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Risky Jump Starts?

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Wed Apr 09, 2008 5:18 am Post subject: Risky Jump Starts?

Greetings,

  

 Someone posted on the rec.aviation.piloting newsgroup a story about damage  caused by jumpstarting the airplane with the alternator switch on.   According to the story, the jump start was from an FBO tug capable of "much more  juice" than the owner's pickup truck that he had successfully used in the past,  and as soon as he turned the key, "every breaker in the plane tripped."   Sensors for fuel quantity, oil temp, ammeter, and oil pressure strip "got fried  and needed to be rebuilt."

  

 I'm having trouble understanding how getting a jump start with the  alternator switch turned on could cause this problem.  Even if it were a 12  volt system and the FBO tug produced 24 volts, I don't see why it would trip the  circuit breakers.

  

 Any ideas?

  

 Dennis Johnson

 Legacy, now flying   

  

  

    [quote][b]

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mprather(at)spro.net
Guest

Posted: Wed Apr 09, 2008 6:09 am Post subject: Risky Jump Starts?

I'm with you..  I don't see how leaving the alternator turned off would

have helped.



OWT  People love to dramatize events.



Hooking 24V to a 12V system can blow breakers on some circuits - like

lighting.  And can damage some items.  Doubling the V, doubles the I -

when a circuit isn't regulated to constant P (which is true for many

things).

Matt-



 	  Quote: 	
		   Greetings,



 Someone posted on the rec.aviation.piloting newsgroup a story about damage

 caused by jumpstarting the airplane with the alternator switch on.

 According to the story, the jump start was from an FBO tug capable of

 "much more juice" than the owner's pickup truck that he had successfully

 used in the past, and as soon as he turned the key, "every breaker in the

 plane tripped."  Sensors for fuel quantity, oil temp, ammeter, and oil

 pressure strip "got fried and needed to be rebuilt."



 I'm having trouble understanding how getting a jump start with the

 alternator switch turned on could cause this problem.  Even if it were a

 12 volt system and the FBO tug produced 24 volts, I don't see why it would

 trip the circuit breakers.



 Any ideas?



 Dennis Johnson

 Legacy, now flying

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Apr 09, 2008 7:10 am Post subject: Risky Jump Starts?

At 06:14 AM 4/9/2008 -0700, you wrote:



 	  Quote: 	
		  Greetings,



Someone posted on the rec.aviation.piloting newsgroup a story about damage 

caused by jumpstarting the airplane with the alternator switch 

on.  According to the story, the jump start was from an FBO tug capable of 

"much more juice" than the owner's pickup truck that he had successfully 

used in the past, and as soon as he turned the key, "every breaker in the 

plane tripped."  Sensors for fuel quantity, oil temp, ammeter, and oil 

pressure strip "got fried and needed to be rebuilt."



I'm having trouble understanding how getting a jump start with the 

alternator switch turned on could cause this problem.  Even if it were a 

12 volt system and the FBO tug produced 24 volts, I don't see why it would 

trip the circuit breakers.


	


   24 volts of normal polarity would not trip all the breakers . . .

   however 24 volts of REVERSED polarity might.  Ground power carts

   are typically capable of 1000 to 2000 amp currents. A set of

   paralleled-series diodes in an alternator would attempt to hold

   off voltage rise of a reversed polarity . . . but the forward

   conduction drop on a highly stressed diode is 1 volt or more. So

   in milliseconds, the bus voltage goes minus to the tune of 2 volts

   or more . . . at MANY amps. The battery would also attempt to stand

   of the event . . . but we don't know if the battery was even on

   line for this scenario. But if the battery were dead, it wouldn't

   be much help.



   The alternator breaker or b-lead fuse would open first and

   probably before the alternator was damaged. As soon as the

   breaker opens, then the bus voltage goes WAAAaayyyy negative

   and really sad things start to happen . . . which may well include

   the opening of a few more breakers.



   I've had line-folks put 24v of the RIGHT polarity to my

   14v airplane and that was scary enough. I've addressed this

   possibility in the designs for ground power jacks in your

   OBAM aircraft project. I've always recommended (1) ov protection,

   (2) reverse polarity protection and (3) giving the pilot

   absolute control of when power is applied to the ship's

   systems. See:



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



   The outcome of this story would not have been altered by

   leaving the alternator switch OFF. I used to contribute to

   the discussions on Usenet groups but had to give up after

   discovering that many folks who hang out there are not

   interested in (or perhaps capable of) understanding

   the simple-ideas behind the physics that controls behavior

   of their electrical systems.



         Bob . . .

        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Apr 09, 2008 7:14 am Post subject: Risky Jump Starts?

At 06:14 AM 4/9/2008 -0700, you wrote:



 	  Quote: 	
		  Greetings,



Someone posted on the rec.aviation.piloting newsgroup a story about damage 

caused by jumpstarting the airplane with the alternator switch 

on.  According to the story, the jump start was from an FBO tug capable of 

"much more juice" than the owner's pickup truck that he had successfully 

used in the past, and as soon as he turned the key, "every breaker in the 

plane tripped."  Sensors for fuel quantity, oil temp, ammeter, and oil 

pressure strip "got fried and needed to be rebuilt."



I'm having trouble understanding how getting a jump start with the 

alternator switch turned on could cause this problem.  Even if it were a 

12 volt system and the FBO tug produced 24 volts, I don't see why it would 

trip the circuit breakers.


	


   24 volts of normal polarity would not trip all the breakers . . .

   however 24 volts of REVERSED polarity might.  Ground power carts

   are typically capable of 1000 to 2000 amp currents. A set of

   paralleled-series diodes in an alternator would attempt to hold

   off voltage rise of a reversed polarity . . . but the forward

   conduction drop on a highly stressed diode is 1 volt or more. So

   in milliseconds, the bus voltage goes minus to the tune of 2 volts

   or more . . . at MANY amps. The battery would also attempt to stand

   of the event . . . but we don't know if the battery was even on

   line for this scenario. But if the battery were dead, it wouldn't

   be much help.



   (UPON FURTHER REFLECTION, THE SCENARIO DESCRIBED MAY NOT HAVE

   INCLUDED REVERSED POLARITY . . . THIS IS AN EXCEEDINGLY RARE

   EVENT THAT IS USUALLY LIMITED TO RECENTLY SERVICED BATTERY

   CARTS THAT HAVE BEEN IMPROPERLY ASSEMBLED. THIS PROBABLY WAS

   A SIMPLE OVER VOLTAGE EVENT SUPPORTED BY A 1000+ AMP CURRENT

   SOURCE. YOU THINK AN OV EVENT FROM A 60A ALTERNATOR IS BAD,

   TRY A 1000A GROUND POWER CART!)



   The alternator breaker or b-lead fuse would open first and

   probably before the alternator was damaged. As soon as the

   breaker opens, then the bus voltage goes WAAAaayyyy negative

   and really sad things start to happen . . . which may well include

   the opening of a few more breakers.



   I've had line-folks put 24v of the RIGHT polarity to my

   14v airplane and that was scary enough. I've addressed this

   possibility in the designs for ground power jacks in your

   OBAM aircraft project. I've always recommended (1) ov protection,

   (2) reverse polarity protection and (3) giving the pilot

   absolute control of when power is applied to the ship's

   systems. See:



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



   The outcome of this story would not have been altered by

   leaving the alternator switch OFF. I used to contribute to

   the discussions on Usenet groups but had to give up after

   discovering that many folks who hang out there are not

   interested in (or perhaps capable of) understanding

   the simple-ideas behind the physics that controls behavior

   of their electrical systems.



         Bob . . .

        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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