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Old 19th Jul 2009, 06:16   #1 (permalink)
 
Join Date: Jul 2008
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Turbulence Intensity - Definition

All of us are aware of the typical definitions of turbulence as stated in various publications (AIM, FOM, etc). I.e.: Slight rhythmic bumps…walking difficult…..passengers thrown violently, etc, etc.

On that subject, a study was conducted about 10 years ago by “Digital Avionics Systems.” That study summarized as follows:

"...The goal of this research was to study how pilots assess turbulence intensity when no “out the window” cues are available (e.g., no clouds or contrails). In this experiment, we explored these subjective differences in turbulence assessments. Pilots provided their turbulence assessments while experiencing various “smooth”, “light”, and “moderate” turbulence intensities…”

Obviously - among other things - individual experience levels will account for at least some of the “Subjective” differences.

Around that same time (1999 – 2000), I read an aerospace publication / engineering document which contained a set of Turbulence definitions which related “Air Speed / Vertical Speed changes,” to specific Turbulence intensities.

The subject of that document seemed an effort to eliminate the "Subjectiveness", by applying a defined aircraft reaction, to a specifed turbulence intensity level (or visa-versa).

I can’t recall the name of the document, nor its publisher. It may have been a NASA (Dryden Flight Center) or Military (US Navy) document, but searching their respective public files, I have not been able to find it.

However, I vaguely recall that it applied to "Transport Aircraft," and that the document classified turbulence somewhere around the following values:
  • Light Chop = 5 – 14 knot changes in IAS
  • Moderate Chop = 15-25 knot changes in IAS
  • Light Turbulence = 5 – 14 knot changes in IAS, AND, 300 – 1199 FPM V/S
  • Moderate Turbulence = 15-24 knot changes in IAS, AND, 1200 – 2400 FPM V/S
  • Severe Turbulence = +25 knot changes in IAS, AND, 2500 - 2999 FPM V/S
  • Extreme Turbulence = + 3000 FPM V/S...
Does anyone recall this document and its publisher?


Cheers
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Old 19th Jul 2009, 08:33   #2 (permalink)

 
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I'm not familiar with the document, but it seems rather pointless. The current defining descriptions of turbulence don't require "looking out the window," and under those definitions, what's seen outside the window is irrelevant.

As far as determining turbulence by airspeed changes...this is a ridiculous concept. I've certainly seen very significant airspeed changes with no associated turbulence, and I've seen significant tubulence with very little airspeed change. Further, one need not be climbing or descending at several thousand feet per minute to be in severe or even extreme turbulence.

I've spent considerable time in severe and extreme turbulence while operating in strong winds very close to mountainous terrain (often with in several feet of mountainous terrain)...turbulence severe enough that the instrument panel became somewhat of a blur and at times aircraft control became very difficult. Sometimes airspeed has been jumping all over the place, but at other times not...and I can tell you that were rates of climb or descent of several thousand feet per minute necessary to experience severe or extreme tubulence, I'd be part of a rock somewhere by now. My own experiences have very clearly met the current defining criteria for various states of turbulence...but whereas I might be five or fifteen feet from the mountainside at the time, a rate of several thousand feet per minute descent is simply a poor descriptor.

With current descriptions such as "makes walking about the cabin difficult," we have a good set of criteria which have never required looking out the window at "contrails or clouds."

I haven't seen your paper, but I'm not sure it would do much to enhance how we interpret turbulence or chop.
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Old 19th Jul 2009, 09:00   #3 (permalink)
 
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Quote:
Does anyone recall this document and its publisher?
"Yes" or "No" would suffice. I just want to find it, read it, and then form a concusion.
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Old 19th Jul 2009, 09:09   #4 (permalink)
 
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Quote:
As far as determining turbulence by airspeed changes...this is a ridiculous concept.
While I have very limited experience of flying in turbulence, I agree. The qualitative guidelines for turbulence severity talk about what happens to unsecured objects. This implies that what we're really measuring is accelerations. Were aircraft to routinely be equipped with g-meters, would this be a more appropriate quantitative measure?
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Old 19th Jul 2009, 09:29   #5 (permalink)

 
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Quote:
"Yes" or "No" would suffice.
Perhaps, but this is a discussion forum, and you presented considerably more material than necessary to merit a "yes" or "no" answer, thus inviting comment. This is what you received.
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Old 19th Jul 2009, 09:39   #6 (permalink)
 
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Hopefully not as many comments that

Quote:
I've spent considerable time in severe and extreme turbulence while operating in strong winds very close to mountainous terrain (often with in several feet of mountainous terrain)...whereas I might be five or fifteen feet from the mountainside at the time
will invite!


RTFQ... In the absence of the name of the document, a little description seemed necessary.

Last edited by Jimmy Do Little; 19th Jul 2009 at 12:14.
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Old 19th Jul 2009, 11:27   #7 (permalink)
 
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Turbulence

Not familiar with any documents on the subject, however my recollection is of three degrees of turbulence. Light, Moderte and Severe.

Light = Light Chop
Moderate = Would/could cause passengers to be ejected from their seat.
Severe = Unable to read instruments due eyeball movement.

I would not put my house on the definitions, however I think they are resonable.
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Old 19th Jul 2009, 13:16   #8 (permalink)
 
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In establishing "Turbulence Tolerance" and associated penetration speeds, the following are used WITH SOME MINOR VARIATION between the regulatory authorities :-

Light Turbulence : 1.3 G tolerance,
Moderate Turbulence : 1.6 G tolerance,
Severe Turbulence : 2.0 G tolerance.

As stated, there is some minor variation, for example the first two quoted are 1.2 G and 1.5 G respectively by some authorities.

Best Regards,

Old Smokey
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Old 19th Jul 2009, 13:25   #9 (permalink)
 
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Thanks Smokey,
As mentioned, I was only looking for the subject document.

However - in your opinion - is it plausible (that since G-load is a function of acceleration(deceleration) / time), that the IAS / VS changes mentioned in that document could have at least some validity?

Cheers

Last edited by Jimmy Do Little; 19th Jul 2009 at 13:40. Reason: Addition of word
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