liedetectors
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Joined: Thu Jul 21, 2005 10:44 am

Siren During DC-9 Engine Start

Thu Jun 06, 2013 1:11 am

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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MesaFlyGuy
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RE: Siren During DC-9 Engine Start

Thu Jun 06, 2013 1:58 am

The siren-like sound during startup comes from the high-energy igniters firing during the ignition phase of engine start.
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N243NW
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RE: Siren During DC-9 Engine Start

Thu Jun 06, 2013 3:04 am

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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AA737-823
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RE: Siren During DC-9 Engine Start

Fri Jun 07, 2013 7:23 pm

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...
 
Goldenshield
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RE: Siren During DC-9 Engine Start

Fri Jun 07, 2013 7:42 pm

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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liedetectors
Topic Author
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Joined: Thu Jul 21, 2005 10:44 am

RE: Siren During DC-9 Engine Start

Fri Jun 07, 2013 10:37 pm

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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