d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; Amendment 25-72, 55 FR 29775, July 20, 1990; 55 FR 37607, September 12, 1990; 55 FR 41415, October 11, 1990; Amendment 25-86, 61 FR 5222, February 9, 1996; and Amendment 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
139 - 147. [RESERVED] 118
Section 4. SUPPLEMENTARY CONDITIONS
148. SECTION 25.361 - ENGINE TORQUE.
a. Rule Text.
(a) Each engine mount and its supporting structure must be designed for the effects of -
(1) A limit engine torque corresponding to takeoff power and propeller speed acting simultaneously with 75 percent of the limit loads from flight condition A of § 25.333(b);
(2) A limit torque corresponding to the maximum continuous power and propeller speed, acting simultaneously with the limit loads from flight condition A of § 25.333(b); and
(3) For turbopropeller installations, in addition to the conditions specified in paragraphs (a)(1) and (2) 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.
(b) For turbine engine installations, the engine mounts and supporting structure must be designed to withstand each of the following:
(1) A limit engine torque load imposed by sudden engine stoppage due to malfunction or structural failure (such as compressor jamming).
(2) A limit engine torque load imposed by the maximum acceleration of the engine.
(c) The limit engine torque to be considered under paragraph (a) of this section must be obtained by multiplying mean torque for the specified power and speed by a factor of -
(1) 1.25 for turbopropeller installations;
(2) 1.33 for reciprocating engines with five or more cylinders; or
(3) Two, three, or four, for engines with four, three, or two cylinders, respectively.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure the engine mounts and supporting structures are capable of withstanding any likely engine torque loads expected in service.
c. Background. This rule remains basically unchanged since it was adopted in part 4b of the Civil Air Regulations (CAR). Amendment 25-23 allowed a rational analysis to be used instead of the fixed factor of 1.6 specified in paragraph (a)(3) for propeller malfunction. If the fixed 1.6 factor is used, the
1.25 factor for limit torque is intended to be combined with the fixed factor to get the limit propeller malfunction torque factor of 2.0. Additional editorial changes have been made to this regulation by Amendment 25-72 to make the latter point clear.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-72, 55 FR 29776, July 20, 1990.
e. References. None.
149 - 157. [RESERVED]
158. SECTION 25.363 - SIDE LOAD ON ENGINE AND AUXILIARY POWER UNIT MOUNTS.
a. Rule Text.
(a) Each engine and auxiliary power unit mount and its supporting structure must be designed for a limit load factor in a lateral direction, for the side load on the engine and auxiliary power unit mount, at least equal to the maximum load factor obtained in the yawing conditions but not less than -
(1) 1.33; or
(2) one-third of the limit load factor for flight condition A as prescribed in
§ 25.333(b).
[Amdt. 25-23, 35 FR 5672, Apr. 8,1970; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the engine mount and its supporting structure are able to withstand any likely side load developed while operating anywhere within the design maneuvering envelope.
c. Background. This rule was carried forward from § 4b.216(b) of the Civil Air Regulations (CAR) with only minor editorial changes. Amendment 25-91 clarified the intent to include auxiliary power unit mounts.
d. Acceptable Compliance Methods. For guidance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8,1970; and Amendment 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
159 - 168. [RESERVED]
169. SECTION 25.365 - PRESSURIZED COMPARTMENT LOADS.
a. Rule Text.
For airplanes with one or more pressurized compartments the following apply:
(a) The airplane structure must be strong enough to withstand the flight loads combined with pressure differential loads from zero up to the maximum relief valve setting.
(b) The external pressure distribution in flight, and stress concentrations and fatigue effects must be accounted for.
(c) If landings may be made with the compartment pressurized, landing loads must be combined with pressure differential loads from zero up to the maximum allowed during landing.
(d) The airplane structure must be designed to be able to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1.33 for airplanes to be approved for operation to 45,000 feet or by a factor of 1.67 for airplanes to be approved for operation above 45,000 feet, omitting other loads.
(e) Any structure, component or part, inside or outside a pressurized compartment, the failure of which could interfere with continued safe flight and landing, must be designed to withstand the effects of a sudden release of pressure through an opening in any compartment at any operating altitude resulting from each of the following conditions:
(1) The penetration of the compartment by a portion of an engine following an engine disintegration;
(2) Any opening in any pressurized compartment up to the size Ho in square feet;
however, small compartments may be combined with an adjacent pressurized compartment and both considered as a single compartment for openings that 195
cannot reasonably be expected to be confined to the small compartment. The size Ho must be computed by the following formula:
Ho = PAs Where Ho = Maximum opening in square feet, need not exceed 20 square feet.
AP = s +.024 6240 As = Maximum cross-sectional area of the pressurized shell normal to the longitudinal axis, in square feet; and
(3) The maximum opening caused by airplane or equipment failures not shown to be extremely improbable.
(f) In complying with paragraph (e) of this section, the fail-safe features of the design may be considered in determining the probability of failure or penetration and probable size of openings, provided that possible improper operation of closure devices and inadvertent door openings are also considered. Furthermore, the resulting differential pressure loads must be combined in a rational and conservative manner with 1-g level flight loads and any loads arising from emergency depressurization conditions. These loads may be considered as ultimate conditions; however, any deformations associated with these conditions must not interfere with continued safe flight and landing. The pressure relief provided by intercompartment venting may also be considered.
(g) Bulkheads, floors, and partitions in pressurized compartments for occupants must be designed to withstand the conditions specified in paragraph (e) of this section. In addition, reasonable design precautions must be taken to minimize the probability of parts becoming detached and injuring occupants while in their seats.
[Amdt. 25-54, 45 FR 60172, Sep. 11, 1980; Amdt. 25-71, 55 FR 13477, Apr. 10, 1990; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990; Amdt. 25-87, 61 FR 28695, Jun 5, 1996]
b. Intent of Rule. The purpose of this rule is to ensure structural integrity of the fuselage pressure vessel for all operating conditions, and to ensure that the airplane will survive after experiencing sudden loss of cabin pressure in any compartment within or outside the pressure vessel.
c. Background. The basic requirement to consider pressure differential loads in combination with flight loads and external aerodynamic loads was contained in § 4b.216(c) of the Civil Air Regulations (CAR). The basic rule was expanded by Amendment 25-54 to include a requirement to consider the pressure differential loads resulting from a specified hole size in passenger and cargo compartments.
The rule was further amended by Amendment 25-71 to require a specified hole size in any compartment and to require that structures outside pressurized areas are also designed to withstand depressurization loads if their failure could interfere with continued safe flight and landing. Amendment 25-87 provided a higher pressure cabin design load factor for high altitude airplanes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-54, 45 FR 60172, September 11, 1980; Amendment 25-71, 55 FR 13477, April 10, 1990; Amendment 25-72, 55 FR 29776, July 20, 1990; and Amendment 25-87, 61 FR 28695, June 5, 1996.
e. References. None.
170 - 177. [RESERVED]
178. SECTION 25.367 - UNSYMMETRICAL LOADS DUE TO ENGINE FAILURE.
a. Rule Text.
The airplane must be designed for the unsymmetrical loads resulting from the failure of the critical engine. Turbopropeller airplanes must be designed for the following conditions in combination with a single malfunction of the propeller drag limiting system, considering the probable pilot corrective action on the fight controls:
(1) At speeds between VMC and VD, the loads resulting from power failure because of fuel flow interruption are considered to be limit loads.
(2) At speeds between VMC and VC, the loads resulting from the disconnection of the engine compressor from the turbine or from loss of the turbine blades are considered to be ultimate loads.
(3) The time history of the thrust decay and drag build-up occurring as a result of the prescribed engine failures must be substantiated by test or other data applicable to the particular engine-propeller combinations.
(4) The timing and magnitude of the probable pilot corrective action must be conservatively estimated, considering the characteristics of the particular engine propeller-airplane combination.
(b) Pilot corrective action may be assumed to be initiated at the time maximum yawing velocity is reached, but not earlier than two seconds after the engine failure. The magnitude of the corrective action may be based on the control forces specified in § 25.397(b) except that lower forces may be assumed where it 197
is shown by analysis or test that these forces can control the yaw and roll
resulting from the prescribed engine failure conditions.
b. Intent of Rule. This rule defines a limit load condition after loss of power in the most critical engine.
c. Background. This rule was carried forward from § 4b.216(d) of the Civil Air Regulations (CAR). Propeller driven airplanes experience a high drag load on the propeller when the engine fails.
For wing mounted engines, this propeller drag load produces high yawing moments on the airplane with the associated high loads on the engine supporting structure and on the vertical tail.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
179 - 187. [RESERVED]
188. SECTION 25.371 - GYROSCOPIC LOADS.
a. Rule Text.
The structure supporting any engine or auxiliary power unit must be designed for the loads including the gyroscopic loads arising from the conditions specified in §§ 25.331, 25.341(a), 25.349, 25.351, 25.473, 25.479, and 25.481, with the engine or auxiliary power unit at the maximum rpm appropriate to the condition. For the purposes of compliance with this section, the pitch maneuver in § 25.331(c)(1) must be carried out until the positive limit maneuvering load factor (point A2 in § 25.333(b)) is reached.
[Amdt. 25-86, 61 FR 5220, Feb. 9, 1996; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the airframe structure will support any likely loads developed from a combination of flight loads and gyroscopic loads while operating at maximum engine speed.
c. Background. This rule was carried forward from § 4b.216(e) of the Civil Air Regulations (CAR). Gyroscopic loads developed from engines and propellers have always been a concern for airplane structural design. Amendment 25-91 added the landing conditions (including dynamic landing).
The rule was also revised to ensure that, notwithstanding the revised 25.331(c)(1) of Amendment 25 91, the gyroscopic forces from the revised “unchecked” pitching maneuver would continue to be 198
developed considering a maneuver time history up to the point the normal load factor was developed on the airplane.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-86, 61 FR 5220, February 9, 1996; and Amendment 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
189 - 199. [RESERVED]
200. SECTION 25.373 - SPEED CONTROL DEVICES.
a. Rule Text.
If speed control devices (such as spoilers and drag flaps) are installed for use in en route conditions
(a) The airplane must be designed for the symmetrical maneuvers prescribed in § 25.333 and § 25.337, the yawing maneuvers prescribed in § 25.351, and the vertical and lateral gust conditions prescribed in § 25.341(a), at each setting and the maximum speed associated with that setting; and
(b) If the device has automatic operating or load limiting features, the airplane must be designed for the maneuver and gusts conditions prescribed in paragraph
(a) of this section, at the speeds and corresponding device positions that the mechanism allows.
[Amdt. 25-72, 55 FR 29776, Jul. 20, 1990; Amdt. 25-86, 61 FR 5222, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane structure is capable of withstanding the loads imposed when the speed control devices are deployed in any approved flight regime.
c. Background. The speed control device rule remains basically unchanged from the requirement of § 4b.217 of the Civil Air Regulations (CAR). Amendments 25-72 and 25-86 made editorial changes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-72, 55 FR 29776, July 20, 1990; and Amendment 25-86, 61 FR 5222, February 9, 1996.
e References. None.
201 - 208. [RESERVED] 200
Section 5. CONTROL SURFACE AND SYSTEM LOADS
209. SECTION 25.391 - CONTROL SURFACE LOADS: GENERAL.
a. Rule Text.
The control surfaces must be designed for the limit loads resulting from the flight conditions in §§ 25.331, 25.341(a), 25.349 and 25.351 and the ground gust conditions in § 25.415, considering the requirements for -
(a) Loads parallel to hinge line, in § 25.393;
(b) Pilot effort effects, in § 25.397
(c) Trim tab effects, in § 25.407;
(d) Unsymmetrical loads, § 25.427; and
(e) Auxiliary aerodynamic surfaces, in § 25.445.
[Amdt. 25-86, 61 FR 5222, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to ensure static strength in the control surfaces sufficient to withstand any likely loads developed either in flight or from ground gust conditions.
c. Background. This rule remains basically unchanged from the requirements of § 4b.220 of the Civil Air Regulations (CAR). Only minor conforming references were changed by Amendment 25-86.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-86, 61 FR 5222, February 9, 1996.
e. References. None.
210 - 217. [RESERVED]
218. SECTION 25.393 - LOADS PARALLEL TO HINGE LINE.
a. Rule Text.
(a) Control surfaces and supporting hinge brackets must be designed for inertia loads acting parallel to the hinge line.
(b) In the absence of more rational data, the inertia loads may be assumed to be equal to KW, where - 275
(1) K=24 for vertical surfaces;
(2) K=12 for horizontal surfaces; and
(3) W= weight of the movable surfaces.
b. Intent of Rule. This rule is intended to ensure adequate strength in the control surfaces and supporting brackets to withstand any likely loading conditions parallel to the control surface hinge line.
c. Background. This rule was carried forward from § 4b.220(e) of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
219 - 227. [RESERVED]
228. SECTION 25.395 - CONTROL SYSTEM.
a. Rule Text.
(a) Longitudinal, lateral, directional, and drag control system and their supporting structures must be designed for loads corresponding to 125 percent of the computed hinge moments of the movable control surface in the conditions prescribed in § 25.391.
(b) The system limit loads, except the loads resulting from ground gusts, need not exceed the loads that can be produced by the pilot (or pilots) and by automatic or power devices operating the controls.
(c) The loads must not be less than those resulting from application of the
minimum forces prescribed in § 25.397(c).
[Amdt. 25-23, 35 FR 5672, Apr. 8,1970; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
b. Intent of Rule. The intent of this rule is to provide design loads for the control system that are sufficient to provide a rugged system for service use, including consideration of jams, ground gusts, taxiing with tailwind, control inertia, and friction.
c. Background. The provisions of the current rule were essentially contained in § 4b.224 of the Civil Air Regulations (CAR). Control systems have remained basically the same, with minor changes for the addition of automatic and power operated flight controls.
_______________________________________________________ _______________________________________________________ _______________________________________________________ 9/1/99 AC 25-21
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8,1970; and Amendment 25-72, 55 FR 29776, July 20, 1990.
e. References. None.
229 - 238. [RESERVED]
239. SECTION 25.397 - CONTROL SYSTEM LOADS.
a. Rule Text.
(a) General. The maximum and minimum pilot forces, specified in paragraph (c) of this section, are assumed to act at the appropriate control grips or pads (in a manner simulating flight conditions) and to be reacted at the attachment of the control system to the control surface horn.
(b) Pilot effort effects. In the control surface flight loading condition, the air loads on movable surfaces and the corresponding deflections need not exceed those that would result in flight from the application of any pilot force within the ranges specified in paragraph (c) of this section. Two-thirds of the maximum values specified for the aileron and elevator may be used if control surface hinge moments are based on reliable data. In applying this criterion, the effects of servo mechanisms, tabs, and automatic pilot systems, must be considered.
(c) Limit pilot forces and torques. The limit pilot forces and torques are as follows Maximum Minimum
Control forces or forces or
torques torques
Aileron:
Stick 100 lbs. 40 lbs.
Wheel1 80 D in-lbs.2 40 D in-lbs.
Elevator:
Stick 250 lbs. 100 lbs.
Wheel(symmetrical) 300 lbs. 100 lbs.
Wheel (unsymmetrical)3 100 lbs.
Rudder 300 lbs. 130 lbs.