Ask The Pilot

petercr

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And the outboard nacelles appear to be smaller in diameter than the inner ones... Due to thrust reversers inboard or some extra equipment mounted on the inboard engines?
 

jb747

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Aero engineer reckons that the wake descends because of air density. It has been compressed and heated by the passage of the wing, but the compression wins. Winglets should reduce wake turbulence, as their sole reason is to reduce drag...in this instance the drag involved in creating the wake.
 

jb747

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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 upÂ*| 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)

The wing bends quite a bit, and the shape on the ground is quite different to that in the air, especially once fuel is pumped out to the outer tanks. So, there may be a bit of difference in the alignment when on the ground, as I'm sure everything is designed to be straight, and at its most efficient, when in flight.

The nacelles are the same size as far as I know...even though the engine accessories are a bit different.
 
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Aero engineer reckons that the wake descends because of air density. It has been compressed and heated by the passage of the wing, but the compression wins. Winglets should reduce wake turbulence, as their sole reason is to reduce drag...in this instance the drag involved in creating the wake.

Can this be an expression of the principle of bouyancy?. A volume of dense air (compressed during its interaction with the airfoil) sinks not because its heavier but less bouyant. And the decay in bouyancy over time is due to the change in density to that of the surrounding atmosphere

Just like a rock sinks in water because its less bouyant. but a boat with the same mass (but less dense) as a rock floats.



..........

The wing bends quite a bit, and the shape on the ground is quite different to that in the air, especially once fuel is pumped out to the outer tanks. So, there may be a bit of difference in the alignment when on the ground, as I'm sure everything is designed to be straight, and at its most efficient, when in flight.

The nacelles are the same size as far as I know...even though the engine accessories are a bit different.


Thanks , possibly also optical effect due to the wide angle lens used in the picture?
 

Boris spatsky

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Can this be an expression of the principle of bouyancy?. A volume of dense air (compressed during its interaction with the airfoil) sinks not because its heavier but less bouyant. And the decay in bouyancy over time is due to the change in density to that of the surrounding atmosphere?

I don't know; i am but a pilot - push forward, trees get bigger; pull back, trees get smaller :)
 

Altair

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I just wanted to enquire about the classification of the 757.
On one of the windless days in WLG I watched the RNZAF 757 come into land, I noticed the formation of two water columns from the vortices on Evans Bay that followed the 757 a few seconds after the plane passed. It is very different to the SQ 777 wake patterns, no noticeable water column but definitely leaves a trail on the water, think of a wake from an invisible boat. Other 737 or A320 not very noticeable patterns on the water. So I do wonder about the statement up thread that the 757 was later determined not to be too different from other aircraft in its classification.
A few years ago while on a training flight I was following a B1900D and my instructor warned me about how that small aircraft caused a few problems with his students due its wake turbulence. Not really matters to anything larger than a 4 seater but had to keep that in mind for me. We extended our leg to give a bit more separation before commencing our final approach.
Oh regarding the different pitch angle of the 4 engine aircraft. This is not just noticeable on the A380, have a look at the A340 and 747 the engines while on the ground all have visible pitch angles. I do not know about earlier jets eg. 707 but from photos of the CFM engined C-135 series aircraft there is no noticeable difference.
 

jb747

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I just wanted to enquire about the classification of the 757.

When I did some reading about the 757, there turns out to be a surprising amount of controversy about the strength, or otherwise, of its wake. On thing that was confirmed though, is that the rotation speed of the vortex cores is about 50% faster than the much larger 767. I've run into 757 wake a few times in the 747 and it gives more of a reaction than you'd expect.

Mostly these days, the only wake that I really notice is from other 380s. An encounter in the flare is particularly interesting.
 

iRobbo

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I just wanted to enquire about the classification of the 757.
In Australia, for the purposes of Wake Turbulence separation, the 757 is classified as a Heavy if leading, and a Medium if following. So, if number 1 in the sequence, you have to think of it as a Heavy and apply the Heavy wake turbulence separation to anything following it. If number 2, you think of it as a Medium and apply appropriate Wake Turbulence separation from whatever is in front.
The FAA has, in some ATC units in the USA, implemented what they call 'Recat' - a recategorization of Wake Turbulence separation. Instead of applying separation based on Maximum Take Off Weight (MTOW), it is applied based on MTOW and wingspan. They did research and came up with 6 categories (Australia currently has 5), and rather than the traditional Heavy, Medium and Light they are categorised as A to F, with A being the largest (A380 and Antonov A225 "Super"). There is no separate category or exception for the B757.
Interesting comments by JB, it would be interesting to read the data they obtained about the 757.
 

Mattg

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Almost every time we see footage of an emergency aircraft evacuation, there are passengers that have taken their hand luggage down the slides with them. Time and time again, it's sparked a discussion about the selfishness of said passengers.

I was wondering, what actually happens to all the bags that do get left behind (as per crew instructions)? Obviously you'd never see them again if the aircraft is totally destroyed, but what about in cases where the aircraft remains intact? (I'm thinking of last week's Peruvian Airlines incident, as an example.) Would passengers be able to retrieve their belongings as soon as it's safe to do so, or would they remain on the aircraft for days while the investigation etc. is conducted?
 

jb747

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Almost every time we see footage of an emergency aircraft evacuation, there are passengers that have taken their hand luggage down the slides with them. Time and time again, it's sparked a discussion about the selfishness of said passengers.

I was wondering, what actually happens to all the bags that do get left behind (as per crew instructions)? Obviously you'd never see them again if the aircraft is totally destroyed, but what about in cases where the aircraft remains intact? (I'm thinking of last week's Peruvian Airlines incident, as an example.) Would passengers be able to retrieve their belongings as soon as it's safe to do so, or would they remain on the aircraft for days while the investigation etc. is conducted?

The passengers certainly wouldn't be able to go back on board. I expect it would need some form of clearance from the investigating team, and they'd first need to ensure that there was nothing that could possibly be of any use to them in explaining the event. I didn't have any luggage in the hold of QF30, so I can't say for sure what happened there, or how long it took.
 
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Boca68

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A statement of fact....I (meaning the 3 of us in the coughpit) had dumped it.


jb, assuming you were HLO only, did you retrieve the hand luggage at some point or did you disembark/evacuate with it? It might go a little way to answering mattg's question.
 

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