CS 25.415 Ground gust conditions

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]