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.
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.