BR715-A1-30
Topic Author
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Sound Of JT8D... What Is The "Whir"? And NW DC9

Thu Sep 20, 2007 4:39 pm

Hello Everyone,

The reason I am making this post is...

The other day, I flew on a DC9-30, and DC9-50. These were my first times sitting in the back of the DC-9, and when I heard the engines start, about 22 seconds into starting, there was a constant "whir-whir-whir-whir-whir-whir-whir-whir" sound. I've been told it may have been a hydraulic pump, but if you listen here
http://myspacetv.com/index.cfm?fusea...n=vids.individual&videoid=18342006

You can hear it.. It can't be too hard. Can ANYONE give me a definitive answer on what this is.


My second question is this.

Does ANYONE know why NW Pilots start #2 engine on the DC-9 first? Of all the Boeing 717s, I've flown on (basically a twin with more fuel efficient engines, and upgraded avionics) Engine #1 was started first, and I thought that the #1 engine supplied all of the electrical and hydraulics to start #2?????????

Please help me figure this stuff out.

Thanks
Puhdiddle
 
Viscount724
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Thu Sep 20, 2007 5:31 pm

Quoting BR715-A1-30 (Thread starter):
My second question is this.

Does ANYONE know why NW Pilots start #2 engine on the DC-9 first? Of all the Boeing 717s, I've flown on (basically a twin with more fuel efficient engines, and upgraded avionics) Engine #1 was started first, and I thought that the #1 engine supplied all of the electrical and hydraulics to start #2?

Not sure how this may have changed recently, but for decades it was traditional to start #2 first on twin-engine aircraft, and on 4-engine aircraft to start them in the order 3-4-2-1. That was true both for piston and turboprop types and all the earlier jets. I think it's still generally done that way but not sure about some of the newer aircraft. I recall reading that the B777 can start both engines simultaneously but don't know if that's done in practice.
 
fr8mech
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Thu Sep 20, 2007 8:01 pm

Quoting BR715-A1-30 (Thread starter):
"whir-whir-whir-whir-whir-whir-whir-whir"

That sound is the sound of the ignition system bleeding over the speakers. Pretty common on the older aircraft.
When seconds count...the police are minutes away.
 
tdscanuck
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Fri Sep 21, 2007 1:16 am

Quoting BR715-A1-30 (Thread starter):
Engine #1 was started first, and I thought that the #1 engine supplied all of the electrical and hydraulics to start #2?????????

It's primarily supplying pneumatics to start #2 (you already had electrical power from somewhere to begin the start at all). And it can, but doesn't have to. The APU can do it, or you can start #2 off the APU and use that to start #1.

Tom.
 
boeingfixer
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC

Fri Sep 21, 2007 1:22 am

Quoting Fr8Mech (Reply 2):
Quoting BR715-A1-30 (Thread starter):
"whir-whir-whir-whir-whir-whir-whir-whir"

That sound is the sound of the ignition system bleeding over the speakers. Pretty common on the older aircraft.

That's exactly what it is.

Quoting BR715-A1-30 (Thread starter):
Does ANYONE know why NW Pilots start #2 engine on the DC-9 first? Of all the Boeing 717s, I've flown on (basically a twin with more fuel efficient engines, and upgraded avionics) Engine #1 was started first, and I thought that the #1 engine supplied all of the electrical and hydraulics to start #2?????????

First off, you don't need hydraulics to start an engine on the DC-9 or 717. On the DC-9, MD-80/90 and 717 you can even do a battery start as long as you have a bleed air supply. There are some basic reasons as to why certain engines are started first. On the DC-9, the #2 is started first mainly due to the way the hydraulic system is designed. While on the ground with the engines not running, an Aux. hyd. pump supplies pressure to the Right Hyd. System. Therefore the #2 engine is started first to reduce the time the Aux. Pump is needed. The second reason is that as the #2 engine is started, there is an oil pressure switch that increases the gain of the cabin audio amplifier.

As another example of differences to the above, on the 747, the #4 engine is started first due to the layout of the hydraulic system as well.

Cheers,

John
Cheers, John YYC
 
tlfd29
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Sat Sep 22, 2007 1:00 am

I thought another reason for starting #2 first on twins was because typically the main doors are on the opposite side so if there is an engine failure/fire during start up it would be easier for passengers to exit the aircraft safely. Obviously there are other reasons with hydraulic systems and what not, but I thought this was a factor as well in the start up procedure.
 
boeingfixer
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Sat Sep 22, 2007 6:39 am

Quoting Tlfd29 (Reply 5):
I thought another reason for starting #2 first on twins was because typically the main doors are on the opposite side so if there is an engine failure/fire during start up it would be easier for passengers to exit the aircraft safely. Obviously there are other reasons with hydraulic systems and what not, but I thought this was a factor as well in the start up procedure.

That's not really a factor in starting the engines as airliners are equipped with exits and slides on both sides of the aircraft. Aircraft evacuation is based upon conditions outside the aircraft and if it's prudent to use exits on the R/H side of the aircraft it doesn't matter which engine is started first. Notable exceptions are the smaller commuter jets and turbo-props which only have smaller emergency exits on the R/H side of the aircraft.

Cheers,

John
Cheers, John YYC
 
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HAWK21M
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Sat Sep 22, 2007 7:48 pm

Quoting Tlfd29 (Reply 5):
Obviously there are other reasons with hydraulic systems and what not

Shouldn't Engine Fire/Hot start during start up be a reason,since #1 & APU have the same Duct.loosing APU will deprive Pneumatic pressure to the Start valve for motoring.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
pilotpip
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Sun Sep 23, 2007 4:54 am

We start the number 2 on the 145 first because of the hydraulic system. The steering is supplied by hydraulic 1 which is supplied by engine 1. We don't want the pump kicking on while our ground crew is near the nosewheel. That could get ugly.

Most starts are done with the APU. This is one of the reasons that the packs turn off and you loose your A/C for a few seconds. To do a crossbleed start (at least on the ERJ) you need the engine doing the engine doing the work to be at a high power setting to provide enough bleed air to turn the other engine.
DMI
 
SPREE34
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Thu Sep 27, 2007 6:45 am

Quoting BoeingFixer (Reply 4):
That's exactly what it is.

Very high voltage spark plugs. In some GA aircraft strobe lights were doing this as well. Igniters and strobes are both high voltage discharge systems. Some of the electronic noise created migrates into the electrical system and is heard in headsets etc. Most everything now has filtering to keep the spurious noise out of the audio systems.

I miss the 400hz hum.
I don't understand everything I don't know about this.
 
twal1011727
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RE: Sound Of JT8D... What Is The "Whir"? And NW DC9

Thu Sep 27, 2007 7:12 am

Quoting BoeingFixer (Reply 4):
On the DC-9, the #2 is started first mainly due to the way the hydraulic system is designed.

Not on Delta DC9 or MD88s... They always started the #1 1st then #2. Spirit/USair/Continental too.

It mainly depends on company policy or capts discretion

KD

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