Ask The Pilot

[jb747]

Why does wake turbulence descend?

'Cos the Earth sucks.

Don't know.

 

[Moody]

'Cos the Earth sucks.

Don't know.

Something to do with "For every action ....."

The properties of the wing produces airflow that provides lift to the wing. Something has to pay for that lift .... and I think "the Earth sucks" is not a bad analogy.

 

[Boris spatsky]

Why does wake turbulence descend?

I always assumed that it was just gravity and cold dense air - never really thought about it. I guess the question is, on a really hot day near sea level, does wake turbulence descend?

 

[Quickstatus]

Newtons 3rd law is more about a push rather than a suck.
Anyway, why doesn't EASA and others mandate minimum vertical and horizontal offsets. Something to do with politics of the industry which has billions invested at anytime? And practically might suddenly reduce capacity at many hubs?

 

[iRobbo]

Newtons 3rd law is more about a push rather than a suck.
Anyway, why doesn't EASA and others mandate minimum vertical and horizontal offsets. Something to do with politics of the industry which has billions invested at anytime? And practically might suddenly reduce capacity at many hubs?

I'm not sure what you mean - there is a wake turbulence envelope which has lateral, vertical and longitudinal dimensions based on the aircraft. Does that answer your question?

 

[Blackbird]

There's a discussion here on wake flows. Section 2.4.4 discusses the vertical motion of the wake.

https://www.faa.gov/training_testing/training/media/wake/04SEC2.PDF

 

[Boris spatsky]

There's a discussion here on wake flows. Section 2.4.4 discusses the vertical motion of the wake.

https://www.faa.gov/training_testing/training/media/wake/04SEC2.PDF

Good article but doesn't explain why wake descends - it alludes to it being due to 'viscous properties' aka fluid dynamics.

 

[Moody]

Good article but doesn't explain why wake descends - it alludes to it being due to 'viscous properties' aka fluid dynamics.

The ubiquitous Wikipedia agrees with Moody .... which has to happen occasionally.

https://en.wikipedia.org/wiki/Lift_(force)

 

[jb747]

Anyway, why doesn't EASA and others mandate minimum vertical and horizontal offsets.

There are many rules regarding wake turbulence separation. Offsets are not routinely assigned but can be used if needed. There don't need to be more rules...

 

[jb747]

The ubiquitous Wikipedia agrees with Moody .... which has to happen occasionally.

https://en.wikipedia.org/wiki/Lift_(force)

Which still doesn't really explain why the vortex descends. It is rotating, and as much of it is moving up, as is flowing down.

 

[Quickstatus]

The ubiquitous Wikipedia agrees with Moody .... which has to happen occasionally.

https://en.wikipedia.org/wiki/Lift_(force)

Yes but its the airfoil thats moving not the air. Sure the velocity of the air over the foil is faster than the air below but the true velocity of the air molecules after the airfoil leaves that particular volume of air is negligible. so essentially the air does not move. Neither does the wake vortex move rearwards. It just descends and moves laterally. (the forward velocity of the aircraft makes the wake vortex look as though its moving backwards). The downward and lateral movement decays over time. I think its Newtons 3rd law. Lift is created by accelerating air downwards. The wake vortex is caught up in this
Thanks to all who have tried to explain this!!

 

[Captain Halliday]

Severe Tropical Cyclone Debbie is now a Category 4.

Hands up which of our contributing pilots would fly ~100km from the eye?

 

[RooFlyer]

I wonder if our pilots have ever come across an ATCer on the air and have had cause to think something along the lines of "Wow, you are pretty bad / you shouldn't be in that chair..." ?

If you did have cause to think something like that in the future, would you report your thoughts to your company or your union, presumably to be taken up in an "appropriate" way with the local authorities?


ps I would have thought that the first motion of wake vortex (broadly) after a large jet would be up, as the air would be heated relative to the surrounding air mass, especially if the surrounding air is cold.

 

[straitman]

Just a reminder from post #1 about the aims and objectives of this thread.

As this is an "ask the pilot" thread, we ask that non-pilot members refrain from answering questions that have been directed to pilots until the pilots members have had a good opportunity to answer the question (i.e. at least 7 days). Posts contrary to this request or discussions that get too far off topic may be removed or moved to a more appropriate thread or forum so we can retain order and respect in this thread.

 

[jb747]

Severe Tropical Cyclone Debbie is now a Category 4.

Hands up which of our contributing pilots would fly ~100km from the eye?

I don't know that you can pick an arbitrary number. What was his radar showing...?

 

[jb747]

I wonder if our pilots have ever come across an ATCer on the air and have had cause to think something along the lines of "Wow, you are pretty bad / you shouldn't be in that chair..." ?

If you felt that strongly about it, then an incident report would probably be the most appropriate response. I've seen plenty of mistakes from controllers...and many more from pilots. 99.999% are corrected quickly and efficiently.

Individual ATC systems certainly vary around the world. Some are quite inflexible. Some, almost abusive. ATC around Heathrow is consistently the best I've seen.

I would have thought that the first motion of wake vortex (broadly) after a large jet would be up, as the air would be heated relative to the surrounding air mass, especially if the surrounding air is cold.

I'm flying with a pilot with an aero-engineering background. We had bit of discussion about this last night, so hopefully I can give a reasonably definitive answer after our next sector.

 

[Quickstatus]

Can you please ask if the wake vortex is mitigated by any wingtip design, and designs such as the raked wing of the 787

 

[Awesom Andy]

Hands up which of our contributing pilots would fly ~100km from the eye?

I don't know that you can pick an arbitrary number. What was his radar showing...?

I would add that the pilots on that flight operate from a hub where it's affected by cyclones/typhoons almost all year round, and while it's impossible for me to say if this flight was safe or not, the crew is likely to be familiar with such weather conditions.

 

[andmiz]

Severe Tropical Cyclone Debbie is now a Category 4.

Hands up which of our contributing pilots would fly ~100km from the eye?

The low and mid level weather associated with a cyclone is different to its upper level weather, and while there are probably some good winds flying around the west of it, it's probably no more than picking their way through a few CB's (and looking at their track, not even having to do that)

 

[Dale Eastham]

This is bugging me.

Photo Gallery - A380 lands in Sydney - Free Travel images | smh.com.au

I was looking at that photo (& then several others of different A380s on the ground) - is it an optical illusion or result of perspective that the outboard engines appear to be ever-so-slightly canted upwards compared to the inboard ones? Head-on pictures of A380s on the ground show it up slightly better perhaps: 10 years in the skies: the A380’s numbers add up | Airbus Press release

If this is real, then what is the reason for it? (I'm theorizing that it has something to do with how the wing deflects when loaded - but if its just an optical illusion then I'm gonna feel like a numpty)