CMH From United States of America, joined Jul 2010, 3 posts, RR: 0 Posted (5 years 10 months 1 week 3 days 22 hours ago) and read 6394 times:
I've wondered for years what this noise is - Embraer RJ-145, and just after coming into ORD yesterday, I hear the captain shut down one engine during taxi, and then I hear a hairdryer-like noise somewhere in the belly, but it changed pitch up and down quite a bit - think of a hairdryer with some massive voltage fluctuation. I thought it might be the APU, but it was clearly coming from underneath instead of the back, and an APU wouldn't change pitch so wildly. All of the flaps and slats were already returned, so it wasn't that either. Ideas?
western727 From United States of America, joined Jan 2007, 885 posts, RR: 4
Reply 5, posted (5 years 10 months 1 week 1 day 1 hour ago) and read 5994 times:
Sorry to go somewhat OT but I've noticed a high-pitch noise in the MD-80 (incl. the -88) and the DC-10 while at the gate, though the MD-80's seemed to be of a higher pitch. Electric hyd pump? Thanks in advance.
jetdoctor From United Kingdom, joined Aug 2001, 260 posts, RR: 0
Reply 8, posted (5 years 10 months 6 days 20 hours ago) and read 5787 times:
Indeed it is the #2 hydraulic pump, and the fluctuation in sound is the demand on the pump, either being the nose wheel steering or brake operation.
I personally find the noise very annoying, and often wonder if the fuel savings from single engine taxi outway the additional hours on an electric backup pump. #2 pumps are always going bad and have to be replaced.
Break ground, and head into the wind. Don't break wind and head into the ground.
ThePinnacleKid From United States of America, joined Feb 2005, 748 posts, RR: 7
Reply 9, posted (5 years 10 months 6 days 19 hours ago) and read 5779 times:
Quoting pilotpip (Reply 7): The captain shut down the #2 engine. The result is the electric driven hydraulic pump for the #2 system turning on to power the nose wheel steering. Quite annoying isn't it?
Ummm... actually, that should be the #1 that was shut down... Most people that commute will single engine taxi on #1 because it is quieter in the back for the people because the engine driven hyd. pump would be running and not the elec. pump under the cabin... it is the #1 hyd. system that controls the nose wheel steering and also the culprit with the noise around mid cabin on the 145...
That said, a lot of guys like to single engine taxi on #2, and #2 is started first (during pushback) because when the engine is turning and the eng. driven hyd. pump is running it does NOT power the steering, there by allowing for simultaneous pushback operations... unlike starting #1 which must be done AFTER pushback... when on a towbar type push (non-electro)
N243NW From United States of America, joined Jul 2003, 1658 posts, RR: 19
Reply 12, posted (5 years 10 months 6 days 7 hours ago) and read 5650 times:
ThePinnacleKid has it spot-on. I've heard the noise many times from the rear of the 145 series, and often I'll try to book a seat farther forward simply to avoid it. Sometimes even a couple rows can make a huge difference.
And yes, it appears that the protocol with many pilots is to taxi out on #2 to enable engine start during pushback and to taxi in on #1 to reduce passenger annoyance.