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Strange Sound On 717 Before Engine Start  
User currently offlineJaylink From United States of America, joined Jun 2006, 41 posts, RR: 0
Posted (7 years 5 months 1 week 6 days 20 hours ago) and read 2837 times:

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.

6 replies: All unread, jump to last
 
User currently offlineCALPilot From United States of America, joined Oct 1999, 999 posts, RR: 13
Reply 1, posted (7 years 5 months 1 week 6 days 10 hours ago) and read 2745 times:

Your right, it's the hydraulic pumps.

User currently offlineN710PS From United States of America, joined Oct 2006, 1166 posts, RR: 3
Reply 2, posted (7 years 5 months 1 week 6 days 10 hours ago) and read 2740 times:

Electric hydraluc pumps pressureizeing the systems.


There is plenty of room for Gods animals, right next to the mashed potatoes!
User currently offlineHangarRat From United States of America, joined Jul 2005, 633 posts, RR: 0
Reply 3, posted (7 years 5 months 1 week 6 days 10 hours ago) and read 2735 times:

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



Spell check is a false dog
User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 4, posted (7 years 5 months 1 week 6 days 6 hours ago) and read 2695 times:

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.



"Luck and superstition wins all the time"!
User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 5, posted (7 years 5 months 1 week 6 days 4 hours ago) and read 2680 times:

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!


"Luck and superstition wins all the time"!
User currently offlineSkoker From United States of America, joined Apr 2006, 440 posts, RR: 1
Reply 6, posted (7 years 5 months 5 days 14 hours ago) and read 2381 times:

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




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