CS 25.391 Control surface loads: general

ED Decision 2005/006/R

The control surfaces must be designed for the limit loads resulting from the flight conditions in CS 25.331, CS 25.341(a) and (b), CS 25.349 and CS 25.351, considering the requirements for:

(a) Loads parallel to hinge line, in CS 25.393;

(b) Pilot effort effects, in CS 25.397;

(c) Trim tab effects, in CS 25.407;

(d) Unsymmetrical loads, in CS 25.427; and

(e) Auxiliary aerodynamic surfaces, in CS 25.445.

[Amdt 25/1]

CS 25.393 Loads parallel to hinge line

ED Decision 2016/010/R

(See AMC 25.393)

(a) Control surfaces and supporting hinge brackets must be designed for inertia loads acting parallel to the hinge line. (See AMC 25.393(a).)

(b) In the absence of more rational data, the inertia loads may be assumed to be equal to KW, where –

(1) K = 24 for vertical surfaces;

(2) K = 12 for horizontal surfaces; and

(3) W = weight of the movable surfaces.

[Amdt 25/18]

AMC 25.393(a) Loads parallel to hinge line

ED Decision 2003/2/RM

The loads parallel to the hinge line on primary control surfaces and other movable surfaces, such as tabs, spoilers, speedbrakes, flaps, slats and all-moving tailplanes, should take account of axial play between the surface and its supporting structure in complying with CS 25.393(a). For the rational analysis, the critical airframe acceleration time history in the direction of the hinge line from all flight and ground design conditions (except the emergency landing conditions of CS 25.561) should be considered. The play assumed in the control surface supporting structure, should include the maximum tolerable nominal play and the effects of wear.

CS 25.395 Control system

ED Decision 2003/2/RM

(a) Longitudinal, lateral, directional and drag control systems and their supporting structures must be designed for loads corresponding to 125% of the computed hinge moments of the movable control surface in the conditions prescribed in CS 25.391.

(b) The system limit loads of paragraph (a) need not exceed the loads that can be produced by the pilot (or pilots) and by automatic or power devices operating the controls.

(c) The loads must not be less than those resulting from application of the minimum forces prescribed in CS 25.397(c).

CS 25.397 Control system loads

ED Decision 2016/010/R

(a) General. The maximum and minimum pilot forces, specified in sub-paragraph (c) of this paragraph, are assumed to act at the appropriate control grips or pads (in a manner simulating flight conditions) and to be reacted at the attachment of the control system to the control surface horn. 

(b) Pilot effort effects. In the control surface flight loading condition, the air loads on movable surfaces and the corresponding deflections need not exceed those that would result in flight from the application of any pilot force within the ranges specified in sub-paragraph (c) of this paragraph. Two-thirds of the maximum values specified for the aileron and elevator may be used if control surface hinge moments are based on reliable data. In applying this criterion, the effects of servo mechanisms, tabs, and automatic pilot systems, must be considered.

(c) Limit pilot forces and torques. The limit pilot forces and torques are as follows:

Control

Maximum forces or torques

Minimum forces or torques

Aileron:

Stick

Wheel*

 

445 N (100 lbf)

356 DNm (80 D in.lb)**

 

178 N (40 lbf)

178 DNm (40 D in.lbf)

Elevator:

Stick

Wheel (symmetrical)

Wheel (unsymmetrical)†

 

1112 N (250 lbf)

1335N(300 lbf)

 

445 N (100 lbf)

445 N(100 lbf)

445 N (100 lbf)

Rudder

1335 N (300 lbf)

578 N 130 lbf

*The critical parts of the aileron control system must be designed for a single tangential force with a limit value equal to 1·25 times the couple force determined from these criteria.

**D = wheel diameter in m (inches)

†The unsymmetrical forces must be applied at one of the normal handgrip points on the periphery of the control wheel.

(d) For aeroplanes equipped with side stick controls, designed for forces to be applied by one wrist and not by the arms, the limit pilot forces are as follows:

(1) For all components between and including the handle and its control stops:

PITCH

ROLL

Nose Up

890 N (200 lbf)

Roll Left

445 N (100lbf)

Nose Down

890 N (200 lbf)

Roll Right

445 N (100 lbf)

(2) For all other components of the side stick control assembly, but excluding the internal components of the electrical sensor assemblies, to avoid damage as a result of an in-flight jam:

PITCH

ROLL

Nose Up

556 N (125 lbf)

Roll Left

222 N (50 lbf)

Nose Down

556 N (125 lbf)

Roll Right

222 N (50 lbf)

[Amdt 25/13]

[Amdt 25/18]

CS 25.399 Dual control system

ED Decision 2006/005/R

(a) Each dual control system must be designed for the pilots operating in opposition, using individual pilot forces not less than –

(1) 0·75 times those obtained under CS 25.395; or

(2) The minimum forces specified in CS 25.397(c).

(b) The control system must be designed for pilot forces applied in the same direction, using individual pilot forces not less than 0·75 times those obtained under CS 25.395.

[Amdt 25/2]

CS 25.405 Secondary control system

ED Decision 2007/010/R

Secondary controls, such as wheel brake, spoiler, and tab controls, must be designed for the maximum forces that a pilot is likely to apply to those controls. The following values may be used:

PILOT CONTROL FORCE LIMITS (SECONDARY CONTROLS).

Control

Limit pilot forces

Miscellaneous:

*Crank, wheel, or lever.

but not less than 222 N (50 lbf) nor more than 667 N (150 lbf) (R = radius in mm).

(Applicable to any angle within 20o of plane of control).

Twist

15 Nm (133 in.lbf)

Push-pull

To be chosen by applicant.

*Limited to flap, tab, stabiliser, spoiler, and landing gear operation controls.

[Amdt. 25/3]

CS 25.407 Trim tab effects

ED Decision 2003/2/RM

The effects of trim tabs on the control surface design conditions must be accounted for only where the surface loads are limited by maximum pilot effort. In these cases, the tabs are considered to be deflected in the direction that would assist the pilot, and the deflections are –

(a) For elevator trim tabs, those required to trim the aeroplane at any point within the positive portion of the pertinent flight envelope in CS 25.333(b), except as limited by the stops; and

(b) For aileron and rudder trim tabs, those required to trim the aeroplane in the critical unsymmetrical power and loading conditions, with appropriate allowance for rigging tolerances.

CS 25.409 Tabs

ED Decision 2003/2/RM

(a) Trim tabs. Trim tabs must be designed to withstand loads arising from all likely combinations of tab setting, primary control position, and aeroplane speed (obtainable without exceeding the flight load conditions prescribed for the aeroplane as a whole), when the effect of the tab is opposed by pilot effort forces up to those specified in CS 25.397(b).

(b) Balancing tabs. Balancing tabs must be designed for deflections consistent with the primary control surface loading conditions.

(c) Servo tabs. Servo tabs must be designed for deflections consistent with the primary control surface loading conditions obtainable within the pilot manoeuvring effort, considering possible opposition from the trim tabs.

CS 25.415 Ground gust conditions

ED Decision 2016/010/R

(See AMC 25.415)

(a) The flight control systems and surfaces must be designed for the limit loads generated when the aircraft is subjected to a horizontal 33.44 m/sec (65 knots) ground gust from any direction, while taxying with the controls locked and unlocked and while parked with the controls locked.

(b) The control system and surface loads due to ground gust may be assumed to be static loads and the hinge moments H, in Newton metres (foot pounds), must be computed from the formula:

where:

K   = hinge moment factor for ground gusts derived in subparagraph (c) of this paragraph

ρo  = density of air at sea level = 1.225 (kg/m3) (0.0023769 (slugs/ft3) = 0.0023769 (lb-sec2/ ft4))

V   = 33.44 m/sec (65 knots = 109.71 fps) relative to the aircraft

S   = area of the control surface aft of the hinge line (m2) (ft2)

c   = mean aerodynamic chord of the control surface aft of the hinge line (m) (ft)

(c) The hinge moment factor K for ground gusts must be taken from the following table:

Surface

K

Position of controls

(a) Aileron

0.75

Control column locked or lashed in mid-position.

(b) Aileron

*±0.50

Ailerons at full throw.

(c) Elevator

*±0.75

Elevator full down.

(d) Elevator

*±0.75

Elevator full up.

(e) Rudder

0.75

Rudder in neutral.

(f) Rudder

0.75

Rudder at full throw.

* A positive value of K indicates a moment tending to depress the surface, while a negative value of K indicates a moment tending to raise the surface.

(d) The computed hinge moment of subparagraph (b) must be used to determine the limit loads due to ground gust conditions for the control surface. A 1.25 factor on the computed hinge moments must be used in calculating limit control system loads.

(e) Where control system flexibility is such that the rate of load application in the ground gust conditions might produce transient stresses appreciably higher than those corresponding to static loads, in the absence of a rational analysis an additional factor of 1.60 must be applied to the control system loads of subparagraph (d) to obtain limit loads. If a rational analysis is used, the additional factor must not be less than 1.20.

(f) For the condition of the control locks engaged, the control surfaces, the control system locks and the parts of the control systems (if any) between the surfaces and the locks must be designed to the respective resultant limit loads. Where control locks are not provided then the control surfaces, the control system stops nearest the surfaces and the parts of the control systems (if any) between the surfaces and the stops must be designed to the resultant limit loads. If the control system design is such as to allow any part of the control system to impact with the stops due to flexibility, then the resultant impact loads must be taken into account in deriving the limit loads due to ground gust.

(g) For the condition of taxying with the control locks disengaged, the following apply:

(1) The control surfaces, the control system stops nearest the surfaces and the parts of the control systems (if any) between the surfaces and the stops must be designed to the resultant limit loads.

(2) The parts of the control systems between the stops nearest the surfaces and the cockpit controls must be designed to the resultant limit loads, except that the parts of the control system where loads are eventually reacted by the pilot need not exceed:

(i) The loads corresponding to the maximum pilot loads in CS 25.397(c) for each pilot alone; or

(ii) 0.75 times these maximum loads for each pilot when the pilot forces are applied in the same direction

[Amdt 25/18]

AMC 25.415 Ground gust conditions

ED Decision 2006/005/R

1. PURPOSE. This AMC sets forth acceptable methods of compliance with the provisions of CS-25 dealing with the certification requirements for ground gust conditions. Guidance information is provided for showing compliance with CS 25.415, relating to structural design of the control surfaces and systems while taxying with control locks engaged and disengaged and when parked with control locks engaged. Other methods of compliance with the requirements may be acceptable.

2. RELATED CERTIFICATION SPECIFICATIONS.

CS 25.415 “Ground Gust Conditions”.

CS 25.519 “Jacking and Tie-down Provisions”

3. BACKGROUND.

a. The requirement to consider the effects of ground gusts has been applied to large/transport aeroplanes since 1950. The purpose of the requirement was to protect the flight control system from excessive peak ground wind loads while the aeroplane is parked or while taxying downwind. For developing the original regulation, the control surface load distribution was considered to be triangular with the peak at the trailing edge representing reversed flow over the control surface. This assumption, along with assumptions about the wind approach angle and typical control surface geometries were developed into a table of hinge moment factors and set forth in the regulation. These hinge moment factors have been carried forward to the existing table in CS 25.415. The maximum design wind speed was originally set at 96 km/h (88 feet per second (52 knots)) under the presumption that higher speeds were predictable storm conditions and the aircraft owner could take additional precautions beyond engaging the standard gust locks.

b. The conditions of CS 25.519 require consideration of the aeroplane in a moored or jacked condition in wind speeds up to 120 km/h (65 knots). In order to be consistent in the treatment of ground winds, the wind speeds prescribed by CS 25.415, concerning ground gust conditions on control surfaces, was increased to 120 km/h (65 knots) at Change 15 of JAR-25.

c. There have been several incidents and accidents caused by hidden damage that had previously occurred in ground gust conditions. Although many of these events were for aeroplanes that had used the lower wind speeds from the earlier rules, analysis indicates that the most significant contributor to the damage was the dynamic load effect. The dynamic effects were most significant for control system designs in which the gust locks were designed to engage the control system at locations far from the control surface horn. Based on these events additional factors are defined for use in those portions of the system and surface that could be affected by dynamic effects.

d. The flight control system and surface loads prescribed by CS 25.415 are limit loads based on a peak wind speed of 120 km/h (65 knots) EAS. In operation, the peak wind speed would most often be caused by an incremental fluctuation in velocity imposed on top of a less rapidly changing mean wind speed. Therefore, an appropriate peak wind speed limitation should be reflected in the applicable documents, when there is a potential risk of structural damage.

4.  COMPLIANCE.

a. The ground gust requirements take into account the conditions of the aeroplane parked with controls locked, and taxying with controls either locked or unlocked. In either of the locked conditions the control surface loads are assumed to be reacted at the control system locks. In the unlocked condition the pilot is assumed to be at the controls and the controls are assumed to be powered, if applicable. In the latter condition, the control surface loads are assumed to be reacted, if necessary, at the cockpit controls by the pilot(s) up to the limits of the maximum pilot forces and torques given in CS 25.397(c).

b. Where loads are eventually reacted at the cockpit controls, the loads in those parts of the control system between the control system stops nearest the control surfaces and the cockpit controls need not exceed those that would result from the application of the specified maximum pilot effort effects. However, higher loads can be reacted by the control system stops. Those parts of the control system from the control surfaces to the control system stops nearest the surfaces should be designed to the resultant limit loads including dynamic effects, if applicable, and regardless of pilot effort limitations. Similarly, pilot effort limitations would not apply to parts of control systems where the loads are not eventually reacted at the cockpit controls, for example an aileron control system where the right hand side aileron loads are reacted by the left hand side aileron, without participation by the pilot(s).

c. In either the taxying condition (controls locked or unlocked) or the parked condition (controls locked), if the control system flexibility is such that the rate of load application in the ground gust conditions might produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application are required to be considered. Manually powered control systems and control systems where the gust lock is located remotely from the control surface are examples of designs that might fall in this category. In such cases the control system loads are required by CS 25.415(e) to be increased by an additional factor over the standard factor of 1.25.

[Amdt 25/2]

CS 25.427 Unsymmetrical loads

ED Decision 2005/006/R

(a) In designing the aeroplane for lateral gust, yaw manoeuvre and roll manoeuvre conditions, account must be taken of unsymmetrical loads on the empennage arising from effects such as slipstream and aerodynamic interference with the wing, vertical fin and other aerodynamic surfaces.

(b) The horizontal tail must be assumed to be subjected to unsymmetrical loading conditions determined as follows:

(1) 100% of the maximum loading from the symmetrical manoeuvre conditions of CS 25.331 and the vertical gust conditions of CS 25.341(a) acting separately on the surface on one side of the plane of symmetry; and

(2) 80% of these loadings acting on the other side.

(c) For empennage arrangements where the horizontal tail surfaces have dihedral angles greater than plus or minus 10 degrees, or are supported by the vertical tail surfaces, the surfaces and the supporting structure must be designed for gust velocities specified in CS 25.341(a) acting in any orientation at right angles to the flight path.

(d) Unsymmetrical loading on the empennage arising from buffet conditions of CS 25.305(e) must be taken into account.

[Amdt 25/1]

CS 25.445 Outboard fins

ED Decision 2003/2/RM

(a) When significant, the aerodynamic influence between auxiliary aerodynamic surfaces, such as outboard fins and winglets, and their supporting aerodynamic surfaces must be taken into account for all loading conditions including pitch, roll and yaw manoeuvres, and gusts as specified in CS 25.341(a) acting at any orientation at right angles to the flight path.

(b) To provide for unsymmetrical loading when outboard fins extend above and below the horizontal surface, the critical vertical surface loading (load per unit area) determined under     CS 25.391 must also be applied as follows:

(1) 100% to the area of the vertical surfaces above (or below) the horizontal surface.

(2) 80% to the area below (or above) the horizontal surface.

CS 25.457 Wing-flaps

ED Decision 2003/2/RM

Wing flaps, their operating mechanisms, and their supporting structures must be designed for critical loads occurring in the conditions prescribed in CS 25.345, accounting for the loads occurring during transition from one wing-flap position and airspeed to another.

CS 25.459 Special devices

ED Decision 2003/2/RM

The loading for special devices using aero-dynamic surfaces (such as slots, slats and spoilers) must be determined from test data.