U.S. Department of Transportation

Federal Aviation

Administration 

# Advisory Circular

Subject: Ground Gust Conditions Date: D R A F T
Initiated By: ANM-115 AC No: 25.415-X

1. PURPOSE. This advisory circular (AC) describes acceptable means for showing compliance with the requirements of Title 14, Code of Federal Regulations (14 CFR) 25.415, Ground gust conditions . Section 25.415 specifies the gust load conditions that are assumed to occur while the airplane is taxiing with control locks engaged and disengaged, and when parked with control lo cks engaged. These load conditions apply to the flight control systems and surfaces.

2. APPLICABILITY.

2.1 The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) transport airplane type certificat ion engineers and their designees.

2.2 This material in this AC is neither manda tory nor regulatory in nature and does not constitute a regulation. While these guide lines are not mandatory, they are derived from extensive FAA and industry experien ce in determining compliance with the relevant regulations. These means are issue d, in the interest of standardization, for guidance purposes and to outline a method that has been found acceptable in showing compliance with the standards set forth in th e rule. If, however, we become aware of
circumstances that convince us that followi ng this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance.

2.3 The material in this AC does not chan ge or create any addi tional regulatory requirements, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.

2.4 Except in the explanations of what the regulations require, the term “must” is used in this AC only in the sense of ensuring app licability of this particular method of compliance when the acceptable method of co mpliance described in this AC is used.XX/XX/XX D R A F T AC 25.415-X

2 3. RELATED 14 CFR REGULATIONS. Section 25.519, Jacking and tie-down provisions

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4. BACKGROUND.

4.1 The requirement to consider the effects of ground gusts has been applied to transport airplanes since 1950. The purpose of the requir ement is to protect the flight control system from excessive peak ground wind loads while the airplane is parked or while taxiing downwind. For developing the orig inal 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 angl e and typical contro l surface geometries, were developed into a table of hinge moment factors and set forth in the regulation. These hinge moment factors ha ve been carried forward to the existing table in § 25.415. The maximum design wind speed was original ly set at 88 feet per second (52 knots) under the presumption that higher speeds we re predictable storm conditions and the aircraft owner could take additional pr ecautions beyond engaging the standard gust
locks.

4.2 Amendment 25-91 incorporated a new requi rement, § 25.519, for jacking and tie-down loads. This requirement was similar to the existing Joint Aviation Requirement 25.519 and required consideration of the airplane in a moored or jacked condition in wind speeds up to 65 knots. In order to be cons istent in the treatment of ground winds, the wind speed requirement in § 25.415 was increased to 65 knots at the same time.

4.3 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 airplanes that had used the lower wind speeds from earlier rules, analysis indicates that the most significant contributor to the da mage 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, Amendment 25-XX revises § 25.415 to include additional factors for use in those portions of the system and surfaces that could be affected by dynamic effects.

4.4 The flight control system and surface loads prescribed by § 25.415 are limit loads based on a peak wind speed of 65 knots equivalent airspeed. In operation, the peak wind speed would most often be caused by an incr emental fluctuation in velocity imposed on top of a less rapidly changing mean wind spee d. Therefore, an a ppropriate peak wind speed limitation should be reflected in th e applicable documents when there is a potential risk of structural damage.

5. COMPLIANCE.

5.1 The ground gust requirements take into account the conditions of the airplane parked with controls locked, and taxiing with controls either locked or unlocked. In either ofXX/XX/XX D R A F T AC 25.415-X

3 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 assume d to be reacted, if necessary, at the flight deck controls by the pilot(s) up to the limits of the ma ximum pilot forces and torques given in § 25.397(c).

5.2 Where loads are eventually reacted at the fli ght deck controls, the loads in those parts of the control system between the control sy stem stops nearest the control surfaces and the flight deck controls need not exceed those that would result from the application of the specified maximum pilot effort forces. However, higher loads can be reacted by the control system stops. Those parts of the cont rol system from the control surfaces to the control system stops nearest the surfaces should be designe d to the result ant limit loads including dynamic effects, if applicable, a nd 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 flight deck controls; for example, an aileron control system where the right hand side aile ron loads are reacted by the left hand side aileron, without particip ation by the pilot(s).

5.3 In either the taxiing conditi on (controls locked or unlocke d) or the parked condition (controls locked), if the control system fl exibility is such that the rate of load application in the ground gust conditions mi ght produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application must be considered. Manually powered cont rol systems and control systems where the gust lock is located remotely from the c ontrol surface are examples of designs that might fall in this category. In such cases, § 25.415(e) requires the control system loads be increased by an additional factor over the standard factor of 1.25.

END