defines Va as “Manoeuvring Speed is the maximum speed you may use abrupt control travel”
Some of the AFM/POH descriptions can present interpretative difficulties unless you know the background. However, the important takeaway is that Va has little to do with the idea of structural protection by stall before break. If the OEM's choice of Va is the minimum value, that may well be a useful outcome so far as pitch input is concerned. Without having undertaken an extensive research exercise, I can only presume that the majority of aircraft are designed on the Va min basis. Va is concerned with the structural design of the controls and related fittings.
Vo was introduced in an attempt to un-confuse the issue for pilots. Vo is defined as Va min. Progressively, with the introduction of newer certifications, we will see Va being the province of the OEM designers and pilots being interested in Vo rather than Va.
it lists Va as a variable of weight
That concern has been discussed earlier in the thread. If we are looking at Va min, then to maintain any value of Va for the pilot, the value on the day needs to reduce as weight reduces from MTOW so that the speed still gives the intersection of the stall line with the limit load factor ie some level of stall before break.
recommend turbulence penetration speeds in accordance with the variable Va
Sounds good to me. If I get into unexpected super severe turbulence, I want to be well below Va min (or Vo). Why ? Pretty simple. If the wings depart company with the aircraft, I'm dead. If I stall, I get to have a second bite at the cherry.
The problem, though, is that it is NO GOOD flying along at Vc and then observing that one should really be a lot slower - you are, very likely, dead before you get the chance to slow down, particularly in a higher performance, slippery machine. It is IMPERATIVE that the pilot be a thinking sort of person, anticipate the possibility of severe turbulence and slow down BEFORE the problem presents itself.
I’m not sure when Vo was defined, I assume it was sometime after 1977?
I'd have to look it up but, as I recall, it came in with an amendment in 2007.
I’d be curious to learn how many fresh 200 hour C210 pilots are aware of these limitations.
Probably, very few. Certainly, that has been my observation in theory instruction and such simulator instruction as I have done in previous years. The pilot text books don't talk about it (although I'm writing several such books and they, most certainly, will), the very great majority of instructors have no idea about it, and so the "blind leading the blind" syndrome just rolls on. There has been an ongoing effort by the Industry to fix this problem but it is going to take a LONG time to achieve that.
And finally, are there any other aircraft with Va or Vo limitations which vary with weight?
Without doing the research, I can only speculate that the majority of light aircraft will have Va based on Va min and, so, need to consider the weight variation thing. Keep in mind that Vo is a relatively new limit and will progressively (and slowly) come into the routine lexicon for pilots. Va will still remain as it is an engineering design limitation. Vo, one hopes, will give the pilot folks a more useful (and simple) limitation. However, the basics of the Va rules will apply. One control input at a time, to the stop (or checked) and then released, and no rocking and rolling on the controls - one input. For this reason, I prefer being quite below Va if things get frighteningly rough.
And indeed, any structural failure is usually from the accumulation of stresses accrued over a hard working lifetime.
Ah, yes and no. If you overload a structure excessively, it will break, even if the aircraft has only just entered service. As the aircraft ages, the reality is that the "fatigue bucket of life" progressively gets emptied. The result is that we need to do structural maintenance stuff to balance this - if a fatigue problem is not detected, for whatever reason, the aircraft can break, progressively at lower and lower applied loads.