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Why Slow Down In Turbulence?  
User currently offlineLHCVG From United States of America, joined May 2009, 1559 posts, RR: 2
Posted (5 years 3 months 1 day 11 hours ago) and read 7467 times:

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 

34 replies: All unread, showing first 25:
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17035 posts, RR: 67
Reply 1, posted (5 years 3 months 1 day 10 hours ago) and read 7467 times:

AFAIK, airliners a "turbulence penetration speed". My guess is that you slow down in order to alleviate potential stress on the airframe.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 2, posted (5 years 3 months 1 day 10 hours ago) and read 7454 times:

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.


User currently offlineSEPilot From United States of America, joined Dec 2006, 6899 posts, RR: 46
Reply 3, posted (5 years 3 months 1 day 9 hours ago) and read 7427 times:

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
User currently offlineLHCVG From United States of America, joined May 2009, 1559 posts, RR: 2
Reply 4, posted (5 years 3 months 1 day 8 hours ago) and read 7386 times:

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)?

User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 5, posted (5 years 3 months 1 day 8 hours ago) and read 7350 times:



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).


User currently offlineSEPilot From United States of America, joined Dec 2006, 6899 posts, RR: 46
Reply 6, posted (5 years 3 months 1 day 8 hours ago) and read 7345 times:



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
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 7, posted (5 years 3 months 1 day 7 hours ago) and read 7335 times:



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!


User currently offlineRedFlyer From United States of America, joined Feb 2005, 4319 posts, RR: 28
Reply 8, posted (5 years 3 months 1 day 6 hours ago) and read 7289 times:



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?



I'm not a racist...I hate Biden, too.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 9, posted (5 years 3 months 1 day 6 hours ago) and read 7270 times:



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.


User currently offlineA342 From Germany, joined Jul 2005, 4681 posts, RR: 3
Reply 10, posted (5 years 3 months 1 day 4 hours ago) and read 7204 times:



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.
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 11, posted (5 years 3 months 1 day 4 hours ago) and read 7198 times:



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
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 12, posted (5 years 3 months 1 day 4 hours ago) and read 7190 times:



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.


User currently offlineFaro From Egypt, joined Aug 2007, 1543 posts, RR: 0
Reply 13, posted (5 years 3 months 1 day 3 hours ago) and read 7136 times:



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



The chalice not my son
User currently offlineFaro From Egypt, joined Aug 2007, 1543 posts, RR: 0
Reply 14, posted (5 years 3 months 1 day 2 hours ago) and read 7065 times:

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



The chalice not my son
User currently offlineA342 From Germany, joined Jul 2005, 4681 posts, RR: 3
Reply 15, posted (5 years 3 months 1 day 2 hours ago) and read 7035 times:



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.
User currently offlineHangarRat From United States of America, joined Jul 2005, 633 posts, RR: 0
Reply 16, posted (5 years 3 months 23 hours ago) and read 6967 times:

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?



Spell check is a false dog
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 17, posted (5 years 3 months 23 hours ago) and read 6942 times:

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
User currently offlineHangarRat From United States of America, joined Jul 2005, 633 posts, RR: 0
Reply 18, posted (5 years 3 months 23 hours ago) and read 6938 times:



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?



Spell check is a false dog
User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 19, posted (5 years 3 months 23 hours ago) and read 6937 times:



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
User currently offlineSEPilot From United States of America, joined Dec 2006, 6899 posts, RR: 46
Reply 20, posted (5 years 3 months 14 hours ago) and read 6736 times:



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
User currently offlineVirginFlyer From New Zealand, joined Sep 2000, 4538 posts, RR: 41
Reply 21, posted (5 years 3 months 7 hours ago) and read 6610 times:



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



"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh
User currently offlineBellerophon From United Kingdom, joined May 2002, 583 posts, RR: 59
Reply 22, posted (5 years 3 months 5 hours ago) and read 6538 times:

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


User currently offlinePilotpip From United States of America, joined Sep 2003, 3150 posts, RR: 11
Reply 23, posted (5 years 3 months 3 hours ago) and read 6486 times:

Yeah, I got my words crossed. It's been a while. Thanks for correcting me guys.


DMI
User currently offlinePropilot83 From United States of America, joined Jan 2001, 598 posts, RR: 0
Reply 24, posted (5 years 3 months 3 hours ago) and read 6481 times:

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.

25 Tdscanuck : 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
26 Post contains images Caryjack : 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.    That's my group and
27 Nomadd22 : 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 doe
28 Klemmi85 : 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 over
29 PhilSquares : 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
30 Klemmi85 : Ah I see, thank you.
31 Faro : Apart from stall/high speed buffet considerations, in mountain wave turbulence is there also a requirement/practice not to fly perpendicular to the d
32 SEPilot : 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 y
33 Faro : 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 signif
34 SEPilot : OK, that sounds like it will work as well. Different technique, but same principle.
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