely robust and rugged and very unlikely to fail as befits a life-safety control. It is also quite compact in terms of panel space when compared with your proposal of replacing it with three separate switches, which if they are small will have minute contact areas compared with the big rotary. Small contact areas = short service life. The advantage to your proposal is possibly reduction in price and a minute advantage in weight. Just because Rotax draws the ignition circuit as two separate switches, does not imply that they do not favor standard aviation practice, as opposite to standard snowmobile practice. As for reliability, consider the relative reliability of three hobbyist grade switches for ignition/start vs a "bulletproof" AN spec'd part. In my Europa, I have a Bendix rotary plus a separate push button starter switch. The duty cycle on the start button is minute, but I have selected a part with can easily handle the 800 mA start Contactor load.
The two power contractors are usually controlled by single pole on-off switches which can vary in form factor, i.e., bat / toggle, rocker, etc. In my PA-22, there are no contactors (Mr Piper was cheap). In this case the Master Power switch is On-Off-On, in case one side of the switch fails. Back to the standard arrangement: It is usually thought to be convenient to semi-gang them since except for very trouble shooting they operate together.
For clarification, there is no such thing as a "standard pole" switch. The term pole refers to the number of circuits controlled by the switch , eg, one pole, double pole, 11-pole.
I hope this helps set out the design issues for this part of panel design.
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italianjon
Joined: 05 Aug 2015
Posts: 68
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| Posted: Thu Jul 21, 2016 7:47 am Post subject: Re: New Panel - Master/Ignition Switches |
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Hi guys,
Interesting food for thought.
So it sees that there is no formal reason not to do what I would like to do, but a fair technical point.
Panel space should not be a problem, as I am cleaning up the "small lower panel" by removing the currently installed engine instrumentation and that is going into a single EMS Xtreme from MGL on the centre panel.
But yes, the reliability is worth thinking about.
The main point that got me started down this "thought route" is why can we turn on our aircraft without a key, but you can't with the car... I started to think would it not be better to put the battery on the key and then everything else on simple toggle switches.
A third point that came up today in a discussion I had at work about this is one of human factors... if I make my aircraft too different that does introduce risk when I fly something else. (I sometimes rent the club aircraft, when I need more than 2 seats)
I am going to keep thinking on this one... some good points raised so far 
Regards
Jon
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rampil
Joined: 04 May 2007
Posts: 870
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| Posted: Sat Jul 23, 2016 2:19 am Post subject: Re: New Panel - Master/Ignition Switches |
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I think the key lock is prevent the aircraft from walking away without its owner. Turning on the radio with the keyless master buss will not steal the aircraft.
Interestingly, on topic, the WWII warbirds I have been in did not have keyed switches at all originally. I guess an armed guard was supposed to served the security purpose. From what I have heard more modern warbirds (F16, A10, UH-1Y, Cobra, etc.) maintain this tradition. Maybe Bud can expound on this, round the campfire this week at KOSH.
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