Exceeding Vno, where is the danger?

[SMO]

Curious as to how others approach exceeding Vno, into the yellow arc on the ASI. The normal cautions are to only be in that regime when in smooth air, but what defines smooth air?

Can I use my g-meter to measure the cutoff between smooth and non-smooth air? For example, if I experience turbulence that shows up on my g meter, say .5 to 1.5, should I be slowing down to Vno? What about if it displays 0.0 - 2.0? Or greater?

Can I safely pull 3 g's in a loop when exceeding Vno if I am in "smooth" air? Or perform a roll where positive g's are maintained and do not significantly exceed 1.0?

Can someone provide an understanding of the aeronautical engineering process that goes into determining Vno?

[BobTurner]

Google "V-g diagram".
Vno is set where the load limit is reached in a instantaneous 30 ft/sec downdraft. (Old definition. I believe there is a revised standard which applies the force in a not-instantaneous fashion, but gives similar results).

[newt]

Otherwise known as a "design gust."

A design gust is significantly stronger than you think. It's enough to exceed a -3 acceleration, which is probably the kind of turbulence that's serious enough to just keep you on the ground, rather than be in the air wondering what speed you should be flying at.

Also note that the yellow arc has nothing to do with Va, the speed at which full control deflection exceeds the aircraft's maximum load factor. That seems to be a point of confusion among some pilots I talk to, so it's worthy of note here.

- mark

[SMO]

Originally Posted by BobTurner

Google "V-g diagram".
Vno is set where the load limit is reached in a instantaneous 30 ft/sec downdraft. (Old definition. I believe there is a revised standard which applies the force in a not-instantaneous fashion, but gives similar results).

Ok, so is there a way to measure this? How would this feel in the cockpit?

FYI, my Rocket has a Vno of 160 knots. My normal cruise at 23 squared, 3000 ft ASL, LOP, is >170 knots. This seems to be the airplanes happy place. I sometimes get into a bit of light chop, especially when crossing converging valleys, and get to wondering - when should I pull the power back? At the onset of any bumps at all?

[snopercod]

Originally Posted by SMO

How would this feel in the cockpit?

Like this:
https://www.dropbox.com/s/zztwztfpdj...20CAT.mp4?dl=0

I was just cruising along at 140 KIAS (in the yellow arc) in smooth air when I hit some CAT. The engine hesitated and I thought something vital had departed the aircraft, but everything smoothed out again. No damage was done, other than to my skivvies.

[sblack]

Originally Posted by SMO

Curious as to how others approach exceeding Vno, into the yellow arc on the ASI. The normal cautions are to only be in that regime when in smooth air, but what defines smooth air?

Can I use my g-meter to measure the cutoff between smooth and non-smooth air? For example, if I experience turbulence that shows up on my g meter, say .5 to 1.5, should I be slowing down to Vno? What about if it displays 0.0 - 2.0? Or greater?

Can I safely pull 3 g's in a loop when exceeding Vno if I am in "smooth" air? Or perform a roll where positive g's are maintained and do not significantly exceed 1.0?

Can someone provide an understanding of the aeronautical engineering process that goes into determining Vno?

You can do anything you want beyond Vno, as long as you don't hit that 30ft/sec gust. Well, you say, how do I avoid that? Well, you can't. Looking at it from another way, the designers have to draw a line somewhere. Somehow, long ago, based on some sample of atmospheric data, they came up with 30 fps as a reasonable gust level to design to, likely thinking that your chances if getting a higher gust level are small. So that is what the chose, and they chose Vno as the speed below which that gust won't bust the plane. It's just like Normal Category airplanes are designed to 3.8g and utility 4.4g (or whatever it is). What is to stop either class of airplane from exceeding these limits? Nothing except the pilot's interest in staying alive. Tnese are arbitrary limits that somebody, or more likeLy a group of people, determined to be reasonable, and it seems to have worked pretty well.

If you want to fly faster than Vno, without your airplane coming apart, you need to be aware that the strength of gust you can cope with is now reduced. Given that you can't tell when and how strong the next gust will be, you had better be damned careful when you encouter any turbulence at speeds higher than Vno.

[SMO]

Originally Posted by sblack

Given that you can't tell when and how strong the next gust will be, you had better be damned careful when you encounter ANY turbulence at speeds higher than Vno.

I bolded/capped the critical piece of your response that starts to answer my primary question - if I read you correctly, for your risk tolerance you would get below Vno at the first sign of turbulence. Or perhaps you would never fly above Vno? Or is this the engineer in you responding versus the pilot ?

I sincerely value your input.

[Reformed SeaSnake]

Scott pretty much hit it on the head. The only thing I would add regards the significance of that 30 FPS gust.

NOAA defines light turbulence as gusts 5-19 FPS and moderate turbulence as gusts of 20-35 FPS. So, that 30 FPS gust is close to the upper end of moderate turbulence.

Everybody has a different tolerance for risk, but it would not be too smart to fly into an area of forecast moderate turbulence above Vno

NOAA Reference

[SMO]

Originally Posted by Reformed SeaSnake

NOAA defines light turbulence as gusts 5-19 FPS and moderate turbulence as gusts of 20-35 FPS. So, that 30 FPS gust is close to the upper end of moderate turbulence.

That helps a lot also. Most of the turbulence I have encountered would be categorized as light.

Would be really interesting to know if there is any correlation between gust levels and what a person is likely to register on a g-meter, assuming an RV-like aircraft. Light chop always seems to show up on my g-meter as an extra +- ~0.5 g.

[kaa]

The current requirement for Vno is to withstand 50 fps gusts (https://www.law.cornell.edu/cfr/text/14/23.333), which according to the NOAA document linked above, corresponds to extreme turbulence.