Structural integrity in severe turbulence

[Terry the bush]

Re: Light twin accident North of YMEL.

A spot for discussions on severe turbulence techniques :ok:

[Sexual Chocolate]

IF you are who you say you are, you're probably the most appropriate person to begin said discussion.....

[ForkTailedDrKiller]

"Severe turbulence techniques"

Technique #1: Avoid at all cost!

if you mess up Technique #1, move quickly to Technique #2 and #3.

Technique #2: Slow to appropriate turbulence penetration speed for your wt. Dropping the gear will slow you quickly. Who gives a rat's arse about the gear doors!

Technique #3: Keep the wings level and try to maintain S & L attitude with smooth movements of the controls. Don't concern yourself with altitude unless you are in danger of hitting something.

If #2 and #3 fail, move on to #4.

Technique #4: Stick your head between your legs and kiss your arse goodbye, while yelling "Faaaaaaaaaaaaaaaark" at the top of your voice.

Having tangled with a standing wave over Cunningham's Gap in Southern Qld, I can vouche for the effectiveness of #2 and #3.

I have yet to test #4.

Dr :cool:

[aussie027]

FTDKiller, summed it up pretty well,
If you suspect you may run into it, eg on descent below a cumulus cloud layer or near building/developed CBs etc slow down early to turb penetration speed if your acft has one or manoeuvring speed for your weight.
Remember Va applies in the longitudinal ie pitching plane only. It DOES NOT APPLY to partial or full deflection of the rudder or ailerons.

FAR 23 and FAR 25 certification standards explain this to designers and it came as a total shock to most pilots after the American Airlines A300 lost its fin and rudder in Nov 2001 climbing out of JFK in New York.

If I remember correctly the NTSB and Airbus engineers calculated that a sideload of 80-90,000 LBS was applied to the fin/rudder which is why it failed. It was not designed for anywhere near that. The left right left deflection of the rudder even below Va generated those staggering sideloads.
That is about 40t +, the weight of a fully loaded big rig???
My point is be aware that to use up to full rudder and/or aileron deflection will require a speed well below Va to be safe. The turbulence induced gust loads you are trying to counter will only add to the aerodynamic loads caused by control deflections.
Please forgive me if my numbers are off, it was from memory.
The FAA and NTSB has published some material on this matter after this terrible crash.

[Triple Captain]

We fly an aging fleet of aircraft. See how much confidence you have left in a 20,000 hour PA31 after you've just taken it through weather from hell (Noted in the log book as 'The Night from Hell') - In the dark, trying to shoot and approach while considering you would not have been the first person to have flown it through such weather.
Remember this accident involving an in-flight break up of a Chieftain?
http://www.atsb.gov.au/publications/investigation_reports/2005/AAIR/aair200506266.aspx Could it just have been the straw that broke the camel's back?


After dealing with TS every evening during summer (no wx radar no storm scope) some good advice was given:
1. Most of the TS activity is in the middle section
2. Get down to your LSALT
3. Stay visual - fly around them. This is not always possible (embedded CB / weather on the deck).


I wonder: How rough does it need to be to consider a reduction in airspeed from the green arc (allowed in rough air) to Vb? My theory is always be a gentle as possible to the airframe. I have made an approach from 10,000 with the undercarriage down in a Navajo (Vb 156kt).


Luckily most twin Cessna (excl 337) now have the SIDS program to go through, still not sure on these 20,000 hour Chieftains.
I find it interesting though, that Aero commanders (http://www.casa.gov.au/airworth/papers/AeroCommander.pdf) were found to have fatigue cracks only after a few thousand hours. I've flew one of these a few years back that had 24,000 hours on it. :eek:


An aircraft accident involving fatalities is always tragic to any pilot whether you knew the persons involved or not. I suppose if we (humans) were meant to fly we'd have our own set of wings.


I have a book that contains aviation related quotes:
"You should never fly through a thunderstorm in peace time"
One I’d like to add (heard around the traps):
"There will be days you wished you didn't have an instrument rating"


I think private pilots are onto something.

[Peter Fanelli]

One I’d like to add (heard around the traps):
"There will be days you wished you doing have an instrument rating"


I think private pilots are onto something.


Huh?

10 characters :hmm:

[Crosshair]

"You should never fly through a thunderstorm in peace time"
There is a funny story that reveals a great truth here.

It may or may not originate with the USAF 555th Fighter Squadron.

Above the squadron operations desk at their home base in Arizona in 1965, a sign: "There is no reason to fly through a thunderstorm in peacetime."

Above their squadron operations desk in Udorn, Thailand, in 1966: "There is no reason to fly through a thunderstorm."

[bushy]

A big thunderstorm can pull ANY airframe apart. They have about as much energy as a big bomb, but they are not just instantaneous.

[tinpis]

I wondered about descent in areas of mechanical/ thermal turb. like around Karratha?


If you were in cloud getting banged around that much you would probably consider slowing down a tad

Dont fly old airplanes from the West :rolleyes:

[Brian Abraham]

The following are extracts from the Beech 36 manual which backs up the Doctors post.

1. Flight in severe turbulence must be avoided. (To remind – severe is defined as – The airplane may be momentarily out of control and occupants are thrown violently against the belts and back into the seat. Unsecured objects are tossed about)

2. You should watch particularly your angle of bank, making turns as wide and shallow as possible. Be equally cautious in applying forward or back pressure to keep the airplane level. Maintain straight and level attitude in either up or down drafts. Use trim sparingly to avoid being grossly out of trim as the vertical air columns change velocity and direction. If necessary to avoid excessive airspeeds, lower the landing gear.

3. For years, Beech POH’s and FAA approved AFM’s have contained instructions that the landing gear should be extended in any circumstance in which the pilot encounters IFR conditions which approach the limits of his capability or ratings. Lowering the gear in IFR conditions or flight into heavy or severe turbulence, tends to stabilise the airplane, assists in maintaining proper airspeed, and will substantially reduce the possibility of reaching excessive airspeeds with catastrophic consequences, even where loss of control is experienced.

4. Know your airplane’s limitations, and your own. Never exceed either.

However you should be cognisant of item 4, as one piston I flew, in the turbulence penetration procedures contained the caution “Do not lower the landing gear during flight in turbulence”. Why so? The aircraft had a gear speed of 140k (note a Beech 36 is 154 except in emergency) and the recommended turbulence penetration speed was 125 to 185k (manoeuvring speed B36 141k). You could in fact take it out to 210k and still be inside the dynamic stall limit. The Vne (no yellow arc on this aircraft) was 325k (B36 top of the green 167k) so you can appreciate from the numbers why the procedures were different. We were fortunate to have “g” meters as well, and the rolling “g” limit was two thirds of the regular limit, and I would assume this is why Beech comments about making wide and shallow turns at 2 above.

Remember too, that severe to a light aircraft will probably get a “light chop” remark from a 747 crew.

[Tmbstory]

Technique #3 as shown in FDTKiller post.
Relating to the discussion on severe turbulence techniques be aware that on occasions the instruments & panel will shake so much that they can be difficult to interpret.

Use smooth control inputs and fly "trends"

[djpil]

Remember Va applies in the longitudinal ie pitching plane only. It DOES NOT APPLY to partial or full deflection of the rudder or ailerons.
FAR 23 and FAR 25 certification standards
NOPE!
Va does indeed apply to all controls but there are some additional factors for the designers:
- many aircraft types are certified to early versions of the regs, or even the old US CAR's so just don't read the current regs, go back to the applicable issue status as many things have changed over the years
- the designer must provide for full control deflection unless limited by "pilot effort". There are specified values of pilot forces for each of the controls which can mean, for many aircraft types, that only partial control deflection is designed for at Va.
I recall working with one test pilot who would easily apply more than the standard rudder pedal load. He couldn't believe it when we put a load cell on the pedal and told him how hard he was pushing.

[aussie027]

DJPIL

- the designer must provide for full control deflection unless limited by "pilot effort". There are specified values of pilot forces for each of the controls which can mean, for many aircraft types, that only partial control deflection is designed for at Va.As you say in many aircraft types that means partial control deflection only at Va.

The standard V/N Diagram is for load factor in the pitching plane only and of course shows the edges of any planes envelope based on its structural design limits and Va at the intersection of the stall limit at max load factor.
The Airbus accident highlighted the complexities of the FARs Part 23 and 25 which designers know well but many pilots do not.
Here in the States after the accident and the investigation there were many good articles in the pilot magazines explaining in detail the basic point I was trying to make which was that full control deflection of the aileron and or rudder at or even below Va down to some given speed will very possibly lead to a structural failure. This came as a big shock to many pilots including extremely experienced airline veterans who were unaware of this fact.
Sorry I have not recently read those FAR paras applicable.

[Brian Abraham]

Va does indeed apply to all controls

NO just to get your attention

Maneuvering speed is usually defined—without regard to asymmetrical loads—as the maximum speed at which full or abrupt combined control movements can be made without damaging the aircraft. The FAA’s AC 61-23C, “Pilot’s Handbook of Aeronautical Knowledge,” says that “any combination of flight control usage, including full deflection of the controls, or gust loads created by turbulence should not create an excessive air load if the airplane is operated below maneuvering speed.” According to the US Navy, “Any combination of maneuver and gust cannot create damage due to excess airload when the airplane is below the maneuver speed.”

The NTSB has pointed out that this broader definition, although widespread among pilots, is incorrect. Engineers consider each axis separately in designing for the air loads accompanying an abrupt, full control input at maneuvering speed. “Full inputs in more than one axis at the same time and multiple inputs in one axis are not considered in designing for these [VA] flight conditions.”
The particular “multiple inputs” that prompted NTSB comment were the rudder reversals leading to a yaw over swing followed by a final reversal that destroyed the vertical tail of American Airlines Flight 587 on November 12, 2001.

Va (maneuvering speed) is the maximum speed at which a full, abrupt elevator movement, a gust, or a combination of the two will cause the wing to stall rather than bend. (The concept of maneuvering speed properly refers to symmetrical flight conditions, meaning no aileron or rudder involved. Somehow, the definition incorrectly got extended to include those surfaces.) Va is a fixed theoretical calculation relative to Vs1. For a utility category light aircraft (whose certificated vertical load limit factor is +4.4g) Va =square root4.4*Vs1, or just over twice Vs1.

[youngmic]

The same as the previous.

Velocity squared divided by Vs1 = G's

For a Shrike, Vs1 approx. 67 kts, therefore 172 squared divided by 67 squared = 6.6 Gs ultimate load factor and 141 kts = 4.4 Gs design load max.

I've always been under the text book impression of all controls full deflection, but gut instinct would have you believe the combined torsional twisting moment of full aileron at a max design load won't help the cause.

Used to be that FAR 23 gust loads were based on a vertical component of 15 m/s believe it latter changed to 30m/s. But could be wrong on this.

M

[Brian Abraham]

Va does indeed apply to all controls

YESmake up yor mind

As they say in "Little Britian", "Yes, but....." and the kicker is in the "but" - no pun intended. Va is certainly used as a benchmark for the design of all controls (aileron, rudder and elevator) but the prevailing pilot view that you can sit there pushing and pulling from one stop to the other while at Va is where you can come to grief big time. For those inclined look up FAR 23 at www.faa.gov - there is a lot of information on the requirements to get your head around, but a very good and simple pilot speak paper discussing this issue can be found at

http://www.flightlab.net/pdf/8_Maneuvering.pdf

[gaunty]

youngmic

Used to be that FAR 23 gust loads were based on a vertical component of 15 m/s believe it latter changed to 30m/s. But could be wrong on this. may be may be not, whatever the case may be, if the aircraft was certified at the "old or previous" vertical component any later change is simply academic.

Think grandfathering and why it is unlikely that the type would not now be certified as is were it to be presented as a newbie, same as the B747 100.

It has always been a mystery to me (not really) why I hear so many youngsters speak in awe of a type that has such a long history. There is an inevitability and confluence of history, events and personalities here that I dare not yet discuss. When the times right perhaps.

Go to the reference to which Brian refers and have a close look at the number of amendments for each section. There is a hard earned reason for every one of them the beneifts of which aircraft certified before each amendment did not necessarily share.

[aussie027]

Thanks Brian, Well put . That looks like a great link. I will have a read when time permits. :ok::ok:

Va (maneuvering speed) is the maximum speed at which a full, abrupt elevator movement, a gust, or a combination of the two will cause the wing to stall rather than bend. (The concept of maneuvering speed properly refers to symmetrical flight conditions, meaning no aileron or rudder involved. Somehow, the definition incorrectly got extended to include those surfaces.)

Good way to define it :D and as you said here

Maneuvering speed is usually defined—without regard to asymmetrical loads—as the maximum speed at which full or abrupt combined control movements can be made without damaging the aircraft. The FAA’s AC 61-23C, “Pilot’s Handbook of Aeronautical Knowledge,” says that “any combination of flight control usage, including full deflection of the controls, or gust loads created by turbulence should not create an excessive air load if the airplane is operated below maneuvering speed.” According to the US Navy, “Any combination of maneuver and gust cannot create damage due to excess airload when the airplane is below the maneuver speed.”

The NTSB has pointed out that this broader definition, although widespread among pilots, is incorrect. Engineers consider each axis separately in designing for the air loads accompanying an abrupt, full control input at maneuvering speed. “Full inputs in more than one axis at the same time and multiple inputs in one axis are not considered in designing for these [VA] flight conditions.”
The particular “multiple inputs” that prompted NTSB comment were the rudder reversals leading to a yaw over swing followed by a final reversal that destroyed the vertical tail of American Airlines Flight 587 on November 12, 2001.
The old definition we were probably all taught at some point is just plain wrong. :=:=

[Oktas8]

If I was a newbie, I could be quite confused by now.

Here's the correct answer as I understand it. Usual caveat about please correct me if I'm wrong, etc.

Va does apply to all the primary controls. It always has. The "BUT" is that it only applies to one control surface at a time, and then only for one single full deflection from neutral, as opposed to multiple L-R-L-R inputs.

Wasn't that the lesson from the oft-quoted Airbus rudder accident? Pretty much everyone knew about the first two phrases in bold, but the third was the surprising one. Guess no-one had sat there hammering away at the footrests in turbulence before.

Thanks ForkTailed one - your post very helpful.

[Centaurus]

Relating to the discussion on severe turbulence techniques be aware that on occasions the instruments & panel will shake so much that they can be difficult to interpret
Actually, its not that the instrument panel shakes that much. The blurring of the instruments during certain frequencies of turbulence is caused by "eye-ball bounce." Like being on the end of a tuning fork. Your eye-balls are oscillating so quickly in turbulence that you cannot focus.
One of the factors in successful handling of severe turbulence - especially at night or in IMC - is confidence and currency at unusual attitude recovery. While PPL and CPL pilots are supposedly instructed on UA recovery in a C150 and even during instrument rating renewals an ATO may require you to demonstrate competence in these manoeuvres, the fact is there is an obvious flight safety limit to demonstrating competence in a real aircraft.

Rather than disregard the problem altogether and rely on past experience at UA's in a light single engine trainer to get you out of future trouble IMC, it is worthwhile practicing UA recovery training in a synthetic trainer or full flight simulator. This way, extreme nose high/low/rolling and inverted attitude recovery can be safely recognised on instruments and no one gets hurt. Better that than nothing at all and simply relying on luck. The control forces experienced in severe turbulence cannot be replicated in simulators but the extreme attitudes displayed on flight instruments certainly can. Again - better than nothing.

[Brian Abraham]

I just had a look at the aerody book of a current CPL student and all it has to say with regard to Va is "Manoeuvring speed - the maximum allowable speed for maximum control deflection". And thats it, so its not surprising many may find themselves confused. If the Mods will pardon the use of bandwidth the following may be educational to the serious/interested student. As you will readily discern it is a complex area, but Oktas8 has it in a nutshell when he says
Va does apply to all the primary controls. It always has. The "BUT" is that it only applies to one control surface at a time, and then only for one single full deflection from neutral, as opposed to multiple L-R-L-R inputs.

§ 23.333 Flight envelope.
(a) General. Compliance with the strength requirements of this subpart must be shown at any combination of airspeed and load factor on and within the boundaries of a flight envelope (similar to the one in paragraph (d) of this section) that represents the envelope of the flight loading conditions specified by the maneuvering and gust criteria of paragraphs (b) and (c) of this section respectively.
(b) Maneuvering envelope. Except where limited by maximum (static) lift coefficients, the airplane is assumed to be subjected to symmetrical maneuvers resulting in the following limit load factors:
(1) The positive maneuvering load factor specified in §23.337 at speeds up to V D;
(2) The negative maneuvering load factor specified in §23.337 at V C; and
(3) Factors varying linearly with speed from the specified value at V Cto 0.0 at V Dfor the normal and commuter category, and −1.0 at V Dfor the acrobatic and utility categories.
(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 f.p.s. at V Cmust be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25 f.p.s. at 50,000 feet.
(ii) Positive and negative gusts of 25 f.p.s. at V Dmust be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at 50,000 feet.
(iii) In addition, for commuter category airplanes, positive (up) and negative (down) rough air gusts of 66 f.p.s. at VΒ must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 66 f.p.s. at 20,000 feet to 38 f.p.s. at 50,000 feet.

§ 23.335 Design airspeeds.
Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds (EAS).
(a) Design cruising speed, V C.For V Cthe following apply:
(1) Where W/S′=wing loading at the design maximum takeoff weight, Vc(in knots) may not be less than—
(i) 33 √(W/S) (for normal, utility, and commuter category airplanes);
(ii) 36 √(W/S) (for acrobatic category airplanes).
(2) For values of W/S more than 20, the multiplying factors may be decreased linearly with W/S to a value of 28.6 where W/S =100.
(3) V Cneed not be more than 0.9 V Hat sea level.
(4) At altitudes where an M Dis established, a cruising speed M Climited by compressibility may be selected.
(b) Design dive speed V D.For V D,the following apply:
(1) V D/MDmay not be less than 1.25 V C/MC; and
(2) With V C min,the required minimum design cruising speed, V D(in knots) may not be less than—
(i) 1.40 V c min(for normal and commuter category airplanes);
(ii) 1.50 V C min(for utility category airplanes); and
(iii) 1.55 V C min(for acrobatic category airplanes).
(3) For values of W/S more than 20, the multiplying factors in paragraph (b)(2) of this section may be decreased linearly with W/S to a value of 1.35 where W/S =100.
(4) Compliance with paragraphs (b)(1) and (2) of this section need not be shown if V D /M Dis selected so that the minimum speed margin between V C /M Cand V D /M Dis the greater of the following:
(i) The speed increase resulting when, from the initial condition of stabilized flight at V C /M C,the airplane is assumed to be upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up with a load factor of 1.5 (0.5 g. acceleration increment). At least 75 percent maximum continuous power for reciprocating engines, and maximum cruising power for turbines, or, if less, the power required for V C/ M Cfor both kinds of engines, must be assumed until the pullup is initiated, at which point power reduction and pilot-controlled drag devices may be used; and either—
(ii) Mach 0.05 for normal, utility, and acrobatic category airplanes (at altitudes where MDis established); or
(iii) Mach 0.07 for commuter category airplanes (at altitudes where MDis established) unless a rational analysis, including the effects of automatic systems, is used to determine a lower margin. If a rational analysis is used, the minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts), and the penetration of jet streams or cold fronts), instrument errors, airframe production variations, and must not be less than Mach 0.05.
(c) Design maneuvering speed V A.For V A,the following applies:
(1) V Amay not be less than V S√ n where—
(i) V Sis a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, C NA ; and
(ii) n is the limit maneuvering load factor used in design
(2) The value of V Aneed not exceed the value of V Cused in design.
(d) Design speed for maximum gust intensity, V B. For VB, the following apply:
(1) VBmay not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ngthe positive airplane gust load factor due to gust, at speed VC(in accordance with §23.341), and at the particular weight under consideration; and
(ii) VS1is the stalling speed with the flaps retracted at the particular weight under consideration.
(2) VBneed not be greater than VC.

§ 23.337 Limit maneuvering load factors.
(a) The positive limit maneuvering load factor n may not be less than—
(1) 2.1+(24,000÷(W+10,000)) for normal and commuter category airplanes, where W=design maximum takeoff weight, except that n need not be more than 3.8;
(2) 4.4 for utility category airplanes; or
(3) 6.0 for acrobatic category airplanes.
(b) The negative limit maneuvering load factor may not be less than—
(1) 0.4 times the positive load factor for the normal utility and commuter categories; or
(2) 0.5 times the positive load factor for the acrobatic category.
(c) Maneuvering load factors lower than those specified in this section may be used if the airplane has design features that make it impossible to exceed these values in flight.

§ 23.349 Rolling conditions.
The wing and wing bracing must be designed for the following loading conditions:
(a) Unsymmetrical wing loads appropriate to the category. Unless the following values result in unrealistic loads, the rolling accelerations may be obtained by modifying the symmetrical flight conditions in §23.333(d) as follows:
(1) For the acrobatic category, in conditions A and F, assume that 100 percent of the semispan wing airload acts on one side of the plane of symmetry and 60 percent of this load acts on the other side.
(2) For normal, utility, and commuter categories, in Condition A, assume that 100 percent of the semispan wing airload acts on one side of the airplane and 75 percent of this load acts on the other side.
(b) The loads resulting from the aileron deflections and speeds specified in §23.455, in combination with an airplane load factor of at least two thirds of the positive maneuvering load factor used for design. Unless the following values result in unrealistic loads, the effect of aileron displacement on wing torsion may be accounted for by adding the following increment to the basic airfoil moment coefficient over the aileron portion of the span in the critical condition determined in §23.333(d):
Δ c m=−0.01δ
where—
Δ c mis the moment coefficient increment; and
δ is the down aileron deflection in degrees in the critical condition.

§ 23.423 Maneuvering loads.
Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.
(b) A sudden aft movement of the pitching control at speeds above VA, followed by a forward movement of the pitching control resulting in the following combinations of normal and angular acceleration:

Condition Normal acceleration (n) Angular acceleration(radian/sec2)
Nose-up pitching 1.0 +39nm÷V×(nm−1.5)
Nose-down pitching nm −39nm÷V×(nm−1.5)
where—
(1) nm=positive limit maneuvering load factor used in the design of the airplane; and
(2) V=initial speed in knots.
The conditions in this paragraph involve loads corresponding to the loads that may occur in a “checked maneuver” (a maneuver in which the pitching control is suddenly displaced in one direction and then suddenly moved in the opposite direction). The deflections and timing of the “checked maneuver” must avoid exceeding the limit maneuvering load factor. The total horizontal surface load for both nose-up and nose-down pitching conditions is the sum of the balancing loads at V and the specified value of the normal load factor n, plus the maneuvering load increment due to the specified value of the angular acceleration.

§ 23.427 Unsymmetrical loads.
(a) Horizontal surfaces other than main wing and their supporting structure must be designed for unsymmetrical loads arising from yawing and slipstream effects, in combination with the loads prescribed for the flight conditions set forth in §§23.421 through 23.425.
(b) In the absence of more rational data for airplanes that are conventional in regard to location of engines, wings, horizontal surfaces other than main wing, and fuselage shape:
(1) 100 percent of the maximum loading from the symmetrical flight conditions may be assumed on the surface on one side of the plane of symmetry; and
(2) The following percentage of that loading must be applied to the opposite side:
Percent=100−10 (n−1), where n is the specified positive maneuvering load factor, but this value may not be more than 80 percent.
(c) For airplanes that are not conventional (such as airplanes with horizontal surfaces other than main wing having appreciable dihedral or supported by the vertical tail surfaces) the surfaces and supporting structures must be designed for combined vertical and horizontal surface loads resulting from each prescribed flight condition taken separately.

§ 23.441 Maneuvering loads.
(a) At speeds up to V A,the vertical surfaces must be designed to withstand the following conditions. In computing the loads, the yawing velocity may be assumed to be zero:
(1) With the airplane in unaccelerated flight at zero yaw, it is assumed that the rudder control is suddenly displaced to the maximum deflection, as limited by the control stops or by limit pilot forces.
(2) With the rudder deflected as specified in paragraph (a)(1) of this section, it is assumed that the airplane yaws to the overswing sideslip angle. In lieu of a rational analysis, an overswing angle equal to 1.5 times the static sideslip angle of paragraph (a)(3) of this section may be assumed.
(3) A yaw angle of 15 degrees with the rudder control maintained in the neutral position (except as limited by pilot strength).
(b) For commuter category airplanes, the loads imposed by the following additional maneuver must be substantiated at speeds from VAto VD/MD. When computing the tail loads—
(1) The airplane must be yawed to the largest attainable steady state sideslip angle, with the rudder at maximum deflection caused by any one of the following:
(i) Control surface stops;
(ii) Maximum available booster effort;
(iii) Maximum pilot rudder force as shown below:
(2) The rudder must be suddenly displaced from the maximum deflection to the neutral position.
(c) The yaw angles specified in paragraph (a)(3) of this section may be reduced if the yaw angle chosen for a particular speed cannot be exceeded in—
(1) Steady slip conditions;
(2) Uncoordinated rolls from steep banks; or
(3) Sudden failure of the critical engine with delayed corrective action.

§ 23.455 Ailerons.
(a) The ailerons must be designed for the loads to which they are subjected—
(1) In the neutral position during symmetrical flight conditions; and
(2) By the following deflections (except as limited by pilot effort), during unsymmetrical flight conditions:
(i) Sudden maximum displacement of the aileron control at V A.Suitable allowance may be made for control system deflections.
(ii) Sufficient deflection at V C,where V Cis more than V A,to produce a rate of roll not less than obtained in paragraph (a)(2)(i) of this section.
(iii) Sufficient deflection at V Dto produce a rate of roll not less than one-third of that obtained in paragraph (a)(2)(i) of this section.

[Zhaadum]

I am so not reading all that. Whew!
Simple answer, don't fly if severe turbulence is forecast.

Z.:ok:

[Barry Bernoulli]

Zhaadum,

It's not that simple. There are areas of Australia where severe turbulence may be forecast for days on end. For example, on descent into Sydney below A100, most of eastern Victoria below A100.

There may be a SIGMET for isolated or occasional severe turb for a wide area for days on end. Not flying may not be an option, and need not be the answer.

The first issue is the correct definition of severe turbulence, and the second issue is the design strength of aircraft. (Both discussed previously)

If you are aware that severe turbulence may be encountered, then you can fly accordingly; ie, everything strapped in/down and not above VA.

Of the many times I've flown through areas of forecast severe turbulence, I've rarely encountered it. I have also encountered severe turbulence where it was not forecast

In my opinion, better to know how to operate in severe turbulence and the implications than to hope it never happens.

It's the same as boating. I wouldn't go fishing in gale force winds, but I need to know how to get the boat to safety if I get stuck in them.