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 locks 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 certification engineers and their designees. 

2.2 This material in this AC is neither mandatory nor regulatory in nature and does not 

constitute a regulation. While these guidelines are not mandatory, they are derived 

from extensive FAA and industry experience in determining compliance with the 

relevant regulations. These means are issued, 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 the rule. If, however, we become aware of 

circumstances that convince us that following 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 change or create any additional 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 applicability of this particular method of 

compliance when the acceptable method of compliance described in this AC is used. 
XX/XX/XX D R A F T AC 25.415-X 

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

provisions. 

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 requirement 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 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 § 25.415. 

The maximum design wind speed was originally set at 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. 

4.2 Amendment 25-91 incorporated a new requirement, § 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 consistent 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 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, 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 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. 

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 of 
XX/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 assumed to be reacted, if necessary, at the flight deck controls 

by the pilot(s) up to the limits of the maximum pilot forces and torques given in 

§ 25.397(c). 

5.2 Where loads are eventually reacted at the flight deck controls, the loads in those parts 

of the control system between the control system 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 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 flight deck 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). 

5.3 In either the taxiing 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 

must 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, § 25.415(e) requires the control system loads 

be increased by an additional factor over the standard factor of 1.25. 

END