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Could This Sound Be The Apu?

Mon May 14, 2001 5:53 am

When I am on aircraft, the next pleasing sound (after the engine being first) is a usually described as a high pitched wine that is normally heard while the aircraft is at the gate while boarding and can be heard sometimes when taxiing. The sound on the MD-80 is a high whine, and the 727 is roughly the same, but some other aircraft have a lower whistle, and low hum. Now the question. Is this sound the APU? Is it the engines running idle? Is it air conditioning? I would be grateful for a concrete answer.

TurboTristar
 
Bryan Becker
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 6:03 am

well when i went on a trip to CT on the way back we were in a MD-80 we would taxi and there would be traffic and we would stop and the wine would stop and then when we started up again the wine would get louder so i would say it is the engines,but i never heard it while at the gate boarding all I could hear was the air conditioning but it was not a wine,and I could also hear the wining in the airbus 319 on the way to CT while taxing.soory if i was not that much of a help Smile

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apuneger
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 6:36 am

It's the APU running. I'm 90% sure of this. I worked as a student on EBBR's (Brussels) ramp, loading and unloading aircraft, and I could hear the APU's running all the time. I find it a nice sound, although it's a pretty high wining, that's true.

Normally we had to wear ear protection, but I sure didn't, unless I had to be really close to the APU, eg passing underneath the aircraft or walking to the back of it.

Ivan
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bio15
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 6:54 am

Yes, it's the APU. Engines are never running idle at the gate (except prior to power pushbacks - using reverse thrust). If you hear it while boarding the plane, it's the APU. Engines sound less whiny and 'roar' a bit more. But once taxiing, the elec. power source is engine generators, so the APU is OFF and you'll be hearing the engines. On some aircraft you can notice the spool-down sound from APU shutdown.

I recall that once a 727's APU was started while I was boarding the plane from the back, and it was a deafening loud whine!!  Smile

-bio
 
AJ
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 8:47 am

The higher pitched whine heard from the front of the aircraft is most likely the air conditioning packs, generally located in the belly of the aircraft. The APU is more audible at the rear of the aircraft, and has the associated rush of exhaust air as well.
 
EssentialPowr
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 11:02 am

Bio15

Many a/c take off with the APU running for additional redundancy (ex - the ERJ).

The APU on the 727, however, is not approved for use in flight, so it's shut down prior to takeoff.

I concur w/AJ regarding the whine. A/C packs cycle off and on while at the gate depending upon cabin temp loads; as a turbine/compressor assembly they make a noticable whine when spun up and during operation.

An APU runs at constant speed (stable electrical frequency is a function of generator speed) so after startup, the noise footprint is fairly constant.
 
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 11:33 am

Thank you guys, now I know! The APU is the small inlet at the tailcone of the planes like on the 747/757/767/777? Is the purpose just to power electric sources?
 
EssentialPowr
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 11:50 am

Typically the APU exhaust is in or on the side of the tailcone on Boeing products (except the 727 - it's over the starboard wing root), near the #2 engine on the DC9 series, and near the nose gear on a P3.

APUs are typically an electric and pneumatic source for the a/c.
 
bio15
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 12:51 pm

I didn't know some aircraft fly with the APU running! It's interesting since it reduces APU life. But if it's done, there must be a good reason. Thanks EssentialPowr for the info.

-bio
 
AerLingus
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RE: Could This Sound Be The Apu?

Mon May 14, 2001 1:51 pm

I have heard this on the 737 and the MD-80 series. This is the APU. The engines, as a rule are shut down at the gate until the tug pushes the jet back for startup or the plane begins a powerback. The pitch is different depending on airplane and soundproofing. On the MD-82 and 734, I have noticed that the whine is lower.
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JT-8D
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RE: Could This Sound Be The Apu?

Tue May 15, 2001 12:14 am

The whining could also be a electric hyd pump. They can be pretty loud. By the way, the apu may also be left running in flight in the event of a generator failure. The MEL states that only one generator can be inoperative. I believe this applies to most A/C, at least all the types we operate. If a generator is inop, The apu gen can power the other bus. On 737, the apu may be used for an air source, for a "bleeds off T/O". As stated, the 727 apu is for grd use only.
 
Ratzz
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RE: Could This Sound Be The Apu?

Tue May 15, 2001 10:22 am

It´s the APU....normally it´s shut down upon completion of the engine start sequence,since the engines supply cabinpressure as well as the electricity..
 
Guest

RE: Could This Sound Be The Apu?

Wed May 16, 2001 1:42 am

Just to clarify the APU use on the CRJ (which may be like the ERJ), the APU is used on Takeoffs, all of which are done bleeds off. It is shut down immediately after transferring the bleeds to the engines, which happens about a minute or so after takeoff.
The APU is restarted passing through 10,000 feet on the way down, and the landing (in case of a go-around) is done bleeds off.
 
VC-10
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RE: Could This Sound Be The Apu?

Wed May 16, 2001 2:28 am

EssentialPowr

The AC packs do not cycle on and off, they run at a constant speed. Temp control is acheived by the amount of hot that is mixed with the cold air comeing out of the air cycle machines.
 
AeroGlobeAir7
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RE: Could This Sound Be The Apu?

Wed May 16, 2001 11:54 am

The whine your hearing is likely the engines. The APU must be running before the engines are started, because it is needed to provide electrical power for the engine startup, which occurs only after push back (unless the plane does a powerback with its thrust-reversers). I was going to suggest the flaps, but the taxiing part ruled that out, as most pilots set the takeoff flaps just before gate pushback. Hope I helped!

Andrew
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EssentialPowr
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RE: Could This Sound Be The Apu?

Thu May 17, 2001 2:39 am

VC10

I concur with your statement. By cycling off and on, I meant exactly that; not steady state ops or air mix valve theory.

As you know, during an engine start, the packs are isolated. As a shutdown and startup, that scenario constitutes a cycle. Also, the #1 APU on the erj is typically selected off during boarding b/c it is extremely loud, and then cycled on after the pax are on and the door is closed. If it is extremely cold or hot, they typically stay on.
 
EssentialPowr
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RE: Could This Sound Be The Apu?

Thu May 17, 2001 2:52 am

One other thing re: constant speed of an AC pack.

At altitude, there is no requirement for cold air, so the compressor turbine assembly is completely bypassed, and is therefore relatively static, even though the packs are selected "on". On the ground in Dallas on a 100F day, the speed is considerably higher...One of the limiting factors in a pack overload condition is the rotor speed.

The rotational speed of the compressor/turbine assembly does vary slightly according to demand, just like a turbo on an automotive engine. In a marcoscopic view, they can be considered "steady state."
 
AJ
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RE: Could This Sound Be The Apu?

Thu May 17, 2001 11:53 am

These poor packs that work so hard are also being accused of heating up centre wing tanks to the degree of flash point, ie TWA B747-100, PAL B737-300, Thai B737-400!
Hydraulics aren't usually pressurised on the B744 until almost ready for pushback, and certainly once refuelling is complete. The controls are checked prior to pushback, but the flaps aren't run until all engines are running.
Cheers.  Smile/happy/getting dizzy

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