QANTAS747-438 From United States of America, joined Jan 2001, 2036 posts, RR: 2 Posted (11 years 2 weeks 5 days 11 hours ago) and read 22023 times:
I was at United today at LAX, and I noticed that an odd event occurs only at United with the A320s and A319s. This sound never happens at NW, US, or AW. The aircraft pulls up to the gate with one engine running prior to installing the power umbilical. The power is connected and the #1 engine is finally shutdown. As it does, it makes a "barking" sound, as if a large dog were in the cargo hold. It does it in pairs: woof-woof......woof-woof.....woof-woof.... The sound gets lower and lower as the engine spools down. Even as the engine is finishing spooling down, the sound finally gives up in a dreadful "woof-woof... woooooof-wooooooooooof" The pace of the sound isn't fast, but exactly resembles a large dog. Also, it doesn't sound like it's coming from the engine, nor does it sound like an engine noise. It sounds like it is coming from the belly. Any ideas?
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
Michi From Singapore, joined Jul 2004, 199 posts, RR: 8
Reply 4, posted (11 years 2 weeks 5 days 5 hours ago) and read 21783 times:
it is the HYD PTU (Hydraulic Power Transfer Unit). It comes on automatically when the differential pressure between the green and the yellow system is greater than 500 PSI (normal system pressure is 3000 PSI). This normally happens, when you start or shut down an engine. The PTU is working only for a few seconds or even less than that, because it stops when the pressure in the associated system is at 3000PSI again (that's the "barking" sound). And there are not to many hydraulic functions needed while shutting down an engine. So the pressure is maintained easily.
There are a few more parameters that affect PTU automatic operation. But that would lead us to far off topic.
Delta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 6
Reply 7, posted (11 years 2 weeks 3 days 1 hour ago) and read 21473 times:
The PTU is indeed the correct answer.
I will shed a bit more light on why the pitch changes as the engine spools down.
As Michi stated above, the PTU comes on when the pressure in the other system drops. The PTU runs just long enough to repressurize the system ... a second or two, then stops. During the next few seconds, the system depressurizes slowly due to "internal leakage" in the hydraulic system. When the difference in pressure agan reaches 500 psi, the PTU cycles.
As the engine spools down, the flow from the engine driven pump available to drive the PTU is reduced, therefore the PTU speed and power transfer to the second system is also reduced. Thus, it runs slower, and takes a bit longer to repressurize the second system - this explains the lower pitch. When the engine stops, the PTU no longer receives flow from the engine pump, so it, too, "dies".
I would be glad to answer any questions on the PTU or other hydraulic components. I worked on the Vickers PTU which is used on some of the A320 series.
Boeing4ever From , joined Dec 1969, posts, RR:
Reply 11, posted (11 years 1 week 4 days 13 hours ago) and read 21109 times:
Why is it that nearly every other thread on this forum is always about another strange noise coming specifically from the A32X series? Are those aircraft possessed or something? Hell, I'd be worried to if the cockpit instruments suddenly started speaking in tongues!
Yokohama1970 From United States of America, joined Jul 2004, 199 posts, RR: 2
Reply 12, posted (11 years 5 days 7 hours ago) and read 20956 times:
quite common. our A319/320's usually taxi-out on engine 2. during taxi-out, they start, turn & ignite the engine 1. here @ AWA, our pilots usually turn engine 2, during pushback & engine 1 during taxi-out.
as posted above, the PTU is hydraulically controlled & required to enable engine starts. next time you are boarding any A319/320, you will also hear the outward-opening, hydraulically controlled cargo doors closing or opening after arrival @ the gate.
the A319/A320 with the IAE V2500 series engines have a very dramatic & distinct sound when starting! Once the engine igniters have been turned off, the engines have a very high-pitched whine, but yet quiet. the idle also audibly increases.
in the US: AWA, jetBlue & United use the IAE V2500 series engines. all the other carriers use the CFM-56-5 series engines.
however, the composite airframe produces that unique high to low pitched "Dolby" effect, during approach. i figured that out here in PHX, by observing all the various A318/319/320 & 321's for carriers on ILS Arrival for Runway 8. my apartment is 5 miles north of PHX & 2 miles south of the final approach pattern. so, the planes are usually @ 6000ft.
Same thing in PIT, my folk's house is on ILS Arrival for Runway 32. they are about 10 miles out. all the US A319/320/321's & UA 319/320's have the same high to low pitch as they pass overhead. i like to show off for my parents! when we are sitting on the deck outside & I announce "airbus", without even looking up.
Klaus From Germany, joined Jul 2001, 21592 posts, RR: 53
Reply 13, posted (11 years 5 days 7 hours ago) and read 20953 times:
Yokohama1970: however, the composite airframe produces that unique high to low pitched "Dolby" effect, during approach.
The A380 is the first Airbus with at least partial use of composites (GLARE) on the actual fuselage structure, as far as I know. All other Airbus models have aluminium fuselages with only a few external components being composite (fairings, stabilizer, ...)
So I´m rather certain that differences in sound would rather be due to different choice of engines / nacelle structures and different wing and flap design.
HorizonGirl From Canada, joined Mar 2005, 808 posts, RR: 15
Reply 14, posted (10 years 4 months 6 days 11 hours ago) and read 20031 times:
Quoting QANTAS747-438 (Thread starter): "barking" sound, as if a large dog were in the cargo hold. It does it in pairs: woof-woof......woof-woof.....woof-woof.... The sound gets lower and lower as the engine spools down. Even as the engine is finishing spooling down, the sound finally gives up in a dreadful "woof-woof... woooooof-wooooooooooof"
I too observed this pattern on an air Canada flight.
Maybe it always woofs in threes?
During taxiing, it did the opposite as you described.
It started with a "Wooooo-ooooof-Wooooooooo-oooooof."
It then got lower and started in with the "Woof-woof......woof's."
When we parked, it the PTU made a stange "RHOAIYUUUUA."
and continued woofing in threes.
Dose anyone know why?
Skywatch From United States of America, joined Mar 2005, 923 posts, RR: 5
Reply 15, posted (10 years 4 months 6 days 4 hours ago) and read 19974 times:
Quoting Yokohama1970 (Reply 12): AWA, jetBlue & United use the IAE V2500 series engines. all the other carriers use the CFM-56-5 series engines.
I am not around Airbuses a whole lot, so I am curious: Do A319's equipped with IAE engines sound better than the CFM's? I hear Fronties's A319, and it sounds high-pitched and whiny. I was wondering if the IAE engines roar. Please note that this is about engines, not Airbus!
Read my profile?
Do you know what I'm trying to do?
It is so hard. Sometimes I don't get even any results!
I have even tried recording it myself during a taxi, but was
asked to put it away.
On my return flight, I managed to hide it, but it came out
with just a strange buzz that sounded nothing like it. Darn.
Anyone have proper recording equipment?
HorizonGirl From Canada, joined Mar 2005, 808 posts, RR: 15
Reply 22, posted (10 years 4 months 1 day 15 hours ago) and read 19554 times:
Quoting SlamClick (Reply 16): You are the first person on here who could spell that sound correctly. Are you sure you haven't been to Airbus school?
Lol, no I haven't. Maybe there should put that word in the textbooks?
But I still don't know exactly why it barks.
I started a thread on it. As you probably know, it didn't
go to well. So I'll ask here where there are more
people. I am sorry if I might be breaking a rule.