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Avionics Master switch

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tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Mon Aug 23, 2010 2:45 am Post subject: Avionics Master switch

Before everyone yawns and says, "Not another question about avionics master switches", I'll ask you to indulge me.



Suppose I had a "friend", who was paranoid about blowing-up his $10,000 WAAS gps, and $20,000 glass panel, and hadn't read Bob's epistles about fault tolerance specs, etc.  But this "friend" didn't want the single point failure of an avionics master switch.  Suppose also, that he had a progressive master switch (Off - Batt only/alternator field off - On/alternator field on).  Why couldn't this "friend" shut down by turning off the master switch before shutting down the engine?  Is this the bugaboo that the non-Aeroelectric crowd fears?  ie. Is the sudden collapse of the alternator field the culprit everyone fears, or is it the limbo-like state of the alternator as the engine rpm winds down after pulling the mixture they fear?  Would this procedure raise more concerns than it eased?



I once had an instructor that shut off the alternator field and looked for a hiccup on the ammeter as part of a pre-takeoff check (avionics on at this point).  This was the same instructor that "enlightened" me about the reason for shutting off all avionics before starting and before shutting down the engine.

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_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Aug 23, 2010 8:38 pm Post subject: Avionics Master switch

At 05:45 AM 8/23/2010, you wrote:

 	  Quote: 	
		  

<Tom(at)CostanzaAndAssociates.com>



Before everyone yawns and says, "Not another question about avionics 

master switches", I'll ask you to indulge me.



Suppose I had a "friend", who was paranoid about blowing-up his 

$10,000 WAAS gps, and $20,000 glass panel, and hadn't read Bob's 

epistles about fault tolerance specs, etc.  But this "friend" didn't 

want the single point failure of an avionics master switch.  Suppose 

also, that he had a progressive master switch (Off - Batt 

only/alternator field off - On/alternator field on).  Why couldn't 

this "friend" shut down by turning off the master switch before 

shutting down the engine?  Is this the bugaboo that the 

non-Aeroelectric crowd fears?  ie. Is the sudden collapse of the 

alternator field the culprit everyone fears, or is it the limbo-like 

state of the alternator as the engine rpm winds down after pulling 

the mixture they fear?  Would this procedure raise more concerns than it eased?


	


    No, the #1 "radio killer" is supposed to be the starter.

    Have you read?



http://aeroelectric.com/articles/avmaster.pdf



    If one really wants an Avionics Master, it's

    easily added in series with the normal feed-path

    diode without major impact to failure mode effects

    analysis.



    There are but four energy sources that the designer

    needs to consider when configuring the electrical

    system.



    (1) Battery: Limited total energy but capable

    of great current due to low internal impedance.



    (2) Engine Driven Power: Usually an alternator

    capable of great total energy but current limited

    due magnetics and internal impedances.



    (3) Direct Lightning Strikes: Most light aircraft

    designers don't strive for resistance to lightning

    strokes. Lots of current, arcing, but again, not

    as much energy in the stroke as your alternator puts

    out in twenty minutes or so or the battery can deliver

    in a complete dumping of charge.



    (4) All other reactive sources are low energy,

    short duration events, routinely trapped off as

    a matter of design and qualification.



    Risks from battery and alternator faults have

    been easily managed by proper wire sizes, fuse/breaker

    sizes, ov-protection on the alternator. Legacy

    design goals call for designing to expect, withstand

    and make graceful recovery from events that lie within

    this envelope



http://www.aeroelectric.com/Pictures/Curves/MSTD704_28V_Trans.jpg



    I can tell you that starting the engine does not

    exceed this recommendation.



    We also limit abnormal events to this envelope



http://www.aeroelectric.com/Pictures/Curves/MSTD704_28V_OV.jpg



     . . . which is no big deal.

 	  Quote: 	
		  I once had an instructor that shut off the alternator field and 

looked for a hiccup on the ammeter as part of a pre-takeoff check 

(avionics on at this point).  This was the same instructor that 

"enlightened" me about the reason for shutting off all avionics 

before starting and before shutting down the engine.


	


    He was simply repeating the mantra handed down to

    him by his teachers. I have gone on spike hunting

    expeditions on airplanes from C-150 to BeechJets

    and failed to see anything that exceeded the design

    goal limits . . . in fact, aside from starter inrush

    brownout . . .



http://www.aeroelectric.com/Pictures/Curves/99_Saturn_SL1_2.gif



http://www.aeroelectric.com/Pictures/Curves/99_Saturn_SL1.jpg



http://www.aeroelectric.com/Pictures/Curves/95_GMC_Safari_3.gif



http://www.aeroelectric.com/Pictures/Curves/95_GMC_Safari_1.gif



http://www.aeroelectric.com/Pictures/Curves/turbine_start_a.jpg



    The rest of the things you see on the bus are exceedingly

    un-exciting.



http://www.aeroelectric.com/Pictures/Curves/Safari_Bus_Noise_1.gif



http://www.aeroelectric.com/Pictures/Curves/Safari_Bus_Noise_2.gif



    Bob . . .

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