Fuel Saving???

[rush4flyer]

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

[highflyer40]

From the thread title are you postulating that they switched one engine off in flight to conserve fuel?

[rush4flyer]

I guess that was my question ... I didn't think that was allowed? Or would there be some freak atmospheric situation that would allow this?

[Hotel Tango]

I might be wrong, but I would have thought that if it was on 3 you wouldn't have been "chasing" it for 40 minutes. What type were you in and at what flight level? Was the KLM traffic below or above?

[back to Boeing]

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.

[DaveReidUK]

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 (https://www.youtube.com/watch?v=fJOyCosRpsM)

Just an optical illusion.

[tdracer]

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.

[El Bunto]

The starboard contrails on the pre-8 model 747s twist very quickly into a single ribbon.

Here's a KLM -400 at FL340.

http://occurs.lineum.org.uk/public/overflights/2014/11/.KLM023_B744_440CF_PH-BFT_6S1E6773_2014-11-18_s.jpg

I've never read an explanation of why this occurs but it is consistent.

[DaveReidUK]

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. Except in that video link I posted, where it's the port ones. Maybe it depends which hemisphere it's flying in. :O

[Flightmech]

Doesn't that depend on how many contrail on/off switches they have in the cockpit? Maybe they left one off by mistake........:-)

[Liobian]

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.

[Tinribs]

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

[Hotel Tango]

Tinribs I don't see that your post has any relevance to the OP's question. And if you were answering Flightmech's comment, please note the smilie at the end of it. ;)

[rush4flyer]

Hey everyone .. thanks many varied answers but makes sense.

I only wish I'd posted enough so I could post the picture ...

[AerocatS2A]

Tinribs I don't see that your post has any relevance to the OP's question. And if you were answering Flightmech's comment, please note the smilie at the end of it. ;)

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.