Think you know what Va means?

TMetzinger

TMetzinger

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Tim
This SAIB was recently released, and contains the latest guidance on what Va does and doesn't mean. For many of us who were originally taught some variant of "do whatever you want as long as you're below Va", this information needs to overwrite what you thought you already knew.

SAFE members will have gotten a private version with an intro by Rich Stowell (which is secured and not for public distribution).
 

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TMetzinger said:
This SAIB was recently released, and contains the latest guidance on what Va does and doesn't mean. For many of us who were originally taught some variant of "do whatever you want as long as you're below Va", this information needs to overwrite what you thought you already knew.

SAFE members will have gotten a private version with an intro by Rich Stowell (which is secured and not for public distribution).
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Wow, Thanks, this is not even close to what I remember my CFI preaching to me verbally.

"The design maneuvering speed (VA) is the speed below which you can move a single flight control,one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane."

That is somewhat eye opening.
 
Interesting...I don't recall calculating new Va in my written.
 
I had a discussion with a former Piper engineering test pilot about this once, and he believes even this is inaccurate if you go into a hard pitchover (ie negative g's).
 
JohnGalt said:
I calculate Va and L/D for each stage of flight as part of normal pre-flight planning. Was something my instructor taught me to do and I assumed was part of everyone's routine.

Good to have on my kneeboard the speeds for turbulence penetration, best glide and max endurance at t/o, each hour of cruise and landing. Just kinda makes sense.
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I sure don't do that. It's pretty unlikely you're going to need those numbers to that level of precision - nor be able to fly them if you did. If you do hit turbulence violent enough to justify Va you won't even be able to read the airspeed indicator. It'll be all over the place. If you're lucky you'll be able to reach teh throttle to pull back to a power setting that you know will get you below Va. All you really can do is try to keep things somewhat level without fighting it too much.
 
JohnGalt said:
Corrrect me if I'm wrong, but holding airspeed +/- 10 kias is PTS minimum for PP, +5/-0 for CP (PTS=best conditions, least skill)
FAA minimums require you to be at least that precise.
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They require you to be that precise in smooth air, BUT if you ever hit severe turbulence you will discover very quickly that there is no way in hell you're going to maintain much of any airspeed, definitely nothing within 10 knots as your indicator will be swinging all over hell.

All you'll be able to do is decrease power to a power that should get you below Va and try to keep the airplane somewhat-sort-of-wings-level.

As far as a power off emergency - I tend to pick a point that I absolutely know without a doubt I can make and then I'll generally fly minimum sink instead so that I have some time to troubleshoot. Trying to stretch a glide somewhere is just a game I don't want to play. I'd rather setup for a good landing into a bad spot then a **** landing into a good spot I ended up not making.
 
JohnGalt said:
I know where you're coming from in that it can be quite hard to maintain precision. However it's eye opening to see that on my specific aircraft, there's up to a 15 kias difference in Va and L/D between max gross and min landing weight (1 hr res). All V speeds change quite a bit. My typical flights equate to about 10 kias difference between origin and destination, with takeoff Va and L/D at 3 and 5 kias lower than the oft memorized Max Gross POH figures.
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Most speeds vary with the square root of weight. A 15 KIAS reduction in an airplane with a 130 KIAS Va would mean operating at about 22% under MGW which is a pretty big swing in weight for most GA airplanes. What are you flying?

Aerodynamic forces would be considerable if you're off by 10 in worst-case tubulence. Real world, maybe it won't make much difference. Aircraft are tested to 150% of their category strengths anyway.
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First of all, it's a good idea to reduce your speed to somewhere about 2/3rds the way from Vs to Va in severe turbulence, trying to hold Va means that you'll spend a lot of time above it and likely way above it. If you slow to what I'm recommending, an error of a few knots won't matter as much. Second, I think you've got the wrong idea about that 50% margin on structural strength. That number represents the force required to break the airplane, it can and will yield (permanently deform)
at anything above the published load limit unless the ultimate limit (150%) requirement makes the yield limit higher than the certification rules mandate.
Plus, a 10 kias too fast best glide could bring you up short.
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The curve of airspeed and glide range is fairly flat near best glide speed so carrying a few extra knots isn't likely to make enough difference to matter.

That said, I see nothing wrong with striving for perfection.
 
While I totally agree with the SAIB, but I feel like they failed to convey an understanding of why this is. A good look at a V-N diagram will help a lot in that you will see that at any speed below Va the aircraft will stall before you can generate enough G-force to damage the aircraft. It really doesn't have anything to do with a full deflection of the control, nor does the control have to be fully deflect to generate a condition where Va will protect the airframe.

Of course applying unusual loads such as full deflection one direction immediately followed by the opposite might apply more load to the structure than it could see in a simple single deflection to the point that the surface stalls. Which of course is the point of the SAIB.

Brian
CFIIG/ASEL
 

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brcase said:
While I totally agree with the SAIB, but I feel like they failed to convey an understanding of why this is. A good look at a V-N diagram will help a lot in that you will see that at any speed below Va the aircraft will stall before you can generate enough G-force to damage the aircraft. It really doesn't have anything to do with a full deflection of the control, nor does the control have to be fully deflect to generate a condition where Va will protect the airframe.
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If you're tooling along at Va and pass through a wind shear that goes from a downdraft/tailwind to a strong updraft/headwind (I'm pretty certain such conditions exist inside ugly TRW and rotors) you can still bend the airplane because your airspeed is gonna go up for a while no matter what you do.
 
JohnGalt said:
All V-speeds vary with square root of weight difference and in doing the math it's easy to see a drop of 10kias or more throughout a routine 4 hour flight in most singles. With just me and full tanks, I'm easily 20% under max gross at take-off and have landed over 900 lbs under, that'll change v-speeds quite significantly.
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I'm gonna stick with "most V-Speeds" as there are many that aren't proportional to weight^.5 such as Vne, Vno, Vle, Vfe, and perhaps Vx. Granted those aren't performance V-speeds, but neither of us included that disclaimer (just doing my part to pick available nits).

JOOC, what are you flying? Few airplanes one thinks of are 900 lbs below MGW with full tanks and one person on board. I do admit I was considering the likes of a C172 or C182 when I made that statement and while a DC3 (or T-Bone) might be part of GA, it's not typical.

I understand quite intimately how to fly in turbulence, forgive me if I implied differently. Rather, my point is that a 10 knot difference in Va will affect structural load and shouldn't be dismissed. If we're gonna shoot for speeds below Va in heavy bumps, we should know the true figure and actively work to stay in the range. I believe many pilots do not even recognize there's a difference between POH Va and their actual weight.
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I think we're on the same page here. I did close with something about being precise as a virtue and agree completely that it's worth knowing your current GW and it's effects on performance and airspeeds.

My mentioning the 150% certification figure was mostly to appease previous post.
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OK. I only brought that up because I hear about that 50% safety margin all too often from folks who don't understand that there's no mandatory design margin at all WRT damaging an airplane with G-loading. There may be and often is some actual margin in that but it's not a regulatory requirement.
 
JohnGalt said:
I agree with your nitpicking (seems like that's what most folks around here do...the my brain is bigger than yours competition...)

V-speeds can actually be pretty misleading. Va, Vs, Vso and L/Dmax actually refer to a constant AOA, but since AOA indicators are pretty rare outside of professional grade equipment, the V-speed is the next best thing. For that reason, I like to know the ones that can make a difference in a bad situation. Notably engine-out and turbulence. Stalls may be good to know, but the airplane will tell you so kias really makes no difference.
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Ironically I do have an AoA gauge and rely on that for many critical speeds (Vs, "Vref", and Vyse). My airplane has a 1700+ lb useful load and landing weights can vary almost 1400 lbs although the only time I'd be landing near MGW would be if I had a problem shortly after takeoff. But using AoA instead of airspeeds compensated for weight is a lot easier.
I fly a nicely equipped C182, and I can take-off with 500+ lbs of fuel and me yet remain a good bit under gross. Notice I said landed at 900 lbs under. With an hour left in the tanks I'll be at 2200 or less.

My guess is that this kind of thinking isn't taught because quite frankly, training aircraft just aren't built with a very big envelope of useful load. Not saying a Skylane is that much more advanced, but it is enough of a truck to make a difference. Crunch the numbers though, a 172 can give you up to a 10 kias difference at the edges.

Unfortunately, there's folks who move into bigger, stronger aircraft with larger performance envelopes who never learn that things like this can make a significant difference.
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Agreed.
 
TMetzinger said:
This SAIB was recently released, and contains the latest guidance on what Va does and doesn't mean. For many of us who were originally taught some variant of "do whatever you want as long as you're below Va", this information needs to overwrite what you thought you already knew.

SAFE members will have gotten a private version with an intro by Rich Stowell (which is secured and not for public distribution).
Click to expand...

Hi Tim,

SAFE has made this available to anyone, linked from the home page (www.SafePilots.org), or directly at http://www.SafePilots.org/documents/SAFE_Training_Aid_MOAs.pdf

Rich
 
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