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Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 5:53 am
by convair880mfan
There is a high pitched noise during this video. It sounds to me as though the APU was operating during taxi, takeoff and climb. Am I wrong? Is it just compressor noise from the JT8Ds?

https://youtu.be/dtx3Hfdc05Q

Don't know if the Boeing 727 APU can be operated in the air. Thanks.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 10:28 am
by Aaron747
The videographer is sitting right over the wing so that must be the hydraulic system. Heard that whine in that location on every 727 flight I can remember.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 3:17 pm
by 113312
I don't disagree with Aaron 747 but the sound in that video is also a result of the audio limitations of the camera used. It is not linear frequency response and enhances certain frequencies. The sound cannot be from an APU in a 727 because it is shutdown after engine start. It may not be operated in flight unlike the 737 and 747. In the 727, the APU is located inside the wheel well and would quickly overheat if running and the landing gear is retracted. You would immediately get a fire warning after takeoff. All of the sound in that video are from the engines and hydraulic pumps (B system mainly)

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 4:05 pm
by tb727
Those are the packs you can hear in that location as well as the pack cooling fans. The pack cooling fans turn off automatically after the flaps are retracted.

The APU exhaust is right outside that window, exiting out the top of the right hand wing right near the fuselage. The APU is not able to be operated in the air and is shut down usually after the 3rd engine is started on the 727. Hydraulic A system pumps are located on the engines and the B system is electric and in the tailcone to the left as you exit down the rear airstairs. The B system pumps are very high pitched and loud and probably a big part of why I can't hear anything! You can't really hear them though once the aft door is closed.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 5:49 pm
by fr8mech
[twoid][/twoid]
tb727 wrote:
Those are the packs you can hear in that location as well as the pack cooling fans. The pack cooling fans turn off automatically after the flaps are retracted.


I was thinking pack fans also. The hydraulic pumps tend to whine and change pitch a little bit, even under steady load.

Though, didn’t the B727 takeoff with the packs off? Can’t remember.

tb727 wrote:
B system is electric and in the tailcone


The B system pumps are behind the left wheel well. The standby rudder pump is in the airstair compartment. I’m thinking aircraft left.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 9:39 pm
by RetiredWeasel
fr8mech wrote:
[twoid][/twoid]
tb727 wrote:
Those are the packs you can hear in that location as well as the pack cooling fans. The pack cooling fans turn off automatically after the flaps are retracted.


I was thinking pack fans also. The hydraulic pumps tend to whine and change pitch a little bit, even under steady load.

Though, didn’t the B727 takeoff with the packs off? Can’t remember.

tb727 wrote:
B system is electric and in the tailcone


The B system pumps are behind the left wheel well. The standby rudder pump is in the airstair compartment. I’m thinking aircraft left.


At the airline I worked for, 727 packs were turned off for takeoff. Turned on after flaps were retracted. Edit: Actually the more I think about it, we turned the packs back on at 1000' before flaps were fully retracted. Same procedure on the 747 classic.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Mon Nov 15, 2021 10:27 pm
by rjsampson
tb727 wrote:
Those are the packs you can hear in that location as well as the pack cooling fans. The pack cooling fans turn off automatically after the flaps are retracted.


I'm guessing the fans don't contribute much too much to the noise, as when the flaps are fully retracted in OP's video, the sound persists.

Presumably different companies had different SOPs with regard to packs on or off during takeoff?

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Wed Nov 17, 2021 12:54 am
by tb727
fr8mech wrote:
[twoid][/twoid]
tb727 wrote:
Those are the packs you can hear in that location as well as the pack cooling fans. The pack cooling fans turn off automatically after the flaps are retracted.


I was thinking pack fans also. The hydraulic pumps tend to whine and change pitch a little bit, even under steady load.

Though, didn’t the B727 takeoff with the packs off? Can’t remember.

tb727 wrote:
B system is electric and in the tailcone


The B system pumps are behind the left wheel well. The standby rudder pump is in the airstair compartment. I’m thinking aircraft left.


Hmm I must be misremembering the location then!

I only did a couple of packs off takeoffs. We generally didn't have a performace problem or we had the horses on board and they needed the airflow.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Wed Nov 17, 2021 1:53 am
by fr8mech
tb727 wrote:

Hmm I must be misremembering the location then!



I have decided that the brain has limited capacity and once that capacity has been reached, new things bump out old things. The catch: it's not a first-in, first-out process. The brain randomly kicks out data to accommodate the new data. I completely expect that some new chunk of data from the -8 will kick out any memory of my children's names anytime now.

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Wed Nov 17, 2021 5:46 pm
by M564038
The sound sounds fine. It is not overloaded and it doesn’t dip in level when boises are made. Background voices sounds as expected. I would say it is a pretty good representation of what was actually heard in that one spot.

113312 wrote:
I don't disagree with Aaron 747 but the sound in that video is also a result of the audio limitations of the camera used. It is not linear frequency response and enhances certain frequencies. The sound cannot be from an APU in a 727 because it is shutdown after engine start. It may not be operated in flight unlike the 737 and 747. In the 727, the APU is located inside the wheel well and would quickly overheat if running and the landing gear is retracted. You would immediately get a fire warning after takeoff. All of the sound in that video are from the engines and hydraulic pumps (B system mainly)

Re: Is this sound from the APU compressor or the main powerplant compressor?

Posted: Wed Nov 17, 2021 6:45 pm
by tb727
fr8mech wrote:
tb727 wrote:

Hmm I must be misremembering the location then!



I have decided that the brain has limited capacity and once that capacity has been reached, new things bump out old things. The catch: it's not a first-in, first-out process. The brain randomly kicks out data to accommodate the new data. I completely expect that some new chunk of data from the -8 will kick out any memory of my children's names anytime now.


I was asking my dad this morning about what was back there I could be thinking of. He was a mech at NW and worked on the 727 for over 20 years. He remembered where the B pumps were, told me about the reservoirs, to watch out for the pack fans and he even remembered my name so he is way better off than me!