747400sp
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Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 8:33 pm

This morning I went to I-hill to watch planes take off of runway 25 at LAX, and I saw the DL 77L on the LAX-ATL route taking off, and I was suprized to see such a long range plane use so much runway ( about same amout it use on the LAX-SYD route) on such a short route. Most of the time, long range planes, use just a little runway to get air born for shorter routes, so why DL use so much runway with ultra long range plane like a 77L ,on such a short route?
 
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seabosdca
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 8:35 pm

Derated takeoff. Why waste fuel and unnecessarily stress the engines if you have enough runway to avoid it?
 
Mir
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 8:51 pm

Quoting seabosdca (Reply 1):
Derated takeoff. Why waste fuel and unnecessarily stress the engines if you have enough runway to avoid it?

IIRC, a derated takeoff actually uses up more fuel than a non-derated one, but the maintenance savings make it worth it.

-Mir
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packcheer
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 10:12 pm

While the most likely answer is a derated take off, what's to say the plane isn't full?

Maybe DL had a large amount of cargo going LAX-ATL on that specific flight and the flight was full of passengers and bags. While lacking the extra fuel to make the trans-pacific flight, that available payload could be used for other things!
Things that fly, Girls and Planes...
 
Fly2HMO
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 10:49 pm

Quoting Mir (Reply 2):
a derated takeoff actually uses up more fuel than a non-derated one, but the maintenance savings make it worth it.

What's the reasoning behind the fuel-saving part? First time I've ever heard such a thing, you'd think you'd be saving fuel as well as you're running at a lower N1 with a derate.
 
AA737-823
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 10:57 pm

Quoting Fly2HMO (Reply 4):
What's the reasoning behind the fuel-saving part? First time I've ever heard such a thing, you'd think you'd be saving fuel as well as you're running at a lower N1 with a derate.

You have to run them longer at 90% N1 to achieve liftoff and initial climb thrust reduction than you do at, say, 98% N1.
That is, it takes a longer amount of TIME to get off of the ground, followed by a longer amount of TIME to get to your initial point of reducing power to cruise thrust.
That said, I suspect that there are multiple curves working on graphs in this situation, and as such, whether or not it uses more or less fuel is dependent on several other factors.
I suspect that, many times, it uses less fuel. But sometimes, probably not.
 
Fly2HMO
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sat Mar 13, 2010 11:45 pm

Quoting AA737-823 (Reply 5):
That is, it takes a longer amount of TIME to get off of the ground, followed by a longer amount of TIME to get to your initial point of reducing power to cruise thrust.
That said, I suspect that there are multiple curves working on graphs in this situation, and as such, whether or not it uses more or less fuel is dependent on several other factors.
I suspect that, many times, it uses less fuel. But sometimes, probably not.

Ah yes, I figured it would be something like that. Makes sense.

What I find even more strange is when the derated T/O thrust is less than the climb thrust. Why even bother derating the T/O thrust if you're gonna bump it back up anyways? All I can think off is that its "easier" on the engines once the plane is on the air...
 
Flighty
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 12:18 am

Quoting Mir (Reply 2):
but the maintenance savings make it worth it.

Yes it seems like maintenance intervals go shorter, exponentially, as you use higher thrust. We can guess, overhauling some of these top thrust GE90-115 engines costs an absolute fortune.
 
A342
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 12:58 am

Quoting AA737-823 (Reply 5):
You have to run them longer at 90% N1 to achieve liftoff and initial climb thrust reduction than you do at, say, 98% N1.
That is, it takes a longer amount of TIME to get off of the ground, followed by a longer amount of TIME to get to your initial point of reducing power to cruise thrust.

All turbine engines becoming more efficient at higher loads might be a reason why a de-rated takeoff uses more fuel.
Exceptions confirm the rule.
 
sunrisevalley
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 2:48 am

Quoting Packcheer (Reply 3):
Maybe DL had a large amount of cargo going LAX-ATL on that specific flight and the flight was full of passengers and bags.

Assuming typical density for belly cargo and full passenger load the typical ZFW for a fully loaded 77L is ~200t. which for something less than a 2000nm jaunt to ATL requites a TOW of about 232t. The takeoff chart shows a a takeoff distance of 5000' for this TOW. Certainly sounds like much less than max power was used.
 
Viscount724
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 3:37 am

Out of curiosity, what would typically be the highest derate from maximum thrust used on current types, for example with light weights and a long runway on a cold day? I noticed, for example, the following item a couple of weeks ago in the Transport Canada daily incident reports referring to a 737-700 taking off at 86.4% of maximum power. How much lower would that percentage normally go?

TSB reported that the WestJet Boeing 737-700, C-GWSE, was operating as flight WJA 1478 from Calgary, AB to Las Vegas, NV. The takeoff was performed with the autothrust engaged and a takeoff thrust setting of 86.4%. At 81% N1 the No. 1 engine ceased to accelerate and the No. 2 engine N1 continued to 86.4%. The crew advanced the No. 1 thrust lever with no effect and then noticed the N1 reducing. The takeoff was rejected at approximately 85 KIAS and the No. 1 engine (CFM56-7B24) failed as the aircraft decelerated through 70 KIAS. The aircraft returned to the gate where maintenance observed fuel leaking from the HMU. The company will advise the TSB of their findings.
 
Mir
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 3:54 am

Quoting AA737-823 (Reply 5):
Quoting Fly2HMO (Reply 4):
What's the reasoning behind the fuel-saving part? First time I've ever heard such a thing, you'd think you'd be saving fuel as well as you're running at a lower N1 with a derate.

You have to run them longer at 90% N1 to achieve liftoff and initial climb thrust reduction than you do at, say, 98% N1.

That, and the fact that turbine engines are more efficient at higher N1s. So even if you use marginally more fuel, you're getting significantly more thrust.

-Mir
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tdscanuck
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 5:48 am

Quoting Viscount724 (Reply 10):
Out of curiosity, what would typically be the highest derate from maximum thrust used on current types, for example with light weights and a long runway on a cold day? I noticed, for example, the following item a couple of weeks ago in the Transport Canada daily incident reports referring to a 737-700 taking off at 86.4% of maximum power. How much lower would that percentage normally go?

A takeoff thrust setting of 86.4% is an N1 setting, not a % of thrust setting. 86.4% is usually more than a 13.6% thrust drop, because the thrust vs. N1 curve isn't linear.

That said, you can easily get takeoff derates of 20%, with an assumed temperature on top of that.

Tom.
 
Rj111
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 8:29 pm

Part of the reason the RB211 is so durable on the 757 is because its excess power is often not needed.
 
Fabo
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Sun Mar 14, 2010 11:09 pm

I guess, that part of reason why lighter plane needs more runway than heavier is, that you need less runway to stop, therefore you can use more of the runway to get to V1 and therefore you need less thrust. If other requirements such as need to get to 35ft (or maybe some other number) on critical engine failure on V1 over threshold warrant V1 to be equal to Vr and to go all the way up.
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deltal1011man
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RE: Why DL 77L Use So Much Runway On The LAX-ATL Route

Mon Mar 15, 2010 1:58 am

Quoting Flighty (Reply 7):
We can guess, overhauling some of these top thrust GE90-115 engines costs an absolute fortune.

Just a note, DL has 110s not 115s.
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