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autobrakes3
23rd Feb 2011, 16:52
On the 777, I noticed that a lot of guys wind down the speed when there is light to moderate turbulence. They claim that by reducing the speed, the ride will be smoother. I'm not talking bout turbulence penetration speed here as that deals with severe turbulence.

Isnt ride relative to wing loading? In this case, wouldn't it be the same as flying through the rough weather with the current airspeed rather than winding it down? In fact since speed has no relation to wing loading, I would think that it would be much better to fly faster so as to leave the turbulent patch quicker.

I would be grateful if someone could provide me with an explanation and not use the "fast car over speed hump" analogy! Thanks! :)

BOAC
23rd Feb 2011, 17:08
not use the "fast car over speed hump" analogy! - ah well! Don't expect too many replies then.

411A
23rd Feb 2011, 17:15
In our L1011, we normally cruise at M.84.
We do not slow down for light/moderate turbulence.
Heavy turbulence...we slow to the M.82-.84 recommended speed, target M.83

beachbumflyer
23rd Feb 2011, 17:56
Turbulence is not heavy, it is severe or extreme.

john_tullamarine
23rd Feb 2011, 18:08
If one views turbulence as being a short distance wind shear interaction with the aircraft, then the speed hump analogy is pretty useful. In the same manner that a car can reduce the vertical acceleration profile by running over the hump slower, rather than faster, the aircraft can adopt a similar strategy to achieve a similar result. This is driven by the desire for a comfortable ride for the occupants.

The turbulence penetration scenario looks at severe situations and endeavours to schedule a balance between certification structure reserves (at the faster end) and loss of control (at the slower end).

Wing loading is useful for comparing the likely ride qualities of one aircraft against another. Encounter speed is more useful for considering variation in accelerations on the one aircraft.

CliveL
23rd Feb 2011, 18:35
On the 777, I noticed that a lot of guys wind down the speed when there is light to moderate turbulence. They claim that by reducing the speed, the ride will be smoother. I'm not talking bout turbulence penetration speed here as that deals with severe turbulence.

Isnt ride relative to wing loading? In this case, wouldn't it be the same as flying through the rough weather with the current airspeed rather than winding it down? In fact since speed has no relation to wing loading, I would think that it would be much better to fly faster so as to leave the turbulent patch quicker.

I would be grateful if someone could provide me with an explanation and not use the "fast car over speed hump" analogy! Thanks! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

They are right. I don't want to get too technical, but the 'delta g' you get for a given vertical gust is proportional to:

Gust velocity * Aircraft velocity * wing lift curve slope / Wing loading

Everything else being equal reducing air speed reduces delta g on a one for one percentage basis. Increasing airspeed may reduce the exposure time but it will be a rougher ride for a shorter time. This is also why recommended speed in turbulence is lower - the loads on the wing are reduced.

If anyone wants a more detailed explanation just ask http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wink2.gif

bfisk
23rd Feb 2011, 20:59
Turbulence is caused by vertical gusts - that means changing vertical components of wind - updrafts and downdrafts. Because your aircraft has inertia, these changed vertical components of the wind will alter your wing's angle of attack, and consequently your lift. This is experienced in the aircraft as positive and negative g-forces.

For a given change in angle of attack, the change in lift is proportional to the true airspeed squared. That means, by reducing speed, you reduce the changes in lift produced by the given vertical gust, and consequently the shaking and rattling.

Capn Bloggs
23rd Feb 2011, 22:06
As bfisk and others have pointed out, it's all related to V in the lift formula:

CL x ½ Rho x V² x S

eg double the speed, 4 times the effect of the bumps.

CliveL
24th Feb 2011, 07:08
eg double the speed, 4 times the effect of the bumps.Not quite true I'm afraid, because if you double the speed you halve the AoA change for a given vertical gust velocity (and therefore halve the CL change) so if you double the speed you double the effect of the bumps not 4 times.

Green Guard
24th Feb 2011, 07:20
I would be grateful if someone could provide me with an explanation and not use the "fast car over speed hump" analogy! Thanks!

OK...If you keep the same speed, the moderate turbulence has a tendency to remain exactly the same.
Reduce the speed..turbulence disapears...nobody knows exactly why.

PS
For light turbulence, do not reduce speed, just switch the "Seat Belts" on.
Cabin Servise stops about the same time the light turbulence stops...

DERG
24th Feb 2011, 10:30
All of the above does use put a mass factor ..
Say if your B74 had less mass, say was 100 tonnes lighter would the effect be the same?

The math tells me that the heavier the aircraft the less the effect at the SAME velocity..yes?

Answer: from posted link in the next post
"So when operating at significantly lower weight (and thus lower wing loading) an encounter with a vertical gust at a particular flight speed will induce greater accelerations than when operating near MTOW, which obviously affects choice of speed in turbulent conditions. "

Was this ever indoubt?

Capn Bloggs
24th Feb 2011, 11:40
Clive,
eg double the speed, 4 times the effect of the bumps.

Not quite true I'm afraid, because if you double the speed you halve the AoA change for a given vertical gust velocity (and therefore halve the CL change) so if you double the speed you double the effect of the bumps not 4 times.

Thanks. I'll have a ponder over that at FL350 tomorrow! :ok:

Greenguard,
OK...If you keep the same speed, the moderate turbulence has a tendency to remain exactly the same.
Reduce the speed..turbulence disapears...nobody knows exactly why.
Explained here:
Wind shear and turbulence (xhtml w3c 12/09) (http://www.recreationalflying.net/tutorials/safety/wind_shear.html)
About half way down: "Effect of Speed".

CliveL
24th Feb 2011, 12:12
Capt. Bloggs

Thanks. I'll have a ponder over that at FL350 tomorrow!

OK - Table 6.1 of that reference you posted says it all nicely

Oh, and the bit on the effect of speed does as well so long as one remembers that I was talking about the increment in g above 1.0, not the total 'g'http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif

de facto
24th Feb 2011, 14:35
They talk about a turbulent airspeed(VA),but i think it is VB for jet pilots,VA being the speed at which use of full control forces will not damage the structure of the aircraft.

Spooky 2
24th Feb 2011, 22:21
Turbulent air penetration speed in the 787 is .85 which is the highest I have seen in a transport category airplane.

DERG
25th Feb 2011, 08:39
SPOOKY

Wow! Gives us some idea just how strong the 787 is..thanks for the info.

CliveL
25th Feb 2011, 10:39
Wow! Gives us some idea just how strong the 787 is..thanks for the info.

Not really; Mach Number on its own tells you nothing about strength. Gust loads depend on airspeed not Mach Number so you need at least to know the altitude that goes with the Mach Number.

And besides, the 'rules' for VB say:

At altitudes where Vc is limited by Mach number –
(i) VB may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,
(ii) VB need not be greater than VC.

So the chances are that the 0.85M value is based on buffet margins not structural capability.

411A
25th Feb 2011, 16:13
Gust loads depend on airspeed not Mach Number so you need at least to know the altitude that goes with the Mach Number.



Indeed so.
With the L1011, just as one example, the turbulence speed is M.82-.84, but limited to 290KIAS, regardless of mach number.
So, at lower altitudes, the IAS value overrides mach number target.
So the chances are that the 0.85M value is based on buffet margins not structural capability.
I would entirely agree, buffet onset margins are very important and offen override other considerations.

DERG
25th Feb 2011, 21:27
I don't see the logic in the above answers. In fact they are based on false premises.

How come you people think you can make up the rules of physics as you go along? Everytime I visit this type of thread I am always surprised at what you believe or have been taught.

If Boeing says the 'plane can do XYZ it will do XYZ : thats why they are late. Better late than have events like the A388 has suffered.

As far as the worry about buffeting goes maybe Boeing uses better math than Airbus does....

I am not saying that Boeing and Airbus know EVERYTHING about the products the make something like the 0,85 value does tell us about the airframe.

Every week there are quite serious events with people being thrown around in turbulence. Do you ever think about the stress on the airframe when these events happen?

Capn Bloggs
25th Feb 2011, 22:38
Turbulent air penetration speed in the 787 is .85 which is the highest I have seen in a transport category airplane.
All that means is that it has such a "critical" wing that it needs to be flown faster than most to prevent low speed buffet! :} IMO strength would have nothing to do with it given we're talking about high altitudes. Whilst .85 is only 260KIAS at 45k, it's 330KIAS ish at 30k. I don't know whether I'd want to be doing 330K in any aeroplane in severe turbulence, regardless of how plasticised it was.

Turbulence (I mean bad turbulence) means bounceys, injuries and structural damage. The lower the turbulence penetration speed, the better.

What is the IAS turb speed of the 787, or does it vary with altitude?

Spooky 2
26th Feb 2011, 01:11
Sorry I should have added:

290 below 25,000
310/.85 Mach (which ever is lower) at and above 25,000
Maintain a minimum speed of 15 knots above min maneuvering speed.

PBL
26th Feb 2011, 07:23
I don't see the logic in the above answers. In fact they are based on false premises.

How come you people think you can make up the rules of physics as you go along? Everytime I visit this type of thread I am always surprised at what you believe or have been taught.


What an astonishing thing to say!

Let me suggest a different attitude. Having worked with CliveL professionally for well nigh 15 years now, I have learned, when I don't quite grasp something he is saying about aerodynamics, to think about it hard, maybe even hit the books, until I do understand. It works every time!

DERG
26th Feb 2011, 07:49
No malice in that statement. Since I found this site I often find bloomers like this. I came to the conclusion that the machines you use are so automated that you forget the basics.

Is it not perfectly obvious that if you slow the velocity the effects of the turbulance will be less? And that the heavier the mass of the 'plane the less the effect.

This is interesting
AvioConsult - Aircraft Expert and Consultant - Home Page (http://www.avioconsult.com/index.html)

Don't take this personal...show me another airframe that is stronger than the 787....per metric tonne. Maybe the B748..?

So many variables here though..don't you agree.?

P.S. I see our colleague "forget" was off line when I posted this, if turns up on this thread please be aware of the current situation between me and him .. not good. Regards :ok:

CliveL
26th Feb 2011, 08:04
DERG

I don't see the logic in the above answers. In fact they are based on false premises.

How come you people think you can make up the rules of physics as you go along? Everytime I visit this type of thread I am always surprised at what you believe or have been taught.

If Boeing says the 'plane can do XYZ it will do XYZ : thats why they are late. Better late than have events like the A388 has suffered.

As far as the worry about buffeting goes maybe Boeing uses better math than Airbus does....

I am not saying that Boeing and Airbus know EVERYTHING about the products the make something like the 0,85 value does tell us about the airframe.

Every week there are quite serious events with people being thrown around in turbulence. Do you ever think about the stress on the airframe when these events happen?Sorry DERG, but it your premises that are completely and utterly false.

As for the principles of physics, I don't make them up but at least I understand them so far as aircraft design is concerned, which is not always true in this thread! When I visit the thread I am in turn surprised as to how people with very little real knowledge can pontificate.

Buffeting has very little to do with mathematics, and if you had the remotest idea of aerodynamics you would know that the criterion on which a design should be judged is not Mach Number but the lift one can develop at a given Mach Number before the wing starts to buffet.

It is an anonymous forum which precludes biographical details, but in the light of your last remark I will just say that I was in overall charge of, inter alia, calculating the design loads for the A320, A330 and A340 aircraft, so yes, I have thought about the stress on the airframe when in turbulence - which incidentally does not result in serious events every week.

PS: Thank for your support PBL!

DERG
26th Feb 2011, 08:20
Well most of those 'planes appear to be in one piece so far...so yes you must have at least a grasp of loadings. Thanks for the advice.

"When I visit the thread I am in turn surprised as to how people with very little real knowledge can pontificate."

Still do not understand how the mass does not play a role...

Well actually the press are reporting quite a lot of injury accidents every week or so....people thrown about etc. No bits dropping off. Not sure about those B737NGs though with rough shod manufacture issues.

"Buffeting has very little to do with mathematics" This is IDEAL territory for the Bayesians...hopefully you can guide them.


Rock:8 On

Just an additional note: you would be surprised how the original documents in German on some of these concepts vary from the accepted translations here in the UK...thats why Lufthansa sometimes rips stuff out..can't blame them either.:E

CliveL
26th Feb 2011, 08:28
No malice in that statement. Since I found this site I often find bloomers like this. I came to the conclusion that the machines you use are so automated that you forget the basics.

Is it not perfectly obvious that if you slow the velocity the effects of the turbulance will be less? And that the heavier the mass of the 'plane the less the effect.

Don't take this personal...show me another airframe that is stronger than the 787....per metric tonne. Maybe the B748..?

So many variables here though..don't you agree.?

OK - no malice http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif But I am not using any machine!

If you read my original post (#6 in this thread) you will find a statement that makes it obvious that low velocity and high mass reduce the effects of turbulence.

I really don't know what you mean by 'stronger per metric tonne'. It is a meaningless concept.
Apart from the fact that nobody outside Boeing knows how strong the 787 will be, all aircraft are designed to the same rules which govern the loads the airframe must withstand. If anybody designs a structure that can withstand considerably more than the requirements then the aircraft will be too heavy (unless they deliberately build in some margin for growth).

I would just add that the critical case for turbulence loads is usually around FL200 for several reasons:
The design gusts are much lower at cruise altitudes
Aircraft generally cruise as high as they can for performance reasons. This means that they fly at a fairly high lift coefficient and are limited in the amount of 'g' they can develop there. I would say the maximum 'g' before running out of lift would be about 2.0 (at cruise AUW, where the aircraft must be designed for 2.5g at MTOW)

All of which is consistent with my remarks that just citing a turbulence penetration Mach Number says nothing about the aircraft strength

DERG
26th Feb 2011, 08:37
Yes .. seeking a measure of ability to withstand turbulance per metric tonne total mass. Basically how good you got the strength/mass ratio...how good a designer you are.

But listen no matter..can see this is not straight forward

Thanks for the reply.

PS. I am big fan of the A34s. Very nice aeroplane. Good job!

bArt2
26th Feb 2011, 08:40
In my experience, it is easier to get rid of the turbulence by switching the fasten belts sign on. I'm not sure were that fits in the equation though. :E

DERG
26th Feb 2011, 08:46
Never take mine off 'cept for the closet. In fact rather get the train these days what with bed bug an all.:sad:

CliveL
26th Feb 2011, 09:03
Yes .. seeking a measure of ability to withstand turbulance per metric tonne total mass. Basically how good you got the strength/mass ratio...how good a designer you are.

Ah, that's a very different question, and one that is not easy to answer because the larger (heavier) the aircraft the more likely it is designed by manoeuvre loads rather than gust loads, because as you say heavier aircraft are less affected by turbulence.
To complicate life a little more, the bottom skins are designed by fatigue or damage tolerance considerations, so the metric tonnes of wing weight are set by how good you are at designing for these as much or more than by pure static strength.

And I will say that traditionally Boeing have been very good at it http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif

henra
26th Feb 2011, 09:24
All that means is that it has such a "critical" wing that it needs to be flown faster than most to prevent low speed buffet! :} IMO strength would have nothing to do with it given we're talking about high altitudes.


I agree that a high turbulence penetration speed in Mach number does not equate directly to airframe strength, that would be rather the case with max penetraionspeed in kts IAS. You can't calculate Lift (which is linked to structural limitations) by purely specifying a Mach number.
Mach limits are Buffet limits.

And regarding entering tubulence with higher speed: I'm not sure your esteemed SLF will really appreciate that feature being bounced around more severely.

To put it more bluntly: This feature might not be a feature, it might be a bug.

Volume
25th Jun 2013, 07:30
And don´t forget that all that theory (v², wing loading and that stuff) is just valid for a rigid body, which an aircraft is not. The same applies to wing loading, it is not the overall wing loading but it also depends on where you do have the mass. High weight due to full wing tanks and low weight due to most of the mass being the payload in the fuselage makes quite a difference. Soft structure allows for "evading" the vertical airspeed hence reducing the effective AoA hence "smoothing out" the gust. That might be the perfect reason why the 787 can fly faster through the same gust, just because the soft wing will dampen it a little more amd hence reduce the peak load on the structure (and the passenger).

outofsynch
25th Jun 2013, 07:30
My argument is that so many pilots reduce by .01-.02mac which I dont think improves the ride at all. Just prolongs it.

However to reduce by 10-20% might improve the ride, but aint possible at most cruise altitudes.

Am I right? Happy to be corrected if 5-10kt reduction at mach .8ish is significant turbulence-wise. It never feels different to me!

bfisk
25th Jun 2013, 08:21
Another reason might have to do with preventing speed excursions. Flying at high speed (ie high cost index in a headwind), you'll be pretty near MMo. With rather large speed fluctuations and a slow-responding (as per design) autothrottle system, it reduces your workload to reduce speed slightly.

Linktrained
25th Jun 2013, 11:50
Aircraft of earlier generations tended to have nearly rigid wings. Now they are designed to be more flexible. " To give a smoother ride..." so we were then told.

Someone published a video of the wingtips of an aircraft in "moderate turbulence" on this thread a year or two ago.

{ As a SLF I try to watch the wing-tips lift, on the opposite side to where I am sitting on the aircraft, as they start to do their job on T/O. This was something I had been unable to see as a pilot. And our wings, then, were supposed to be (very nearly) rigid, anyway!)

Perhaps the " smoother ride " story may have been at least partially true !

Agaricus bisporus
25th Jun 2013, 12:42
On the 777, I noticed that a lot of guys wind down the speed when there is light to moderate turbulence. They claim that by reducing the speed, the ride will be smoother

I don't think anyone has really addressed the original question or the "reasoning" behind it.

The only aircraft I've flown have a stated "turbulence penetration speed" published. This is a speed decided by the manufacturer that maximises buffet margins (high and low) in the event of extreme IAS excursions - in other words is specified for structural and control reasons. It surely has nothing whatsoever to do with comfort/ smooth ride.

I frequently fly with FOs who, as soon as a bump is felt, wind the speed back to turbulence speed "because we're in turbulence". My impression is they do this in the belief that it is required for pax comfort rather than being limited to structural limitations in extreme turbulence as opposed to light chop. This technique is so widespread I'd say it is almost universal in my company so there are clearly misunderstandings in the practical use of Turb speed.
"But we're required to use turbulence speed in turbulence" is the usual response - usually accompanied by a "everyone knows that, what the &%$£'s the matter with you?" look. (It is an airline noted for pavlovian training methods)

Uplinker
5th Jul 2013, 15:39
E=0.5 x mv² so if you go a little slower, you will reduce the kinetic energy effects from the turbulence.

In our company, and I assume in most others, we go to turbulence speed which is halfway between VLS and MMO to give us the maximum speed excursion protection between those two limits.


But as mentioned, the seatbelt switch seems to stop most turbulence.........

president
6th Jul 2013, 01:34
Relative to the air mass you are moving at 800 kph. Reducing to 795 kph does not improve the ride. I guess people do it to fly between the red bands which has never been recommend by Boeing. Who honestly cares if the airplane overspeeds for a second or two? Just fly it like the cooking book tells you to.

Capn Bloggs
6th Jul 2013, 01:59
Reducing to 795 kph does not improve the ride.
Speed reduction does improve the ride, Mr President.

I guess people do it to fly between the red bands which has never been recommend by Boeing.
So how do you explain why the turbulence penetration speed is soemtimes less than normal cruise?

NSEU
6th Jul 2013, 02:09
@president

Who honestly cares if the airplane overspeeds for a second or two?

If you report an overspeed, then someone is forced to care. There are overspeed checks to be done on the ground, often leading to delays and $$$ ;)

Capn Bloggs
6th Jul 2013, 03:15
Who honestly cares if the airplane overspeeds for a second or two?
Probably the regulator when it finds out that you oversped without writing it up in the tech log... not to mention Safety and the chief pilot from the FDAP...

president
6th Jul 2013, 11:04
Who said I wouldn't write in the book? The ground check is negligible. What's worse: overspeeds in the range of 5 knots OR low speed at hight alt with possible forced descends to keep on flying. Alternatively sticks shakers/pushers activating. As I said: follow the manufacturers guidelines. Nowhere does Boeing prescribe speed reduction in cruise. If we are talking about climb/descend at 280 kts iso 335 kts it another matter.

mikedreamer787
7th Jul 2013, 08:29
The only aircraft I've ever heard of where reducing to Vturb actually
improves the ride comfort in moderate turbulence is the 757. All the
others - nope.

Shrike200
14th Jan 2014, 11:44
Ok, so for us slower people (the Captain and I are busy setting up between legs, can't spare too much time - at least that's our excuse!) are wondering if slowing from (say) M.76 to M.73 will make any practical difference to perceived ride quality?

The Captain :mad::mad: says he reckons so. I reckon it's more a mental thing (M.76 to .73 is only a couple of knots, ie only a small percentage?)

Settle this for us - and not with 'the Captain is always right!' :)

misd-agin
14th Jan 2014, 14:03
Bravo Shrike!

Skip the theory and tell us the practical considerations. How much does a reduction from .84 or .83 to .82 decrease the turbulence impact in a 777-200 @FL370 @ 500,000 lbs? 3%? 25%? 67%? CliveL??

Graph online that shows 787 load reduction reduces vertical G's by approx. 2/3's in turbulence. In other words, if a 787 complains it might be really rough ahead.

Long talk with manufacturer test pilot. He expressed some disdain at how much airline pilots slow for chop - it's for "turbulence penetration". From the discussion he seemed to link it to the random big stuff we encounter and not the everyday stuff.

Mad (Flt) Scientist
14th Jan 2014, 14:57
Bravo Shrike!

Skip the theory and tell us the practical considerations. How much does a reduction from .84 or .83 to .82 decrease the turbulence impact in a 777-200 @FL370 @ 500,000 lbs? 3%? 25%? 67%? CliveL??

As mentioned already, to a first approximation the decrease in the 'g' bumps is of the order of the %change in speed. (There are secondary effects due to things like the CL-alpha curve changing with changing Mach, but they ARE secondary). So slowing from 0.84 to 0.82 would reduce the bumps by a factor of 82/84 i.e. about 2.5%

However, that would likely "feel" a lot more, because there's a threshold below which we are physiologically insensitive to bumps - after all, there are ALWAYS some low level of disturbance, even in what we might think of as calm conditions. Since we're now talking of the subjective human reaction, different people are going to evaluate the same objective response differently.

So maybe you are both right!

FullWings
14th Jan 2014, 16:21
So slowing from 0.84 to 0.82 would reduce the bumps by a factor of 82/84 i.e. about 2.5%
And means you experience them for 2.5% longer... :p

arismount
14th Jan 2014, 17:55
From Peter Garrison's *Flying* Magazine article, "A Violent Sky," p. 24, relating to a small aircraft (U.S. Utility category), but applicable in principle to Transport category aircraft as well:

"...Strength requirements for certification [of this category] are based on a 50 foot per second vertical gust. The effect of such a gust is both to increase the indicated airspeed slightly and, more important, to change the wing's angle of attack. An airplane moving horizontally at a true 170 knots would experience an effective increase in angle of attack of about 10 degrees. This would be equivalent to a 4.7 G pull-up--enough to wrinkle [the aircraft]..."

CliveL
14th Jan 2014, 17:57
How much does a reduction from .84 or .83 to .82 decrease the turbulence impact in a 777-200 @FL370 @ 500,000 lbs? 3%? 25%? 67%? CliveL??

Mad (Flt) Scientist has it exactly right - everything else being equal the magnitude of the 'g' from a given gust is proportional to EAS.

phiggsbroadband
14th Jan 2014, 18:52
Indirectly connected to airspeed is the ability to fly at different altitudes to avoid the turbulent patches. This requires a knowledge (or guess.) of where the turbulent layers are from traffic further up the route.
If you can descend then you can also slow down, but usually the only way to avoid turbulence, or mountain wave for example, is to climb over the top of it.

misd-agin
14th Jan 2014, 20:43
I assumed it was 82/84's but wanted to make sure.

So a 2.5% increase will feel stronger than that so to unknown degree. 5%? 10%?

Seems to me that the Boeing guy was right, sliding down is overrated for most chop/turbulence.

Mad (Flt) Scientist
14th Jan 2014, 20:58
And means you experience them for 2.5% longer... :p

Indeed. But since the human response to gusts is not linear, in practice its usually better to be exposed for longer at a slightly reduced 'g' level.

ISO2631 provides some design guidelines for human reaction to and exposure to vibrations.

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRZBaA2N9zIkw6IWU4Rb9ixXCE1Sfj5cdzhZcZCxiv _c0dHi3cbmQ

Not sure how well that comes out, but the important thing to notice is that the vertical scale - acceleration in 'g' or in m/sec^2 - is logarithmic. If we use a frequency of about 50Hz as an example (and I accept that turbulence frequency varies, but the curve is similar in behaviour elsewhere) we see that 1 min at +/-0.2'g' is equivalent to more than 8 hours at 0.02'g', whereas a linear exchange would have said 1 min at 0.2'g' equals 10 min at 0.02'g'. So its better to be exposed for longer, if you can get the amplitudes down, at a linear exchange.

Incidentally, I'm not sure which of the ISO2631 graphs that is - it's a costly document so finding a public domain image wasn't easy. Theer are (IIRC) a series of charts which show maximum exposure time as a function of 'g' and frequency, with the charts corresponding to different levels of impact (everything from mild nausea to imminent death). Charts like these are used in design assessments of both normal ops and failures cases, where oscillations might be expected.

CliveL
14th Jan 2014, 21:00
So a 2.5% increase will feel stronger than that so to unknown degree. 5%? 10%?

Perhaps I should have made it clear that the 'g' there was at the CG. At the ends of the airplane it will still be only a 2.5% reduction, but it will be 2.5% of a bigger number and therefore more significant in absolute terms and the level of discomfort alleviated.

misd-agin
15th Jan 2014, 00:40
Clive - thanks.

stilton
15th Jan 2014, 08:58
I thought 411A had been resurrected when I saw his contribution to this thread :eek: then I noticed it was in February of 2011..


Anyway he was right then and still is, many pilots slow down for the slightest of bumps and by such a small amount it wouldn't make a difference anyway. I think it's mostly nervousness and a lack of understanding of basic aerodynamics. Normal speed should be flown unless you hit severe turbulence.

underfire
15th Jan 2014, 09:43
Curious if it is simply reducing speed, or the resultant change in altitude that is the goal?
EDIT: I would guess that each ac has a certain harmonic resonance, but that would be difficult to determine, and , even for the same ac, would vary with a exponential number of factors..

harmonics, and how to deal with them, are little understood.

gerago
15th Jan 2014, 16:32
It is all about the fear brought about by the " punishment culture " of the Koreans. Snooping by management via AIMS and foqa monitoring which at the minimum means a nasty e-message from the chiefs or maybe tea + bickies with the chiefs. Turbulence can easily cause speed excursion beyond the amber or red speed bands. This excursions are recorded in AIMS and are used by the ever diabolical foqa people to humiliate pilots.

Some managements are easy with such speed excursions( as long as they are less than 20 kts beyond limit ) but some just come down hard on the " offending " pilots. So most take the obvious way out and reduce speed to avoid speed excursions.

Prince Rupert
15th Jan 2014, 18:10
It is all about the fear brought about by the " punishment culture " of the Koreans. Snooping by management via AIMS and foqa monitoring which at the minimum means a nasty e-message from the chiefs or maybe tea + bickies with the chiefs. Turbulence can easily cause speed excursion beyond the amber or red speed bands. This excursions are recorded in AIMS and are used by the ever diabolical foqa people to humiliate pilots.

Very true about the punishment culture of the Koreans. Years ago in another life at KAL, I had an 8kts speed excursion beyond Mmo/Vmo due to sudden drop in tailwind and decreasing OAT. Huge argument with FOQA chiefs ending with me punished with a SIM retraining. During SIM retraining, I tried enlightening Alteon instructor on what had happened was no big deal...well Alteon instructor toed KAL FOQA team line singing the punishment tune. All the expat instructors and " advisers " were of no help. Only one ex-LIP SEAsian expat stood up for me pointing out that any speed excursion less than 20 kts was a non event, and 270/280/.82 turbulence penetration speed is just that...turbulence penetration speed for SEVERE turbulence! The company brushed all that off. What do you know...next I was made to undergo a " non scheduled " line check! And my supporter ended up with a fail in his next PC for very dubious reasons! Go figure!

Calvin Hops
15th Jan 2014, 18:48
Prince, that was probably a long time ago. Now I believe KAL flight operations have wise up to that. No more sim retraining for speed excursions less than 20 kts, but lots of crewlink advisories to WATCH THAT SPEED!

About the alteons blokes, sigh! The less said the better!

ngapsayot
15th Jan 2014, 20:51
Calvin Hops

About the alteons blokes, sigh! The less said the better!


Now what do you mean by that?

ngapsayot
15th Jan 2014, 20:56
Calvin Hops

About the alteons blokes, sigh! The less said the better!


Now what do you mean by that?

Chuck Canuck
21st Jan 2014, 19:33
Now what do you mean by that?

It means washed up A300/B757 drivers who had never physically flown a real B777 trying to teach T-7 vets how to land one in 10kts XW!

Years ago in another life at KAL, I had an 8kts speed excursion beyond Mmo/Vmo

Yep, you would be punished for that. It is the rule that someone needed to be punished so that the small bosses seemed to be doing something! They get a small pat of the back for " good job ".:ugh:

totempole
6th Feb 2014, 18:14
Sometimes, the " technique " of immediately reducing to 0.82M comes from bar room/ pub talk. At other times it is just mindless aping of what monkeys do! And of course the monkeys at KAL-ALTOON training and their punishment culture...mind you, I had seen Korean coaches whacking their charges with bamboo canes at school gyms!

sodapop
6th Feb 2014, 19:44
AB3,

As the originator of this thread, which has generated some good tech discussion, where have you gone? As it is obvious that speed does play an important role in the effects of turbulence, I find it strange that you are amiss from the banter.

Kengineer-130
6th Feb 2014, 20:38
What worries me as a licenced engineer & private pilot is people saying they would not report an overspeed :ooh:, not very fair on the next 300+ people who will be flying on the aircraft when you are tucked up in bed asleep is it? The checks are there for a reason.....

point76
6th Feb 2014, 23:02
The maths behind slowing in turbulence to reduce G load seems logical. On the Airbus 320 /321 though there is the anomaly of turbulence penetration speed ( granted thats for severe turbulence )being 275Kts for the 320 but 305 Kts for the 321( basically the same wing but of course heavier aircraft ).It means you actually have to speed up in the 321 rather than slow down. Wondering what the logic behind that is ? The Mach number for turbulence penetration is the same at M0.76.

Owain Glyndwr
7th Feb 2014, 03:49
Originally posted by point 76

On the Airbus 320 /321 though there is the anomaly of turbulence penetration speed ( granted thats for severe turbulence )being 275Kts for the 320 but 305 Kts for the 321( basically the same wing but of course heavier aircraft ).It means you actually have to speed up in the 321 rather than slow down. Wondering what the logic behind that is ? The Mach number for turbulence penetration is the same at M0.76. Those are sea level numbers for EAS I think? The logic is simply the increased weight - the rules defining Vb specify that it cannot be less than K*Vs1g where K is the square root of the 'g' developed under the influence of the design gust velocity. In other words the aircraft must not stall when hit by the design gust. So greater weight, greater Vb. That is today's wording; the rules as written in 1985 said the same thing in different words.

Prince Rupert
7th Feb 2014, 17:31
A log-book entry is still required.

Of course a flight log entry was made with an ASR as well. In other carriers it would have been a non event in the event of a Vmo exceedence of less than 20 kts. The KAL B777-200s with those PW4090 donks suck big time in turbulence and windshifts.

The LIP who stood up for me even suggested that KAL revisit Boeing autoflight algorithms wrt the PW engines; instead of appreciating good inputs they did him in with the connivance of some " alcartoon " douchebags.

Now I suppose almost everyone in KAL is so :mad: scared of any tiny speed exceedence that at the first sign of a bump, they wind back the speed to .82 M without thinking. Absolute baboon operating culture!:ugh:

misd-agin
8th Feb 2014, 00:30
"In other carriers it would have been a non event in the event of a Vmo exceedence of less than 20 kts."

Our guidance is to write up any overspeeds - speed and time above redline.

Chuck Canuck
8th Feb 2014, 04:55
I thought Prince clearly stated he wrote up the exceedence in the log and even filed an ASR!:ugh::ugh::ugh: sheesh!

What I believe he was advised by his LIP friend that no maintenance action required for Vmo exceedence less than 20 kts. The maintenance will access the QAR to ascertain that the exceedence is within limits after the mechanic had read the flight/maintenance log report. If it is less than 20 kts, no action required and the aircraft goes back into service without further ado. If the exceedence is above 20 kts, then physical and diagnostic maintence actions are required.

How did the 20 kts thingy came about? I believe it is in the maintenance manual and that individual knew about it as he had done many maintenance acceptance and delivery acceptance flights before, whereby the Boeing factory pilots and engineers had advised him so.

Stupidbutsaveable
9th Feb 2014, 18:55
Gents. Great thread.
Clive. Very many thanks, I am much the wiser. I assumed a V squared relationship until you squared me away.