#### § 25.415 Ground gust conditions.

(a) The flight control systems and surfaces must be designed for the limit loads generated when the airplane is subjected to a horizontal 65-knot ground gust from any direction while taxiing and while parked. For airplanes equipped with control system gust locks, the taxiing condition must be evaluated with the controls locked and unlocked, and the parked condition must be evaluated 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 must be computed from the formula:

H = K (1/2) ρo V2 c S

Where—

K = hinge moment factor for ground gusts derived in [paragraph (c)](/on/2024-09-19/title-14/section-25.415#p-25.415(c)) of this section;

ρo = density of air at sea level;

V = 65 knots relative to the aircraft;

S = area of the control surface aft of the hinge line;

c = mean aerodynamic chord of the control surface aft of the hinge line.

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

| Surface | K | Position of controls |
| --- | --- | --- |
| (1) Aileron | 0.75 | Control column locked or lashed in mid-position. |
| (2) Aileron | \* ±0.50 | Ailerons at full throw. |
| (3) Elevator | \* ±0.75 | Elevator full down. |
| (4) Elevator | \* ±0.75 | Elevator full up. |
| (5) Rudder | 0.75 | Rudder in neutral. |
| (6) 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 [paragraph (b)](/on/2024-09-19/title-14/section-25.415#p-25.415(b)) of this section 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 substantiating a different dynamic factor, an additional factor of 1.6 must be applied to the control system loads of [paragraph (d)](/on/2024-09-19/title-14/section-25.415#p-25.415(d)) of this section to obtain limit loads. If a rational analysis is used, the additional factor must not be less than 1.2.

(f) For the condition of the control locks engaged, the control surfaces, the control system locks, and the parts of any control systems between the surfaces and the locks must be designed to the resultant limit loads. Where control locks are not provided, then the control surfaces, the control system stops nearest the surfaces, and the parts of any control systems 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 taxiing with the control locks disengaged, or where control locks are not provided, the following apply:

(1) The control surfaces, the control system stops nearest the surfaces, and the parts of any control systems 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 flight deck 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 [§ 25.397(c)](/on/2024-09-19/title-14/section-25.397#p-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. 25-141, [79 FR 73468](https://www.federalregister.gov/citation/79-FR-73468), Dec. 11, 2014]