Green, Va and turbulence

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Boatguy
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Green, Va and turbulence

Post by Boatguy »

Flying across the SW deserts the last two afternoons I really got bounced around. And the bouncing was separate from the big up/down drafts that caused the AP to pitch up/down 5˚ in ALT mode since I was on an IFR plan.

Since Yellow on the airspeed is "only in smooth air", then green is for everything else. But green extends well above and below maneuvering speed Va (which is Vo in the AFM). The AFM says the g's that the airframe can take at Va is -1.5g positive and 3.8g negative.

For starters, how does negative / positive relate to the drops and updrafts?

But then, other than my own tolerance for getting bounced around (which is not that high), how do we know when to slow from normal cruising speed in the green (e.g., 125KIAS) to Va? I've looked at the data logs from some particularly bumpy flights and the worst was -.7g and +.9g. Would that have suggested slowing to Va?
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smoss
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Re: Green, Va and turbulence

Post by smoss »

As well, what is tolerable in the yellow? An instructor once told me I was way too conservative with the yellow, and really it is fine for all but full deflection control inputs, despite the AFM saying smooth air only, and then with caution. I'm sure many of us have been cruising along in a smooth air descent, near vNE, when we got walloped by unexpected turbulence. Clearly, the wings don't routinely fall off when that happens. What do most people tolerate in the low yellow, which often corresponds to cruise speed at 8,500 MSL or so?
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CFIDave
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Re: Green, Va and turbulence

Post by CFIDave »

First, try really hard to avoid flying over the US desert southwest in the afternoon. Once time in our DA40 on our way westbound to Albuquerque we got our "asses kicked so hard" by afternoon turbulence that we diverted to land at Las Vegas, NM and spent the night there.

Your DA40NG (like all Diamonds) is certified as a "standard category" (as opposed to utility or aerobatic) aircraft, with +3.8g positive and 1.5g negative load limits.

During certification flight testing, the factory will determine what combination of maximum positive and negative wind gusts (i.e., 50 fps updrafts/downdrafts) and airspeed will hit those load limits -- in order to arrive at Vno, the bottom of the yellow arc. Similarly, they'll determine what speeds full control deflection might damage the airframe (or the plane will stall before it breaks) in determining Va or Vo maneuvering speed.

The good news is that there's never been an in-flight breakup of a Diamond (I'd like to hear if anyone has heard differently). So your body is likely to tell you when to slow down (or get down on the ground) way before you're likely to damage your plane.
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Colin
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Re: Green, Va and turbulence

Post by Colin »

I don't know how you spent the night Las Vegas, NM. I've landed there twice and didn't see anything anywhere near the airport.

There's nothing like exploring the countryside by general aviation. You see things you are just never going to see otherwise.
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CFIDave
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Re: Green, Va and turbulence

Post by CFIDave »

Colin wrote: Fri May 13, 2022 8:57 pm I don't know how you spent the night Las Vegas, NM. I've landed there twice and didn't see anything anywhere near the airport.

There's nothing like exploring the countryside by general aviation. You see things you are just never going to see otherwise.
Agreed. We got lucky -- after fleeing turbulence and landing at the deserted airport outside Las Vegas, NM we learned of a hotel in town offering a "pilot's special;" we got a suite there (the Plaza Hotel) for next to nothing. The hotel manager came out to the airport to pick us up, we stayed in a historical building from the 1800s with a saloon on the first floor located right on the town's main plaza, so we were able to walk around to see the town and go to dinner. The next day the hotel manager offered to drive us back to the airport, and took photos of our DA40. :)

This was years ago, but It was a very enjoyable diversion...
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Re: Green, Va and turbulence

Post by dant »

I remember reading somewhere (presumably this forum) that stall speed is an input in to Vno, and that was the governing factor for DA40s. Structurally they were sound well in to the yellow arc.

I don't remember feeling comfortable with the sourcing of that fact so I'm pretty conservative about when I fly in the yellow, but I'd be curious if someone else has heard the same.
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Re: Green, Va and turbulence

Post by Boatguy »

Yes, don't fly in the afternoon is good idea, but almost impossible when flying east and trying to cover more than 500nm in a day. The compromise is of course to take more days for the trip. In any case my question is not about flying in the yellow which I can only get into when descending or when flying under 6,000' MSL.

To be specific, Va is 101-113 depending on weight and the top of the green is 130. How much turbulence is enough to pull back the throttle and slow to Va? What I'm hearing is that my discomfort level will be reached a lot earlier than anything that might harm the plane.
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Re: Green, Va and turbulence

Post by CFIDave »

Boatguy wrote: Sat May 14, 2022 7:33 pmWhat I'm hearing is that my discomfort level will be reached a lot earlier than anything that might harm the plane.
Yes.

Since most desert southwest afternoon turbulence is caused by thermals (hot air rising from the ground) rather than mountain waves, there's some benefit to flying as high as possible. And I've found it's not quite as bad in a DA62 due to higher wing loading, compared to a DA40. But the turbulence can still get uncomfortable.
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Re: Green, Va and turbulence

Post by mfdutra »

Boatguy wrote: Sat May 14, 2022 7:33 pm How much turbulence is enough to pull back the throttle and slow to Va?
If you're not used to high Gs, you'll be in serious distress anywhere above 3 Gs. The airplane supports 3.8 just fine. Va means you will stall at 3.8G. Above Va, you may break something and the wing will still not stall.

If things start floating around, including you feeling your weight on the seat belts, you're experiencing negative Gs. I think that's a very clear signal to slow down a lot.

If you are on autopilot, put it in pitch mode and ask for an altitude block if you are IFR. Don't let the autopilot manipulate the elevator to maintain altitude.
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Re: Green, Va and turbulence

Post by Paul »

Above the puffy clouds on a summer day, you will find smooth air. The problem is that those clouds can be 18,000 feet or higher in the SW on a warm day. Even so, I will still fly at 12,000 to 14,0000 feet in the DA40. It’s fuel efficient and indicated airspeed is almost always going to be below the yellow arc. Afternoon thermal turbulence sucks but it’s not enough to break the plane so I don’t worry too much if I’m experiencing a few bumps in the bottom part of the yellow arc. As others have said, DA40s don’t break and Va is the speed where the most aggressive control input will result in a stall before breaking the plane. That’s severe turbulence that you experience in a TS or a mountain rotor or a bad wake encounter. You don’t get that from thermal turbulence unless the wind is blowing hard.
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