14 CFR § 25.361 - Engine and auxiliary power unit torque.
(a) For engine installations—
(1) Each engine mount, pylon, and adjacent supporting airframe structures must be designed for the effects of—
(i) A limit engine torque corresponding to takeoff power/thrust and, if applicable, corresponding propeller speed, acting simultaneously with 75% of the limit loads from flight condition A of § 25.333(b);
(ii) A limit engine torque corresponding to the maximum continuous power/thrust and, if applicable, corresponding propeller speed, acting simultaneously with the limit loads from flight condition A of § 25.333(b); and
(iii) For turbopropeller installations only, in addition to the conditions specified in paragraphs (a)(1)(i) and (ii) of this section, a limit engine torque corresponding to takeoff power and propeller speed, multiplied by a factor accounting for propeller control system malfunction, including quick feathering, acting simultaneously with 1g level flight loads. In the absence of a rational analysis, a factor of 1.6 must be used.
(2) The limit engine torque to be considered under paragraph (a)(1) of this section must be obtained by—
(i) For turbopropeller installations, multiplying mean engine torque for the specified power/thrust and speed by a factor of 1.25;
(ii) For other turbine engines, the limit engine torque must be equal to the maximum accelerating torque for the case considered.
(3) The engine mounts, pylons, and adjacent supporting airframe structure must be designed to withstand 1g level flight loads acting simultaneously with the limit engine torque loads imposed by each of the following conditions to be considered separately:
(i) Sudden maximum engine deceleration due to malfunction or abnormal condition; and
(ii) The maximum acceleration of engine.
(b) For auxiliary power unit installations, the power unit mounts and adjacent supporting airframe structure must be designed to withstand 1g level flight loads acting simultaneously with the limit torque loads imposed by each of the following conditions to be considered separately:
(1) Sudden maximum auxiliary power unit deceleration due to malfunction, abnormal condition, or structural failure; and
(2) The maximum acceleration of the auxiliary power unit.