How does reducing speed in turbulence improve the ride?
Originally Posted by President
Who honestly cares if the airplane overspeeds for a second or two?
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How does reducing speed in turbulence improve the ride?
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.
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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.
improves the ride comfort in moderate turbulence is the 757. All the
others - nope.
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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
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!'
The Captain
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!'
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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.
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.
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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!
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Timely: *Flying* Magazine, Feb. 2014
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]..."
"...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]..."
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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??
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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.
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.
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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.
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.
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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.
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.
ISO2631 provides some design guidelines for human reaction to and exposure to vibrations.
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.
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So a 2.5% increase will feel stronger than that so to unknown degree. 5%? 10%?
I thought 411A had been resurrected when I saw his contribution to this thread 
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.
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.
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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.
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.
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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.
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.
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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.
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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!
About the alteons blokes, sigh! The less said the better!