lhcvg
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Why Slow Down In Turbulence?

Thu Jun 04, 2009 2:23 pm

I've read in some of the articles about the AF crash that the pilot may have caused stall due to slowing down in turbulence. This is not intended to be a spat over the AF crash, but rather the principle behind why you would slow down in turbulence. From my layman's position, it would seem that you would want to accelerate when heading into turbulence in order to counteract the momentary loss of effective airspeed when going through updrafts/downdrafts. I have no engineering knowledge whatsoever so this is just my intuition from my limited understanding of aerodynamics.  footinmouth 
 
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Starlionblue
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 2:36 pm

AFAIK, airliners a "turbulence penetration speed". My guess is that you slow down in order to alleviate potential stress on the airframe.
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PhilSquares
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 2:55 pm

Turbulence penetration speed is a trade off. For instance on the 744, it's 290-310KIAS/.82-.85IMN. This speed provides a compromise between high speed buffet on one end and a stall on the other end. In addition, it provides sufficient margins to ensure over G is not an issue.

On the 744, it is suggested to slow to those speeds in sever turbulence. With that kind of turbulence you will see excursions of airspeed, now you have to worry about high speed buffet and you will encounter "g" loading as the aircraft tries to maintain level flight and with you will experience an increase on stall speed.
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SEPilot
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 3:52 pm

Consider what you do on a bumpy road. Do you go as fast as possible to smooth the ride (which it does)? Or do you slow down to reduce the damage to the car? The physics is simple; the faster you are going the less effect that the ground excursions have on your path of motion, but the higher the impacts on the structure. Going slower causes much greater excursions of the vehicle, but less stress is imparted to it. The difference is that with a car there is no minimum speed, while in an aircraft there is. So as PhilSquares points out, you must have a speed high enough so as not to be in danger of a stall, but you want to reduce it to both to reduce impact loads and to avoid overspeed.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
lhcvg
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 4:39 pm

What is overspeed exactly? Is it just as simple as air moving too fast for the airfoil to effectively provide lift anymore (as in beyond the optimum design envelope of the airfoil)?
 
PhilSquares
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 5:22 pm



Quoting LHCVG (Reply 4):
What is overspeed exactly?

Overspeed is when the aircraft exceeds an airpeed limitation, either KIAS (Knots Indicated Airspeed) or IMN (Indicated Mach Number).
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SEPilot
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 5:31 pm



Quoting LHCVG (Reply 4):
What is overspeed exactly?

One consequence of overspeed (that I am surprised PhilSquares didn't mention) is that it can by itself inflict structural damage. So it is very important to avoid.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
PhilSquares
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 5:48 pm



Quoting PhilSquares (Reply 5):
limitation,



Quoting SEPilot (Reply 6):
One consequence of overspeed (that I am surprised PhilSquares didn't mention) is that it can by itself inflict structural damage. So it is very important to avoid.

Isn't that fairly obvious? That's why it's a limitation!
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redflyer
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 6:41 pm



Quoting PhilSquares (Reply 2):
Turbulence penetration speed is a trade off. For instance on the 744, it's 290-310KIAS/.82-.85IMN.

Phil, how much different than cruise speed is that? Isn't the big girl's typical IMN in cruise around .85? Does a reduction of .03 make that much of a difference when encountering turbulence?
My other home is in the sky inside my Piper Cherokee 180.
 
PhilSquares
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 7:05 pm



Quoting RedFlyer (Reply 8):
Phil, how much different than cruise speed is that? Isn't the big girl's typical IMN in cruise around .85? Does a reduction of .03 make that much of a difference when encountering turbulence?

The 400 tends to stabilize around .865 when heavy. The problem is in smooth air it's very stable and rock solid at that IMN. However, a 10 KIAS change increas in speed will get you an overspeed. In reality, everyone will open the Mach/Airspeed window and crank it back to .82.
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A342
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 8:32 pm



Quoting PhilSquares (Reply 9):
In reality, everyone will open the Mach/Airspeed window and crank it back to .82.

But at high weights, this could sometimes spell a descent to a lower altitude, couldn't it? What if turbulence is even worse there?
Exceptions confirm the rule.
 
pilotpip
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 8:41 pm



Quoting A342 (Reply 10):
But at high weights, this could sometimes spell a descent to a lower altitude, couldn't it? What if turbulence is even worse there?

This may still be advisable because of the higher buffet margin at lower altitudes.
DMI
 
PhilSquares
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 8:48 pm



Quoting A342 (Reply 10):
But at high weights, this could sometimes spell a descent to a lower altitude, couldn't it? What if turbulence is even worse there?

Not really. You will be limited as you are heavy, most like to 310-320 and that's really about it. So, you could stay at your current altitude and just slow down. Or, if there were better ride reports down low, you could certainly descend.

If that's not what you're asking, then I am missing your point.
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Faro
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 9:49 pm



Quoting PhilSquares (Reply 2):
Turbulence penetration speed is a trade off. For instance on the 744, it's 290-310KIAS/.82-.85IMN. This speed provides a compromise between high speed buffet on one end and a stall on the other end. In addition, it provides sufficient margins to ensure over G is not an issue.

For information, from DP Davies's book "Handling the Big Jets", the above turbulence penetration speed is predicated on a target gust velocity of 66 feet per second in the vertical sense (in the UK I believe).

Faro
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Faro
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 10:53 pm

Link copied from the AF 447 thread 10 in Civil Aviation:

http://www.timesonline.co.uk/tol/news/world/europe/article6430398.ece

Tentative: there may be evidence that the A330 may have slowed down "too much", ie way below the turbulence penetration speed resulting in a high altitude stall in turbulence. Source of "evidence" is construed as being the last four minutes of the actual FDR recording, via automatic satellite uplink to AF maintenance.

Faro
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A342
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RE: Why Slow Down In Turbulence?

Thu Jun 04, 2009 11:25 pm



Quoting PhilSquares (Reply 12):
So, you could stay at your current altitude and just slow down.

My point is that when you lower speed, lift decreases. So you either add some nose-up trim or you descend.
Correct me if I'm wrong.
Exceptions confirm the rule.
 
hangarrat
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 1:34 am

Just to be clear, I don't intend to engage in speculation about what happened to AF 447, but rather, I'm trying to understand the theory of how the combination of unreliable air data and turbulence might cause an upset during cruise.

For those unfamiliar with the concept of the coffin corner, here's the wikipedia article:

Quote:
The coffin corner or Q-Corner is the altitude at or near which an aircraft's stall speed is equal to the critical Mach number, at a given gross weight and G loading. At this altitude the aircraft becomes nearly impossible to keep in stable flight. Since the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Since the critical Mach number is maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch heavily nose-down, and lose altitude. The "corner" refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number lines come together.

http://en.wikipedia.org/wiki/Coffin_corner_(aviation)

I looked it up myself to refresh my understanding.

The way I understand this theory of the accident is that the aircraft flew into an area of CBs, where it encountered severe turbulence. The turbulence increased the g-loading and pushed airspeed either above critical mach or below stall.

Do I have that right?


My questions are how close would AF 447 have been to the conjunction of stall and critical mach?

And how sustained would an excursion to either side of the flight envelope need to be in order to cause an unrecoverable stall?
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pilotpip
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 2:01 am

Not so much airspeed, but the rate of acceleration. The "G" forces if you will.

Remember, an aircraft can stall at any airspeed. It stalls when critical angle of attack is exceeded.

If it did in fact enter a storm, it's not impossible that there was structural damage preventing recovery. That would explain the aircraft breaking up mid-air as some of the speculation is pointing to.
DMI
 
hangarrat
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 2:09 am



Quoting Pilotpip (Reply 17):

Remember, an aircraft can stall at any airspeed. It stalls when critical angle of attack is exceeded.

Right, but it can also stall at a very low angle of attack at high altitude with a high wing loading.

Perhaps someone can point to an example of a flight envelope chart showing the stall/critical mach curves?
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pilotpip
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 2:17 am



Quoting HangarRat (Reply 18):
Right, but it can also stall at a very low angle of attack at high altitude with a high wing loading.

That's because you're exceeding the critical AOA for that loading. Critical AOA is not a fixed number. It decreases with load factor.
DMI
 
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SEPilot
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 11:20 am



Quoting PhilSquares (Reply 7):
Isn't that fairly obvious? That's why it's a limitation!

Very obvious to you, as you deal with it every day. I am also aware of it, but being an amateur pilot and not professional I am also aware that most non pilots do not realize this. I find that it is often the case that things that are so obvious to me that I don't even think about them are not at all obvious to many who do not have my background and experience.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
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VirginFlyer
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 5:32 pm



Quoting Pilotpip (Reply 19):
Critical AOA is not a fixed number. It decreases with load factor.

That's the first I've heard of there being a relation between load factor and critical AoA. I was always taught (and consequently have always taught to my students) that critical AoA is a function of the shape of the wing, and that an increased load factor results in a greater angle of attack for a given speed, which in turn results in the critical angle of attack being reached at a higher speed i.e. increased load factor will give an increased stall speed, but the same stall angle of attack. Have I misunderstood what you are getting at here?

V/F
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Bellerophon
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 7:59 pm

Pilotpip

I wonder if on reflection you might want to edit part of your post!  Wink

...an aircraft can stall at any airspeed. It stalls when critical angle of attack is exceeded...

Correct.


...Critical AOA is not a fixed number...

For a given wing, the critical angle of attack, at which the wing will stall, is fixed.


...It decreases with load factor....

Increasing the load factor on the wing, either by pulling g or increasing the mass of the aircraft, will not alter the critical angle of attack.

It will alter the speed at which this critical angle of attack is reached.

The stalling speed will increase in proportion to the square root of the wing loading, but the angle of attack at which the wing stalls will remain the same, whether at 1g or 6g.


Best Regards

Bellerophon
 
pilotpip
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 9:52 pm

Yeah, I got my words crossed. It's been a while. Thanks for correcting me guys.
DMI
 
propilot83
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 9:58 pm

I thought planes are supposed to fly faster in turbulence to compensate for headwind and stall speed. I guess they do go a little faster, but not too fast or too slow.
 
tdscanuck
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RE: Why Slow Down In Turbulence?

Fri Jun 05, 2009 10:50 pm



Quoting Propilot83 (Reply 24):
I thought planes are supposed to fly faster in turbulence to compensate for headwind and stall speed.

If you're going slowly, like on approach, you add speed to compensate for convergence between gusts and stall speed.

However, in cruise, that's a non-issue. The bigger problem then is keeping loading acceptably low, so you want to slow down.

Tom.
 
Caryjack
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RE: Why Slow Down In Turbulence?

Sun Jun 07, 2009 7:32 am

Quoting SEPilot (Reply 3):
Consider what you do on a bumpy road. Do you go as fast as possible to smooth the ride (which it does)? Or do you slow down to reduce the damage to the car?

Thanks. I couldn't possibly count the number of times I've done that with a pickup truck but I can with an airliner - zero.

Quoting SEPilot (Reply 3):
The difference is that with a car there is no minimum speed, while in an aircraft there is. So as PhilSquares points out, you must have a speed high enough so as not to be in danger of a stall, but you want to reduce it to both to reduce impact loads and to avoid overspeed.

  

Quoting SEPilot (Reply 20):
I find that it is often the case that things that are so obvious to me that I don't even think about them are not at all obvious to many who do not have my background and experience.

That's my group and thanks for another down to earth explanation. Looking down just this one short thread I see 4 guys, including PhilSquares, who have taken time to answer my questions in terms that I can understand. Keeps me coming back.
Thanks all,
Cary   

[Edited 2009-06-07 00:40:28]
 
nomadd22
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RE: Why Slow Down In Turbulence?

Sun Jun 07, 2009 1:17 pm

It's a lot easier recovering from a stall than it is putting the airplane back together when it breaks up from excessive loads. And at mach .85 it doesn't take much of an increase before airflow starts going supersonic over certain places which can have unpleasant consequences if they weren't designed for it.
Anon
 
klemmi85
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RE: Why Slow Down In Turbulence?

Wed Jun 10, 2009 10:54 am

If there is such a narrow path to play on, isn't it extremely dangerous in turbulences all the time?

I mean, shifting winds can pretty fast cause overspeed or stall speed if changes are significant. Given the small headroom you imply, it sounds quite dangerous all the time because as far as I know A/T does not respond THAT quickly nor can engines spool up with the blink of an eye.

Hitting the speedbrakes at crusing altitude to prevent overspeed seems rather uncomfy, too as lift would be reduced, propably putting you in a stall stituation right away, wouldn't it?
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PhilSquares
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RE: Why Slow Down In Turbulence?

Wed Jun 10, 2009 11:13 am



Quoting Propilot83 (Reply 24):
I thought planes are supposed to fly faster in turbulence to compensate for headwind and stall speed. I guess they do go a little faster, but not too fast or too slow.



Quoting Klemmi85 (Reply 28):
If there is such a narrow path to play on, isn't it extremely dangerous in turbulences all the time?

You only see those type of airspeed excursions in moderate or greater turbulence. You do have a fairly good spread of airspeed, so it's not quite as bad as you think.

Quoting Nomadd22 (Reply 27):
And at mach .85 it doesn't take much of an increase before airflow starts going supersonic over certain places which can have unpleasant consequences if they weren't designed for it

Vmo for the 744 is .92, so there is quite a spread there. I have seen it bump up to about .89 or so but never close to .92.
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klemmi85
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RE: Why Slow Down In Turbulence?

Wed Jun 10, 2009 11:31 am



Quoting PhilSquares (Reply 29):
You only see those type of airspeed excursions in moderate or greater turbulence. You do have a fairly good spread of airspeed, so it's not quite as bad as you think.

Ah I see, thank you.
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Faro
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RE: Why Slow Down In Turbulence?

Wed Jun 10, 2009 12:22 pm



Quoting PhilSquares (Reply 2):
Turbulence penetration speed is a trade off. For instance on the 744, it's 290-310KIAS/.82-.85IMN. This speed provides a compromise between high speed buffet on one end and a stall on the other end. In addition, it provides sufficient margins to ensure over G is not an issue.

Apart from stall/high speed buffet considerations, in mountain wave turbulence is there also a requirement/practice not to fly perpendicular to the direction of the wave? I recall that in the case of BOAC 911 near Mt Fuji in 1966, the aircraft was clobbered by a violent *lateral* gust that instantly ripped away the fin and engine pylons.

Faro
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SEPilot
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RE: Why Slow Down In Turbulence?

Thu Jun 11, 2009 6:27 pm



Quoting Faro (Reply 31):

Apart from stall/high speed buffet considerations, in mountain wave turbulence is there also a requirement/practice not to fly perpendicular to the direction of the wave?

I don't know of any requirement, but any course on mountain flying will tell you to ALWAYS cross a ridge at about a 45 deg angle, as that will give you a much better chance of turning back if the turbulence gets too severe. You still might get overwhelmed, but you have a lot more options than if you were perpendicular to the ridge.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
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Faro
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RE: Why Slow Down In Turbulence?

Fri Jun 12, 2009 9:26 am



Quoting SEPilot (Reply 32):
I don't know of any requirement, but any course on mountain flying will tell you to ALWAYS cross a ridge at about a 45 deg angle, as that will give you a much better chance of turning back if the turbulence gets too severe. You still might get overwhelmed, but you have a lot more options than if you were perpendicular to the ridge.

In France where I got my basic licence, they teach you to get as close as possible to being parallel to the ridge, ascertain that there are no significant downdrafts then cross over rapidly.

Faro
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SEPilot
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RE: Why Slow Down In Turbulence?

Fri Jun 12, 2009 9:59 am



Quoting Faro (Reply 33):
In France where I got my basic licence, they teach you to get as close as possible to being parallel to the ridge, ascertain that there are no significant downdrafts then cross over rapidly.

OK, that sounds like it will work as well. Different technique, but same principle.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

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