Wake Turb manuvering speed

I wouldn't say it's guaranteed but the chances of safe flight during turbulence increase substantially at or below Va FOR your current aircraft weight. Published Design Maneuvering Speed (Va) in the POH is almost always for maximum takeoff weight.

His better option would have been to never get near and certainly not below a heavy aircraft. Flying traffic, I have C-130s and C-5s below me but not a chance in San Francisco will I continue if they are above my flight path.
 
He might have been fine had he not been flying in the yellow arc, too. In the green arc, he would have had a much better chance. I'm one of those people who only fly at Va when I anticipate having to make substantial control movements (of course, at altitude, I'm at/below Va anyways) . Turbulence alone doesn't bother me as much.

-Felix
 
Va is supposed to ensure that your airplane isn't over stressed even if you use full controls (except for the rudder as seen in some transport accidents) I forget the gust loading in feet per second that is the standard for Va, but just like in a thunderstorm, I wouldn't count on wake turbulence to be less than that standard.

There's a reason avoidance of wake turbulence is taught, instead of "watch your airspeed when operating around slow, heavy, clean aircraft".

The only other contribution I have is that I understand the folks who do upset emergency manuevers training teach students to continue the roll to the upright position in a wake encounter, instead of fighting it with opposing controls.
 
TMetzinger said:
The only other contribution I have is that I understand the folks who do upset emergency maneuvers training teach students to continue the roll to the upright position in a wake encounter, instead of fighting it with opposing controls.
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I once heard the same thing from an older CFI. I don't recall the type but he said it had flapperons. One side failed while flaps were extended during a turn to final. The plane began to roll and very quickly he was at ninety degrees bank. The student happen to be former fighter pilot regaining his currency. The CFI said he started to right the plane level and the student told him to continue the roll. A new change of underwear was required for the CFI, I'm sure.

The former fighter pilot later told him the momentum was already going into the roll. Continue it and you'll roll out level with less loss of altitude than if attempted to return to wings level from a ninety degree bank.

Some of the aerobatic folks could contribute more to that one.
 
KennyFlys said:
The former fighter pilot later told him the momentum was already going into the roll. Continue it and you'll roll out level with less loss of altitude than if attempted to return to wings level from a ninety degree bank.
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I wouldn't teach that from a ninety degree bank, but would from a roll that had the airplane inverted.
 
gibbons said:
I wouldn't teach that from a ninety degree bank, but would from a roll that had the airplane inverted.
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If you're at a slow airspeed and low altitude (roughly 500 AGL on final), wouldn't you risk losing substantially more altitude? The idea of continuing the roll makes me picture a dart spinning to maintain direction.
 
I of course don't suggest that its ok to fly in Wake Trub if you at Va. no way no how. I'm just wondering if there is anyway to determine what percantage of the failre was the PICs flying in the Yellow Arc. ie if he had it down around 100kts he would have had a rough as hell ride but would the plane remained in one peice? :dunno:
 
After 8 years I finally had a "caution, wake turbulance" warning from the tower at OLM as I was following an L-39 on Saturday. Easy to avoid, just stay above his flight path and land a touch long. Would have saved that guy and his passenger.

Now, I know there's a heck of a difference between an L-39 and a 737, but the idea is the same.

Stay awake up there.
 
Doing most of my training out of MDW, that's my excuse to my CFI when I land long at other airports: "I'm so used to accounting for wake turbulence that I..." :D

Kidding aside, though, wake turbulence does seem to be one of those things that can easily slip to the back of one's mind if you're around the big guys routinely. As my CFI has mentioned to me, if you're operating at an airport that can accommodate an aircraft big enough to concern you about WT, then you've likely got plenty of runway with which to avoid it.

You don't get docked style points for staying high and landing long when you're behind or parallel to a 737, but you can definitely get docked something a lot more important if you don't.
 
TMetzinger said:
Va is supposed to ensure that your airplane isn't over stressed even if you use full controls (except for the rudder as seen in some transport accidents) I forget the gust loading in feet per second that is the standard for Va, but just like in a thunderstorm, I wouldn't count on wake turbulence to be less than that standard.
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Gust loading is not part of the Va standard -- only the full application of control mentioned above. The top of the green arc (where it meets the yellow) is the speed set by a gust load standard, and I've never seen a light plane whose maneuvering speed was in the yellow arc, so Va should be more conservative and keep you as safe as speed can keep you in event of a wakr turbulence encounter.
 
TMetzinger said:
Ron,

http://www.flyingmag.com/article.asp?section_id=12&article_id=527&print_page=y

"Va is a calculated airspeed based on the actual gross weight of the airplane and the wing’s response to a 50-foot per second wind gust, or movement of the elevator. "
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From 14 CFR 23.335:
c) Design maneuvering speed V A.For V A,the following applies:
(1) V Amay not be less than V S√ n where—
(i) V Sis a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, C NA ; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of V Aneed not exceed the value of V Cused in design.
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I don't see anything about a wind gust in there. The discussion of gust loads (which includes that 50 ft/sec figure) in 14 CFR 23.333 does not mention Va.
 
Ron, none of what you quoted speaks to how V(a) is derived, only that it can't be less than Vs times the square root of n,

And that it need not exceed the value of Vc

I don't know how V(a) is derived, and it may differ for part 23 and part 25 aircraft, but I'll take Mac's word for now.

As to the point of the thread, I'm in full agreement that Va is "as safe as speed can keep you" for turbulent air. I'm just reminding folks that you might enounter turbulence that will break up your airplane at or below Va - it's not a foolproof speed.

Certainly flying above Va didn't help the guy in the wake encounter. I'm just not going to say that if he'd been at Va he'd have come out ok.
 
TMetzinger said:
I don't know how V(a) is derived, and it may differ for part 23 and part 25 aircraft, but I'll take Mac's word for now.
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Tim,

Give up on that. What Ron quoted says that Va is determined by the load-limit stall speed. For normal category that's 3.8G. Gust factor has nothing to do with it. If you don't believe me, follow the references and do the math. Mac didn't bother. That doesn't make him right.

All,

With respect to the incident in the O.P.: How many of us slow to Va for visual approaches in smooth air? Especially going into a busy Class B ("Keep your speed up!") airport? Are you at Va descending through 4,000? I didn't think so. Flight at Va might have saved the incident aircraft but that's probably not the right question. Would flight below Vno have saved the day? That's a decent question. So what are the gust loads in a 737 wake vortex?

Regards,
Joe
 
Joe B said:
Tim,

Give up on that. What Ron quoted says that Va is determined by the load-limit stall speed. For normal category that's 3.8G. Gust factor has nothing to do with it. If you don't believe me, follow the references and do the math. Mac didn't bother. That doesn't make him right.

All,

With respect to the incident in the O.P.: How many of us slow to Va for visual approaches in smooth air? Especially going into a busy Class B ("Keep your speed up!") airport? Are you at Va descending through 4,000? I didn't think so. Flight at Va might have saved the incident aircraft but that's probably not the right question. Would flight below Vno have saved the day? That's a decent question. So what are the gust loads in a 737 wake vortex?

Regards,
Joe
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OK, I give up. I've looked up the references and agree - Va is the max speed where the airplane can be stalled without exceeding the G envelope for the category of aircraft. nothing to do with wind gusts.

But the flight envelope (Which is what I think Mac was addressing and called it Va) is specified in :
23.333 Flight envelope.
(a) General. Compliance with the strength requirements of this subpart must be shown at any combination of airspeed and load factor on and within the boundaries of a flight envelope (similar to the one in paragraph (d) of this section) that represents the envelope of the flight loading conditions specified by the maneuvering and gust criteria of paragraphs (b) and (c) of this section respectively.

Which means that the FAR-23 compliant airplane must be able to be flown throughout the combined envelope of maneuvering and gust loads at or below the various speeds..

So, why are so many of us taught that Va is the speed for turbulent air? Is it because for most airplanes the gust load envelope isn't published (with Vc and Vb), or because much of the GA training fleet isn't/wasn't certified under FAR-23?

Is there general agreement that slower (even below Va but above stall) is better for turbulence, since Va may not provide "full" protection in rough air?

Thanks for another good learning experience.
 
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I was always taught that the Vno was the max speed for rough air, which is why it is the top of the green arc.
 
gibbons said:
I wouldn't teach that from a ninety degree bank, but would from a roll that had the airplane inverted.
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I think that the significant issue is whether or not you are still in the vortex and if the plane is still rolling with neutral aileron that must be true. I see little reason to try reversing the roll in that case since you'll reach wings level much sooner by continuing in the same direction as the upset. Notice I said "wings level" not upright. Virtually any airplane will be controllable enough while inverted to maintain altitude while exiting the wake that caused the upset. And as you know, the real problem with pilot's who've never been upside down is that they may not be willing to push the yoke.
 
Given that what Steve says appears correct (Vno is driven by Vc and Vne, and Vc IS driven by the gust load), why is Vno higher than Va? Is Vno ever less than Va? Are they ever equal (say at max gross weight)?

These Va/Vno/Vb/Vc numbers are for straight and level flight, and vary with load factor, don't they (they seem to based on the V-n diagram)? I've been looking at this for a while now and I'm more confused than ever. However past experience tells me that max confusion is normally the point where a new insight appears.
 
TMetzinger said:
So, why are so many of us taught that Va is the speed for turbulent air? Is it because for most airplanes the gust load envelope isn't published (with Vc and Vb), or because much of the GA training fleet isn't/wasn't certified under FAR-23?
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Neither -- it's because Va is a more conservative value that will protect the plane not only from the gust load but also from the pilot's potential overreaction to the upset.
 
Thanks Ron - do you agree that Va isn't absolute protection against gust load? Say a thunderstorm or other severe turbulence would damage your airplane regardless of airspeed?
 
TMetzinger said:
Thanks Ron - do you agree that Va isn't absolute protection against gust load? Say a thunderstorm or other severe turbulence would damage your airplane regardless of airspeed?
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There are no absolutes. I have no doubt that a thunderstorm is capable of generating enough loads to tear your plane apart regardless of the speed at which it's flying. However, Va will give you more load margin than the top of the green.
 
you guys are thinking about this way too hard.

Va is simply the maximum speed that a full control movement will still stall the airplane without breaking anything. Its the curved line on the V-N diagram.

Vno is the speed that a certain vertical gust will cause a structural failure. I have drawn this in on this particular V-N diagram, it is a diagonal line. IIRC the standard is a 24 foot per second gust. Thats over a 1400 fpm thermal, a hell of an updraft.

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tonycondon said:
you guys are thinking about this way too hard.

Va is simply the maximum speed that a full control movement will still stall the airplane without breaking anything. Its the curved line on the V-N diagram.

Vno is the speed that a certain vertical gust will cause a structural failure. I have drawn this in on this particular V-N diagram, it is a diagonal line. IIRC the standard is a 24 foot per second gust. Thats over a 1400 fpm thermal, a hell of an updraft.
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The 23.333 Flight Envelope states
(c) Gust envelope.
(1) The airplane is assumed to be subjected to symmetrical vertical gusts in level flight. The resulting limit load factors must correspond to the conditions determined as follows:
(i) Positive (up) and negative (down) gusts of 50 fps at VC must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 50 fps at 20,000 feet to 25 fps at 50,000 feet.


23.1505(b) The maximum structural cruising speed VNO must be established so that it is -
(1) Not less than the minimum value of VC allowed under § 23.335; and
(2) Not more than the lesser of -
(i) VC established under § 23.335; or
(ii) 0.89 VNE established under paragraph (a) of this section

So it seems to me that for FAR 23 airplanes, a 50 fps gust is used.

But since 50 fps is only 34 Mph, so it doesn't seem like that big a gust to me, at least when we're talking about severe turbulence. Another reason why Va is less.

I think - I've been looking at this stuff all day and hoping I didn't make another error.
 
tim - its a VERTICAL gust. think thermal. a 50 fps thermal is 3000 feet per minute!! thats something you are going to find in building cu-nims.
 
I know it's vertical, but as I've said, compared to what you'll find in severe weather it's not that big (Scott can chime in if he has good vertical gust info).

So Vno is a pretty safe speed for most operations (even light turbulence though it's not pleasant), but I don't think a 50 fps gust is out of the question in rough air.

And a thermal isn't a gust in my mind, it's a current.
 
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tonycondon said:
Vno is the speed that a certain vertical gust will cause a structural failure. I have drawn this in on this particular V-N diagram, it is a diagonal line. IIRC the standard is a 24 foot per second gust. Thats over a 1400 fpm thermal, a hell of an updraft.

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Tony,

Can you explain why it's a diagonal line? And, why there's green above that line, even below Va? I'm missing something.

Thanks,
 
my glider buddy who flies in building cumulus clouds reports typical climbs of 2-3000 fpm in the clouds in florida that will develop into Tstorms later in the afternoon. building Tstorms of course could be worse. staying below Vno is definitely not a guarantee against in flight breakup around Tstorms, as many former pilots have shown.
 
flyingcheesehead said:
Tony,

Can you explain why it's a diagonal line? And, why there's green above that line, even below Va? I'm missing something.

Thanks,
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kent -

let me get home this evening and look through my aircraft structures and flight dynamics book. then ill be able to get you the answers straight from the source
 
Dive speed I think - I assume thats where the wings rip off. There's a safety margin between Vne and Vd.
 
tonycondon said:
my glider buddy who flies in building cumulus clouds reports typical climbs of 2-3000 fpm in the clouds in florida that will develop into Tstorms later in the afternoon. building Tstorms of course could be worse. staying below Vno is definitely not a guarantee against in flight breakup around Tstorms, as many former pilots have shown.
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One of the guys at the NWS in Romeoville said that you could see vertical updrafts of up to 6000 fpm in some thunderstorms.:hairraise:

Edit: This article (http://ams.allenpress.com/archive/1520-0469/36/9/pdf/i1520-0469-36-9-1747.pdf) talks about some updrafts maxing at about 50 m/s, or around 9000 fpm! (That one turned into a tornado.) Now the article is WAY beyond me, so Scott or someone will have to tell me if I'm misinterpreting it.
 

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TMetzinger said:
Dive speed I think - I assume thats where the wings rip off. There's a safety margin between Vne and Vd.
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From 14 CFR 1.2: "V Dmeans design diving speed." It is generally control surface flutter, not wing structure failure, which sets the limit on Vd.
 
gprellwitz said:
One of the guys at the NWS in Romeoville said that you could see vertical updrafts of up to 6000 fpm in some thunderstorms.:hairraise:

Edit: This article (http://ams.allenpress.com/archive/1520-0469/36/9/pdf/i1520-0469-36-9-1747.pdf) talks about some updrafts maxing at about 50 m/s, or around 9000 fpm! (That one turned into a tornado.) Now the article is WAY beyond me, so Scott or someone will have to tell me if I'm misinterpreting it.
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A little off topic but one thing that can help judge the updraft strength in a thunderstorm is hail size... according to the chart at the bottom of this NWS site you need 24 mph updrafts for pea sized hail and 64 mph updrafts for golf ball sized hail.
 
skyflyer8 said:
A little off topic but one thing that can help judge the updraft strength in a thunderstorm is hail size... according to the chart at the bottom of this NWS site you need 24 mph updrafts for pea sized hail and 64 mph updrafts for golf ball sized hail.
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So I think we could all agree it would be prudent to slow below Vno whenever you encounter hail any larger than a pea.:)
 
skyflyer8 said:
A little off topic but one thing that can help judge the updraft strength in a thunderstorm is hail size... according to the chart at the bottom of this NWS site you need 24 mph updrafts for pea sized hail and 64 mph updrafts for golf ball sized hail.
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Off topic, but interesting. The radar returns during the recent storms to hit Atlanta indicated 4" hail according to the meteorologists:hairraise:. I was watching this on a TV in my basement. I ain't no dummy.
 
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