ultrapig
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777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 3:56 am

I was at Logan yesterday-taking an AA MD-80 from B-33. I was there about 1.5 hours before departure time. Next door at Gate B-32 is a T7 which I assumed was eventually going to LHR. I noted that both of the engines were going at about 30 rpm-and continued to do so until I enplanned next door.

Help me-I assume the engines were running to generate electricity to run the airconditioning-but isn't this normally done using ground based service? It was hot-would the grounded service not be sufficient to cool the plane? But would two engines be needed and if my guess is correct why cool the plane at 3:30 for a 7 pm flight?

Even at idle these engines must use a significant amount and chalk up several hours operating time.

I'm sure one of you will tell me what I am missing.
 
graphic
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 3:58 am

30 RPM is effectively 1 rotation every 2 seconds, the rotors were probably just windmilling, as even at idle power, the fans would appear to be a blur.
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FriendlySkies
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 3:58 am

Just for clarification...did you hear them or just see the fans blowing? A gas turbine cannot operate at 30 rpm.
 
ultrapig
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:08 am

At first I assumed they were spinning down after engine shut off. But they kept spinning at 30 rpm (I timed them using the "squiggles".) It did not occur to me that the wind was spining them- in fact I don't think there was much wind-You gentlemen may know better but I thought it would take a pretty strong win to spin a turbine and the spinning seemed pretty consistent.
 
unattendedbag
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:14 am

Quoting Ultrapig (Reply 3):
I thought it would take a pretty strong win to spin a turbine and the spinning seemed pretty consistent.

It takes a good 10-15 knot wind to spin the blades. They are heavy, but they are also free spinning and those fans can easily catch air if the wind is blowing in the right direction. At engine idle, you would never had been able to count the revolutions per minute.
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flyf15
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:16 am

Quoting Ultrapig (Reply 3):
At first I assumed they were spinning down after engine shut off. But they kept spinning at 30 rpm (I timed them using the "squiggles".) It did not occur to me that the wind was spining them- in fact I don't think there was much wind-You gentlemen may know better but I thought it would take a pretty strong win to spin a turbine and the spinning seemed pretty consistent.

Turbofans have extremely good lubrication and bearings. It doesn't take much at all to spin them and this happens fairly commonly.

Yesterday I was at an airport that had winds gusting to 30kts and we were parked directly into the wind. Those fans were spinning very fast just windmilling.... it was showing almost 3% N1!
 
unattendedbag
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:16 am

Quoting Ultrapig (Reply 3):
But they kept spinning at 30 rpm (I timed them using the "squiggles".)

I am not familiar with that measuring method.
Slower traffic, keep right
 
ultrapig
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:25 am

Quoting UnattendedBag (Reply 6):
am not familiar with that measuring method.

It was invented by me yesterday-Counting the number of turns of the squiggles marking on the engine hub per minute.

I'm still confused-I'm not doubting anyoen that the wind can spin the turbines but the wind was pretty slow yesterday.
 
EMBQA
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:33 am

Quoting Ultrapig (Reply 7):
I'm still confused

Just shut your window fan off and watch the blades... they'll start spinning for the reason. It's just the wind passing through the fan case. Very-very-very-very common.....

The blades are also very efficient... I hand spun a CF34-10E once and I could fell the air around me getting sucked in. It doesn't take much to start the blades moving.

[Edited 2007-08-02 21:36:12]
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darkblue
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 4:50 am

Quoting Ultrapig (Reply 7):
I'm still confused-I'm not doubting anyoen that the wind can spin the turbines but the wind was pretty slow yesterday.

It doesn't take much. I assume it was an AA 777, which means it had Trent800 engines. The 110in diameter fan (66sqft) does a good job of catching any bit wind there is. On top of that, fan blades may be designed to compress the air, but when the fan is unpowered, they make really good wind turbines.

Just for comparison, a Trent800 at idle would have fan speed around 500rpm and would make significant noise.
 
ultrapig
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 5:20 am

OK all I think I've got the answer! Thanks
 
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HAWK21M
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RE: 777 Parked At Gate-Engines On Idle

Fri Aug 03, 2007 6:05 pm

Quoting Ultrapig (Reply 3):
I timed them using the "squiggles"

Interesting term.

Quoting UnattendedBag (Reply 4):
It takes a good 10-15 knot wind to spin the blades

Depends on the direction of the wind too.
regds
MEL
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boeing767mech
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RE: 777 Parked At Gate-Engines On Idle

Wed Aug 08, 2007 4:01 am

Quoting Ultrapig (Thread starter):
I was at Logan yesterday-taking an AA MD-80 from B-33. I was there about 1.5 hours before departure time. Next door at Gate B-32 is a T7 which I assumed was eventually going to LHR. I noted that both of the engines were going at about 30 rpm-and continued to do so until I enplanned next door.

Gate 32 is at the end of the terminal so you get the wind blowing around causing a high pressure area next to the jetbridge. I has seen B32 almost ripped off the side of the building in a snow storm because of the way the wind blows around the building

David
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sv2008
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RE: 777 Parked At Gate-Engines On Idle

Wed Aug 08, 2007 6:15 am

Quoting Ultrapig (Reply 3):
spinning seemed pretty consistent.

This (random guess) might be caused by the mass of the blades which smooths out the irregularity of the wind speed (a sort of flywheel affect). For example a car engine is smooth(ish) because it has a flywheel at one end of the crankshaft despite distinct, large irregular forces caused by the combustion process/timing.

Quoting Flyf15 (Reply 5):

Turbofans have extremely good lubrication and bearings.

But are they lubricated without the engine running? Most machines have engine driven oil pumps don't they? I have no idea about this just wondered.........
 
tdscanuck
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RE: 777 Parked At Gate-Engines On Idle

Wed Aug 08, 2007 6:28 am

Quoting Sv2008 (Reply 13):
Quoting Flyf15 (Reply 5):

Turbofans have extremely good lubrication and bearings.

But are they lubricated without the engine running?

Sort of. Most jets run the lube pump off of the accessory gearbox, which is hard-connected to the inner spool (or one of the inner spools in the case of a RR engine). So, if the inner spool turns (for any reason) the lube pump turns too. However, most windmilling will get the fan (outer spool) going but unless the wind is really kicking I doubt it gets the inner spool turning much at all.

So yes, they can be lubricated without the engine running but no, in the case of normal windmilling at the gate, they're not well lubricated.

Tom.
 
Goldenshield
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RE: 777 Parked At Gate-Engines On Idle

Wed Aug 08, 2007 8:38 pm

Quoting HAWK21M (Reply 11):
Quoting Ultrapig (Reply 3):
I timed them using the "squiggles"

Interesting term.

He's talking about the spiral on the engine spinner which is a visual cue to rampers when an engine is safe to approach.
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HAWK21M
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RE: 777 Parked At Gate-Engines On Idle

Wed Aug 08, 2007 10:49 pm

Quoting Goldenshield (Reply 15):
He's talking about the spiral on the engine spinner which is a visual cue to rampers when an engine is safe to approach.


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Is that an official term."Squiggles" or just a nickname.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
flyf15
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RE: 777 Parked At Gate-Engines On Idle

Thu Aug 09, 2007 12:22 am

Quoting Sv2008 (Reply 13):
But are they lubricated without the engine running? Most machines have engine driven oil pumps don't they? I have no idea about this just wondered.........

Well, there won't really be any oil pressure when the engine is not running, but there will be residual oil present that is more than adequate to lubricate the engine during windmilling.
 
airfoilsguy
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RE: 777 Parked At Gate-Engines On Idle

Thu Aug 09, 2007 12:49 am

Quoting Goldenshield (Reply 15):
He's talking about the spiral on the engine spinner which is a visual cue to rampers when an engine is safe to approach.

It's supposed to keep the birds out too.  duck   spin 
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