RobertL18C wrote:
Va I believe is only relevant for pitching around the lateral axis. The Harvard publishes Va for each of the axis, with yaw being the lowest, and rolling being the highest – for this type, YMMV for your type. Why a WW2 advanced trainer publishes this, but you can’t find the same information in GA types AFM, or why the concept of different Va for different axes is not taught am not sure.
Do you know what 3.8g is like in a light GA type? It is REALLY bad turbulence.
Absolutely, but it ought to be at least +6 g before anything actually breaks, because engineers add a whole heap of Jesus factors to cover variations in material qualities, workmanship, and other stuff we don’t know. That’s why our products don’t break as often as they otherwise might. Ultimately, we add unknown reserves of strength because our customers are willing to pay for it.
Slowing down in turbulence is surely not a bad idea, but if it still gives you the willies, why not thumb a lift with your local demented Pitts or Extra driver to experience +6 g?
ArcticChiller wrote:
Maybe I am just stating the obvious, but I believe that the average pilot underestimates the possible consequences of exceeding Va in turbulence.
For more advanced aircraft there is a turbulence penetration speed published Vb. This speed should be used when encountering turbulence, and not Va. For aircraft where no Vb is defined, it is usually encouraged that the pilot will use Vb = Va.
You may be right however that not everyone flies by the latter principle.
“engineers add a whole lot of Jesus factors”
We don’t always and I would suggest that no-one bets their life on it. The reserve factors are specified in the design codes and vary for different features but do not bet on higher than 1.5. It’s also no good looking at CS-23 if you are flying something with a Type Certificate issued in the 1950s. Stating the obvious but those reserve factors are to cover not only design and manufacturing but deterioration so if you are flying something built in 1974 just muse on the life it has had.
The general rule is that a structure may not deform up to the limit load (the service load) then up to the ultimate load (1.5 times the limit load usually) it may permanently deform but not break. Typically, to pass the certification test the ultimate load must be held once for three seconds. When we did this recently on a composite airframe, we also heated it to its maximum allowed flight temperature for the ultimate test.
Why people mix maneuvering with turbulence? I really don’t get it (although you might want to control the plane while in turbulent air, just like you might want to control the plane in more than one axis at a time). They are separate things and they’re dealt with separately – you have maneuvering envelope and gust envelope which comprise a flight envelope. Part 23 specifies several gust intensities at different speeds and altitudes (including 3,000 fpm at design cruise speed at altitudes up to 20,000 feet).
If Va (design maneuvering speed) is lower than Vc (design cruise speed), flying at Va gives you a bigger margin (no kidding, right? 
). But misunderstanding what Va is can lead to people killing themselves and plenty have.
If you want to look it up, it’s in Subpart C of Part 23. Just a few select sections:
§23.333 Flight envelope.
§23.335 Design airspeeds.
§23.337 Limit maneuvering load factors.
§23.341 Gust loads factors.
§23.423 Maneuvering loads.
§23.425 Gust loads.
Peter wrote:
IIRC this stuff got redefined IIRC after the 2001 US accident where the tail came off an Airbus, due to a certain type of rudder input.
I think that recommendation was to simply change wording in manuals so that it’s more clear to pilots that they can’t just do whatever they want with controls below Va which some apparently think/ thought (which shows a hole in training).
Martin wrote:
I think that recommendation was to simply change wording in manuals so that it’s more clear to pilots that they can’t just do whatever they want with controls below Va which some apparently think/ thought (which shows a hole in training).
IIRC the problem in the Airbus case was the dynamic effect of repeatedly giving large alternating rudder inputs. A single large rudder input would not have caused the failure.
Airborne_Again wrote:
IIRC the problem in the Airbus case was the dynamic effect of repeatedly giving large alternating rudder inputs. A single large rudder input would not have caused the failure.
Correct (well, that’s how I recall it as well). I believe analysis shoved that particular example performed substantially better than it was designed to (even after including a safety factor). And the pilot still managed to break it. But I think they wanted to change wording used in manuals so it’s harder to interpret it as you can yank (or in this case stomp) at the controls to your heart’s content and it won’t break up.
Jacko wrote:
Absolutely, but it ought to be at least +6 g before anything actually breaks, because engineers add a whole heap of Jesus factors to cover variations in material qualities, workmanship, and other stuff we don’t know. That’s why our products don’t break as often as they otherwise might.
If memory works, during my days with the slide ruler we have added only 25%, i.e everything was calculated at 1.25 safety factor.
This Flying magazine article has some useful comment on Va – note its applicability to 50 fps vertical gust speed and stabilator/elevator strength.
Va is a regulatory number as well. Case in point is the Commander 690-series. Due to two in-flight breakups the NTSB could not entirely explain – although at least in one, the test flight of a new prop and the fact it was almost 1000lbs above gross was a contributing factor – they still petitioned the FAA to lower Va from 170-180kt to 136kt on the 690A and B models via AD. So Va is what FAA says it is, it’s not always a purely aerodynamic number. I’m sure there are other aircraft that has had the same FAA treatment.
