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[Sir Tigger KC]

Coming home from Bangkok on Tuesday I had a window seat on the second leg from Dubai, and flying across Germany where the skies were busy, I noticed that all of the other planes were belching out trails of dirty black smoke. 

So a couple of questions for the aviationists here. 

A) Are there any sort of emissions standards/regulations for aeroplanes?

B) Why do these emissions look like a trail of fluffy white cloud at ground level, yet at 40,000 ft it's a trail of dirty exhaust smoke?

 

TIA! 

[Mister Rog]

I always thought it was vapour not smoke.

I have strong views on air travel. We are being constantly berated over pollution from cars, told to recycle this and that, save energy on heating etc blah blah, and yet little is said about huge airctaft burning tons of fuel at 35,000 ft, and in fact air travel is actually encouraged with airport expansion etc.

[Kevin Wood]

Pretty much everything in a "vapour trail" is just that - water vapour.

Fuel burns hot and pretty efficiently in a turbine engine running at cruise power, much more so than in an internal combustion engine, so the hydrocarbons break down pretty much to carbon dioxide and water with perhaps a trace of carbon monoxide and NOx.

Black smoke implies soot, which is only generated when burning very inefficiently, something that would be avoided on cost grounds and because it would coat the internal surfaces in the engine and cause mechanical problems.

Even a minute amount of water vapour released into the atmosphere at 30,000+ feet will immediately condense and freeze, because the air can't sustain very much humidity at all at the low temperature and pressure found there.

I suspect the colour you saw was probably down to the angle at which sunlight was striking the vapour trail.

[Entwood]

Pretty much as Kevin says... except .... at high altitude (low pressure) and cold temperatures (-40 odd) water vapour occurs in a "supercooled" state and is basically hanging about doing not a lot, in order to form a water droplet it has to condense onto something, it cannot just form a drop ... (this does occur when water crystallises and forms snow but at very different pressures and temperatures) ....

The exhaust from the engine contains unburned hydrocarbons and soot, emitted at high temperatures, this causes the "local" temperature to rise and the soopercooled droplets can then form water droplets by condensing onto the soot particles, and you get the "vapour trail" you see .. starts about 100 metres behind the aircraft. How long it "lingers" depends on the meteorological conditions .. some last hours, some just a few seconds, but as conditions change the water droplets return to vapour and the trail vanishes.

A military pilot will take a great interest in when this occurs as the trail is a huge give-a-way as to his position, vapour trails being visible from many miles away, even we lowly Herc boys took an interest !!

[YZ250]

Pretty much as Kevin says... except .... at high altitude (low pressure) and cold temperatures (-40 odd) water vapour occurs in a "supercooled" state and is basically hanging about doing not a lot, in order to form a water droplet it has to condense onto something, it cannot just form a drop .......

Regarding these 'supercooled droplets' I recall watching the Air Crash Investigation series and one of them was about 'supercooled' droplets. The investigators found, in their search for reasons why there had been a couple of near miss 'plunges' and a fatal plunge, that these supercooled droplets were hanging around in the atmosphere in a plane's 'waiting area' as they circled around awaiting clearance to land. Flying around in this area a couple of times was not a problem as they just used the rubber inflators on the front of the wing to disperse the ice but the planes affected had been circling for a while. Unknown to the pilots the supercooled droplets, which had previously had nothing to latch on to, flowed over the wing and settled on the Aileron. The eventual build up of droplets jammed the Aileron causing loss of control.
Every day is a school day, you learn something new.

[steve6367]

Coming home from Bangkok on Tuesday I had a window seat on the second leg from Dubai, and flying across Germany where the skies were busy, I noticed that all of the other planes were belching out trails of dirty black smoke. 

So a couple of questions for the aviationists here. 

A) Are there any sort of emissions standards/regulations for aeroplanes?

B) Why do these emissions look like a trail of fluffy white cloud at ground level, yet at 40,000 ft it's a trail of dirty exhaust smoke?

 

TIA! 

Absolutely, very tough ones - look at a 737 from a few years ago against a current model and look at all the changes. Winglets to save fuel, much larger vey high bypass turbofans etc

[Doctor Gollum]

As alluded...

A. Yes.
B. All about perspective. The English channel always looks really inviting from the air... Stand in it and the colour is more akin to sewage

[Kevin Wood]

A military pilot will take a great interest in when this occurs as the trail is a huge give-a-way as to his position, vapour trails being visible from many miles away, even we lowly Herc boys took an interest !!

Not something I've ever worried about, unless dumping water ballast, when it used to be customary to aim for the patio outside the club house - until the CFI started labouring the implications of the 500' rule. 

[Doctor Gollum]

More of a challenge from further away

[omega2018]

no tax on their fuel either not even VAT - even trains pay tax on fuel

CO₂ dumped at altitude has about double the global warming impact of CO₂ at ground level I believe.

[Sir Tigger KC]

no tax on their fuel either not even VAT - even trains pay tax on fuel

CO₂ dumped at altitude has about double the global warming impact of CO₂ at ground level I believe.

That's why you can fly from London to Bangkok for about £400 return then! 

[STEMO]

On the days after 9/11, when there were no planes in the sky, people said the skies were actually blue, instead of murky grey.

[Doctor Gollum]

no tax on their fuel either not even VAT - even trains pay tax on fuel

CO₂ dumped at altitude has about double the global warming impact of CO₂ at ground level I believe.

Stop reading the Daily Mail... you'll give yourself an ulcer. Commercial fuel is cheap because it's bought in vast quantities, duty remains the same. It is then written of as a legitimate expense. Which is nice

[omega2018]

no tax on their fuel either not even VAT - even trains pay tax on fuel

CO₂ dumped at altitude has about double the global warming impact of CO₂ at ground level I believe.

Stop reading the Daily Mail... you'll give yourself an ulcer. Commercial fuel is cheap because it's bought in vast quantities, duty remains the same. It is then written of as a legitimate expense. Which is nice

that's just the usual dangleberries .. type "tax on airline fuel" into google .

or http://www.ethicalconsumer.org/commentanalysis/ethicaleconomics/taxhavensinthesky.aspx :

"If global aviation emissions were a country, it would be ranked 7th in the list of global carbon emitters, between the UK and Japan. Yet aviation is the only means of transportation that doesn’t pay a penny of tax on the fuel it burns. This is an unfair advantage that airlines have over trains, coaches and cars, making it the fastest growing form of transport while also being the most carbon intensive. "

[Doctor Gollum]

I would question the motives of such a piece The implication being that airlines contribute nothing...