Fuel Saving???
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From: Wales
Fuel Saving???
On a recent flight LHR to AUS we chased what was quite clearly a KLM747 over the northern route for a good 40 minutes
It only had 3 contrails though ... and was defo a 747. I have pictures but can't post them
It only had 3 contrails though ... and was defo a 747. I have pictures but can't post them
Joined: Aug 2015
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From: Sunnydale
Fuel Saving???
You will burn more fuel on 3 engines than you will on four. The increase in fuel burn is approximately 10%. If it was BA LHR to AUS then it would have been a 787 so similar speeds to a 747.
Joined: Jan 2008
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From: Reading, UK
The first example here (coincidentally another KLM B744) looks like it's only leaving 3 trails as well:
http://www.youtube.com/watch?v=fJOyCosRpsM
Just an optical illusion.
http://www.youtube.com/watch?v=fJOyCosRpsM
Just an optical illusion.
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From: Everett, WA
It's quite possible for a 747-400 to cruise 37k/Mach .84 on 3 engines if they're not heavy (been there, done that during flight test). So that part is quite plausible.
But they wouldn't have shut down an engine to save fuel - as others have mentioned on a 747 will burn more fuel on 3 engines than it will with 4 burning. So if they were cruising on 3, it wasn't to save fuel.
But they wouldn't have shut down an engine to save fuel - as others have mentioned on a 747 will burn more fuel on 3 engines than it will with 4 burning. So if they were cruising on 3, it wasn't to save fuel.
Joined: Jan 2008
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From: Reading, UK
The starboard contrails on the pre-8 model 747s twist very quickly into a single ribbon.
I've never read an explanation of why this occurs but it is consistent.
I've never read an explanation of why this occurs but it is consistent.
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From: Scotland
Rush - no one appears to want to mention vortex wake. Your answer lies (I believe) in the airflow coming off the upper wing surface which rotates the outer contrail upwards and inwards where it can appear to merge with that from the inner engine - it does that eventually too.
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From: UK
To trail or not?
Aircraft leave contrails because fuel burn creates moisture which freezes before it evaporates leaving a trail of frozen moisture droplets which slowly dissipate.
Military pilots know what levels their aircraft will contrail, sometimes they avoid these levels but trails can be used for tactical purposes for example a pair may have one aircraft trailing as it passes well clear of a target causing a false sense of security unaware the other is turning in behind. A group of aircraft may contrail approaching a border for all to see and then visibly turn away. Defenders are unaware one aircraft is continuing the attack. Contrail levels are named according to MINTRA levels which are briefed before flight. Sometimes tankers will dump a small amount of fuel perhaps in a pattern say three short lines to attract the attention of a receiver who is having trouble locating them
To return to the point the science is not exact, it is possible for a pair of aircraft to trail or not or one to trail and the other be clear. Engines on the same aircraft can trail or not though both have the same design and power setting when near the critical point.
There is no significance to the presence or absence of trailing on any flighjt
Military pilots know what levels their aircraft will contrail, sometimes they avoid these levels but trails can be used for tactical purposes for example a pair may have one aircraft trailing as it passes well clear of a target causing a false sense of security unaware the other is turning in behind. A group of aircraft may contrail approaching a border for all to see and then visibly turn away. Defenders are unaware one aircraft is continuing the attack. Contrail levels are named according to MINTRA levels which are briefed before flight. Sometimes tankers will dump a small amount of fuel perhaps in a pattern say three short lines to attract the attention of a receiver who is having trouble locating them
To return to the point the science is not exact, it is possible for a pair of aircraft to trail or not or one to trail and the other be clear. Engines on the same aircraft can trail or not though both have the same design and power setting when near the critical point.
There is no significance to the presence or absence of trailing on any flighjt
Joined: Jan 2004
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From: Here and there
He is saying it's possible for an engine to simply not leave a trail even though others on the same aircraft are. Seems to address the OP.
