jaylink
Posts: 37
Joined: Wed Jun 07, 2006 2:06 am

Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 2:48 am

Does anyone know what the unusual sound is on a 717 that happens approximately one minute before the engines start? Perhaps it is a pump -- it sounds a little like a pump, or even a brief, muffled electric drill. It goes "wrrrt!" for about a second, then stops for 2 - 4 seconds, and then does it again a few more times. The cycle repeats maybe 5 times in total.

I mostly notice the sound towards the rear of the aircraft, right around push-back time.
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calpilot
Posts: 881
Joined: Sun Oct 17, 1999 5:16 am

RE: Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 12:23 pm

Your right, it's the hydraulic pumps.
 
n710ps
Posts: 1117
Joined: Tue Oct 10, 2006 7:09 am

RE: Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 12:36 pm

Electric hydraluc pumps pressureizeing the systems.
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hangarrat
Posts: 428
Joined: Wed Jul 06, 2005 12:24 pm

RE: Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 12:47 pm

There was a thread along these lines last year, with regard to the DC9/MD80. The sound was described as a whoop-whoop sound. The greatly revered and respected SlamClick opined that it was something to do with the ignitors charging, as discussed below.
RE: WHOOP-WHOOP Sound On Engine Start UP (by SlamClick Nov 22 2005 in Tech Ops)

Far as I am from arguing with a man who flew the DC9, the hydraulic PTU sound often noted on the A32X series sounds similar. I've noticed both the sound on start up on the DC9 and the MD80, and the "barking" sound on the A32x often heard during taxi and shutdown. I wonder if they have similar explanations.

HR
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FBU 4EVER!
Posts: 980
Joined: Sat Jan 27, 2001 6:53 am

RE: Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 5:03 pm

It is the electrical hydraulic pump.It is usually switched on before pushback to pressurize the right hydraulic system, which is needed to raise the aft stairway (not on the 717 as it doesn't have one!), and prssurize the brakes.The "barking",or groaning,sound results from the pump operating to top up the hydraulic pressure to the 3000 lbs.
The pump is almost identical on all DC-9's and it's developments.

The "whoop-whoop" sound referred to above,is the ignition capacitors recharging during engine start.It interferes with the PA-system.
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FBU 4EVER!
Posts: 980
Joined: Sat Jan 27, 2001 6:53 am

RE: Strange Sound On 717 Before Engine Start

Mon Jul 09, 2007 6:36 pm

Having re-read my reply above,I realize that in my jet-lagged state,I've forgot to mention the Power Transfer Unit.It is this one that gives the "barking" sound.It is powered by the electrical hyd. pump and charges the left hydraulic system until the left engine driven pump takes over after engine start.Sorry!
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skoker
Posts: 279
Joined: Sun Apr 09, 2006 5:49 am

RE: Strange Sound On 717 Before Engine Start

Tue Jul 17, 2007 8:51 am

Are you talking about the same sound heard at :25-:30 here?


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