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Siren During DC-9 Engine Start  
User currently offlineliedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0
Posted (10 months 2 weeks 4 days 20 hours ago) and read 2398 times:

I flew on a pair of DL DC9-50s this past weekend. I've noticed it on previous flights....during engine start there is a distinctly noticeable siren horn sound during the engine start sequence. What is this?


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5 replies: All unread, jump to last
 
User currently offlinemesaflyguy From United States of America, joined Dec 2012, 2489 posts, RR: 4
Reply 1, posted (10 months 2 weeks 4 days 19 hours ago) and read 2367 times:
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The siren-like sound during startup comes from the high-energy igniters firing during the ignition phase of engine start.


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User currently offlineN243NW From United States of America, joined Jul 2003, 1609 posts, RR: 20
Reply 2, posted (10 months 2 weeks 4 days 18 hours ago) and read 2336 times:

Quoting mesaflyguy (Reply 1):
The siren-like sound during startup comes from the high-energy igniters firing during the ignition phase of engine start.

   It's the ignition circuit recharging between firing the ignitors, heard through the PA system because of wires in close proximity and poor electromagnetic shielding.



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User currently offlineAA737-823 From United States of America, joined Mar 2000, 5639 posts, RR: 11
Reply 3, posted (10 months 2 weeks 3 days 2 hours ago) and read 1917 times:

Indeed.
You can also hear the red flashing beacon light, but it's much more faint, and you have to be plugged in to the aircraft audio system to discern it.
But then, I have good hearing...


User currently onlineGoldenshield From United States of America, joined Jan 2001, 5845 posts, RR: 15
Reply 4, posted (10 months 2 weeks 3 days 2 hours ago) and read 1912 times:

Quoting N243NW (Reply 2):
It's the ignition circuit recharging between firing the ignitors, heard through the PA system because of wires in close proximity and poor electromagnetic shielding.


The sound you hear is that of an induction coil charging a capacitor. It's much like how you'd hear the flash charging in a camera. A little freaky, but cool at the same time.



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User currently offlineliedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0
Reply 5, posted (10 months 2 weeks 2 days 23 hours ago) and read 1857 times:

good info guys.

It almost seemed like a warning siren for the guys on the ground letting them know stand clear.



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