Yesturday we went from Denver to Chicago on Frontier Airlines and we heard on the ground these noises that sounded like a saw going back and forth only when we were on the ground or Taxiing.
What is that?
Please give a Detailed Explanation.
Thanks
It was in the middle of the plane.
here is another supplment to Sul’s info
http://www.airliners.net/aviation-forums/tech_ops/read.main/93654/
skimming through the posts i copy&pasted three important ones
this one is from Michael
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.
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.
and another by Pete
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.
another signed tom
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.
click link at start of answer for the original source site , where all the info, plus some speculation and witness accounts as well as a little humor about Dash-8s farting and speculation that 32x series are possessed