Structural integrity in severe turbulence
Join Date: Aug 2003
Location: Sale, Australia
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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
§ 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.
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.
(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.
Join Date: Nov 2006
Location: Oz
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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.
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.
