1 The critical parts of the aileron control system must be designed for a single tangential force with a limit value equal to 1.25 times the couple force determined from these criteria.
2 D = wheel diameter (inches).
3 The unsymmetrical forces must be applied at one of the normal handgrip points
on the periphery of the control wheel.
[Amdt. 25-38, 41 FR 55466, Dec. 20, 1976; Amdt. 25-72, 55 FR 29776, Jul. 20,
1990]
b. Intent of Rule. The purpose of this rule is to establish the maximum and minimum design values for the pilot forces to be used in design of the control systems. Special conditions have been issued for novel designs such as side stick controllers.
c. Background. The limit pilot force and torque requirements were carried forward from § 4b.227 of the Civil Air Regulations (CAR) with minor changes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-38, 41 FR 55466, December 20, 1976; and Amendment 25-72, 55 FR 29776, July 20, 1990.
e. References. A320 Special Conditions for side stick controllers, 54 FR 3986, January 27, 1989.
240 - 249. [RESERVED]
250. SECTION 25.399 - DUAL CONTROL SYSTEM.
a. Rule Text.
(a) Each dual control system must be designed for the pilots operating in
opposition, using individual pilot forces not less than -
(1) 0.75 times those obtained under § 25.395; or
(2) The minimum forces specified in § 25.397(c).
(b) The control system must be designed for pilot forces applied in the same direction, using individual pilot forces not less than 0.75 times those obtained under § 25.395.
b. Intent of Rule. The purpose of this rule is to ensure that the control system is sufficiently robust to withstand any likely pilot forces that could be applied when two pilots are acting in unison and in opposition.
c. Background. This rule remains basically unchanged from § 4b.225 of the Civil Air Regulations (CAR).
____________________________________________________________ ____________________________________________________________ ____________________________________________________________ 9/1/99 AC 25-21
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
251 - 259. [RESERVED]
260. SECTION 25.405 - SECONDARY CONTROL SYSTEM.
a. Rule Text.
Secondary controls, such as wheel brake, spoiler, and trim tab controls, must be designed for the maximum forces that a pilot is likely to apply to those controls.
The following values may be used:
PILOT CONTROL FORCE LIMITS (SECONDARY CONTROLS)
Control Limit pilot forces
(1 + R ) Miscellaneous: x50 in. lbs., but not less than Ł 3 ł *Crank, wheel, or 50 lbs. nor more than 150 lbs. (R=radius).
lever. (Applicable to any angle within 20o of plane of control).
Twist 133 in.-lbs.
Push-pull To be chosen by applicant.
*Limited to flap, tab, stabilizer, spoiler, and landing gear operation controls.
b. Intent of Rule. The purpose of this rule is to ensure that secondary control systems are designed to withstand the maximum forces that a pilot is likely to apply to these controls.
c. Background. This rule was carried forward from § 4b.227 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.
261 - 268. [RESERVED] 280
269. SECTION 25.407 - TRIM TAB EFFECTS.
a. Rule Text.
The effects of trim tabs on the control surface design conditions must be accounted for only where the surface loads are limited by maximum pilot effort.
In these cases, the tabs are considered to be deflected in the direction that would assist the pilot, and the deflections are -
(a) For elevator trim tabs, those required to trim the airplane at any point within the positive portion of the pertinent flight envelope in § 25.333(b), except as limited by the stops; and
(b) For aileron and rudder trim tabs, those required to trim the airplane in the critical unsymmetrical power and loading conditions, with appropriate allowance for rigging tolerances.
b. Intent of Rule. The purpose of this rule is to ensure that the additional loads produced by the trim tab deflections are accounted for in the structural design loads.
c. Background. This rule was carried forward from § 4b.220(b) of the Civil Air Regulations (CAR) with only minor editorial changes.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
270 - 275. [RESERVED]
276. SECTION 25.409 - TABS.
a. Rule Text.
(a) Trim tabs. Trim tabs must be designed to withstand loads arising from all likely combinations of tab setting, primary control position, and airplane speed (obtainable without exceeding the flight load conditions prescribed for the airplane as a whole), when the effect of the tab is opposed by pilot effort forces up to those specified in § 25.397(b).
(b) Balancing tabs. Balancing tabs must be designed for deflections consistent with the primary control surface loading conditions.
(c) Servo tabs. Servo tabs must be designed for deflections consistent with the primary control surface loading conditions obtainable within the pilot maneuvering effort, considering possible opposition from the trim tabs.
b. Intent of Rule. The purpose of this rule is to ensure that control surface tabs will withstand any likely loading conditions experienced in service.
c. Background. This rule was carried forward from § 4b.222 of the Civil Air Regulations (CAR) with minor editorial changes.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. Reference. None.
277 - 286. [RESERVED]
287. SECTION 25.415 - GROUND GUST CONDITIONS.
a. Rule Text.
(a) The control system must be designed as follows for control surface loads due to ground gusts and taxiing downwind:
(1) The control system between the stops nearest the surfaces and the cockpit controls must be designed for loads corresponding to the limit hinge moments H of paragraph (a)(2) of this section. These loads need not exceed -
(i) The loads corresponding to the maximum pilot loads in § 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.
(2) The control system stops nearest the surfaces, the control system locks, and the parts of the systems (if any) between these stops and locks and the control surface horns, must be designed for limit hinge moments H, in foot pounds, obtained from the formula, H = .0034KV 2cS, where - V = 65 (wind speed in knots) K = limit hinge moment factor for ground gusts derived in paragraph
(b) of this section.
c = mean chord of the control surface aft of the hinge line (ft);
S = area of the control surface aft of the hinge line (sq.ft);
_____________________________________________________ _____________________________________________________ _____________________________________________________ 9/1/99 AC 25-21
(b) The limit hinge moment factor K for ground gusts must be derived as follows:
Surface K Position of controls
(a) Aileron 0.75 Control column locked or lashed in mid-position.
(b) Aileron 1± 0.50 Ailerons at full throw.
(c) Elevator 1±0.75 (c) Elevator full down.
(d) Elevator 1±0.75 (d) Elevator full up.
(e) Rudder 0.75 (e) Rudder in neutral.
(f) Rudder 0.75 (f) Rudder at full throw.
1A 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.
[Amdt. 25-72, 55 FR 29776, Jul. 20, 1990; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the control systems and surfaces will not be damaged by likely ground gust conditions.
c. Background. This rule was carried forward from § 4b.226 of the Civil Air Regulations (CAR) with minor 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 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
288 - 297. [RESERVED] 284
298. SECTION 25.427 - UNSYMMETRICAL LOADS.
a. Rule Text.
(a) In designing the airplane for lateral gust, yaw maneuver and roll maneuver conditions, account must be taken of unsymmetrical loads on the empennage arising from effects such as slipstream and aerodynamic interference with the wing, vertical fin and other aerodynamic surfaces.
(b) The horizontal tail must be assumed to be subjected to unsymmetrical loading conditions determined as follows:
(1) 100 percent of the maximum loading from the symmetrical maneuver conditions of § 25.331 and the vertical gust conditions of § 25.341(a) acting separately on the surface on one side of the plane of symmetry; and
(2) 80 percent of these loadings acting on the other side.
(c) For empennage arrangements where the horizontal tail surfaces have dihedral angles greater than plus or minus 10 degrees, or are supported by the vertical tail surfaces, the surfaces and the supporting structure must be designed for gust velocities specified in § 25.341(a) acting in any orientation at right angles to the flight path.
(d) Unsymmetrical loading on the empennage arising from buffet conditions of § 25.305(e) must be taken into account.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-86, 61 FR 5220, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane is able to withstand any likely maneuver or gust condition that imposes unsymmetrical loads on these surfaces. The rule is intended to provide for round the clock gusts for geometries where directions other than vertical and lateral could be more critical. The rule is intended to account for unsymmetrical buffet loads on the horizontal tail, including those arising from stalls.
c. Background. This rule was carried forward from § 4b.220(c) of the Civil Air Regulations (CAR) with minor editorial changes in Amendment 25-23. Amendment 25-86 provided the round the clock gust criterion and required the consideration of buffet conditions on the horizontal tail.
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 25-86, 61 FR 5220, February 9, 1996.
e. References. None.
299 - 307. [RESERVED] 285
308. SECTION 25.445 - AUXILIARY AERODYNAMIC SURFACES.
a. Rule Text.
(a) When significant, the aerodynamic influence between auxiliary aerodynamic surfaces, such as outboard fins and winglets, and their supporting aerodynamic surfaces, must be taken into account for all loading conditions including pitch, roll, and yaw maneuvers, and gusts as specified in § 25.341(a) acting at any orientation at right angles to the flight path.
(b) To provide for unsymmetrical loading when outboard fins extend above and below the horizontal surface, the critical vertical surface loading (load per unit area) determined under § 25.391 must also be applied as follows:
(1) 100 percent to the area of the vertical surfaces above (or below) the
horizontal surface.
(2) 80 percent to the area below (or above) the horizontal surface.
[Amdt. 25-86, 61 FR 5222, Feb. 9, 1996
b. Intent of Rule. The purpose of this rule is to account for the redistribution of loads due to the interaction of intersecting aerodynamic surfaces.
c. Background. This requirement was carried forward from § 4b.220(d) of the Civil Air Regulations (CAR). In Amendment 25-86 the title was changed from “Outboard fins” to “Auxiliary Aerodynamic Surfaces” and the intent was extended to cover auxiliary aerodynamic surfaces such as winglets.
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.
309 - 318. [RESERVED]
319. SECTION 25.457 - WING FLAPS.
a. Rule Text.
Wing flaps, their operating mechanisms, and their supporting structures must be designed for critical loads occurring in the conditions prescribed in § 25.345, accounting for the loads occurring during transition from one flap position and airspeed to another.
b. Intent of Rule. The purpose of this rule is to ensure that the wing flaps and their supporting structures are able to withstand any loading condition likely to occur in service.
c. Background. This rule was carried forward from § 4b.221 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Advisory Circular (AC) 25-14, or latest revision, provides guidance for showing compliance with structural and functional safety standards for high lift and drag devices and their operating systems.
e. References. Advisory Circular 25-14, or latest revision, High Lift and Drag Devices.
320 - 328. [RESERVED]
329. SECTION 25.459 - SPECIAL DEVICES.
a. Rule Text.
The loading for special devices using aerodynamic surfaces (such as slots, slats and spoilers) must be determined from test data.
[Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure the loads used for design of these systems and surfaces are verified by test evidence.
c. Background. This rule was carried forward from § 4b.223 of the Civil Air Regulations (CAR).
Amendment 25-72 added slats to the list of items to which this rule applies.
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. Advisory Circular (AC) 25-14, or latest revision, provides guidance information for showing compliance.
e. References. Advisory Circular 25-14, or latest revision, High Lift and Drag Devices.
330 - 339. [RESERVED] 287
Section 6. GROUND LOADS.
340. SECTION 25.471 - GENERAL.
a. Rule Text.
(a) Loads and equilibrium. For limit ground loads -
(1) Limit ground loads obtained under this subpart are considered to be external forces applied to the airplane structure; and
(2) In each specified ground load condition, the external loads must be placed in equilibrium with the linear and angular inertia loads in a rational or conservative manner.
(b) Critical centers of gravity. The critical centers of gravity within the range for which certification is requested must be selected so that the maximum design loads are obtained in each landing gear element. Fore and aft, vertical, and lateral airplane centers of gravity must be considered. Lateral displacements of the c.g. from the airplane centerline which would result in main gear loads not greater than 103 percent of the critical design load for symmetrical loading conditions may be selected without considering the effects of these lateral c.g.
displacements on the loading of the main gear elements, or on the airplane structure provided
(1) The lateral displacement of the c.g. results from random passenger or cargo disposition within the fuselage or from random unsymmetrical fuel loading or fuel usage; and
(2) Appropriate loading instructions for random disposable loads are included under the provisions of § 25.1583(c)(1) to ensure that the lateral displacement of the center of gravity is maintained within these limits.
(c) Landing gear dimension data. Figure 1 of Appendix A contains the basic landing gear dimension data.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, landing gear and its supporting structure are able to withstand any load distributions likely to occur in service.
c. Background. This rule was carried forward from § 4b.230 of the Civil Air Regulations (CAR).
Amendment 25-23 added the requirement to account for lateral c.g. locations on ground loads.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5673, April 8, 1970.
e. References. None.
341 - 349. [RESERVED]
350. SECTION 25.473 - LANDING LOAD CONDITIONS AND ASSUMPTIONS.
a. Rule Text.
(a) For the landing conditions specified in §§ 25.479 to 25.485 the airplane is assumed to contact the ground -
(1) In the attitudes defined in § 25.479 and § 25.481;
(2) With a limit descent velocity of 10 fps at the design landing weight (the maximum weight for landing conditions at maximum descent velocity); and
(3) With a limit descent velocity of 6 fps at the design take-off weight (the maximum weight for landing conditions at a reduced descent velocity).
(4) The prescribed descent velocities may be modified if it is shown that the airplane has design features that make it impossible to develop these velocities.
(b) Airplane lift, not exceeding airplane weight, may be assumed unless the presence of systems or procedures significantly affects the lift.
(c) The method of analysis of airplane and landing gear loads must take into account at least the following elements:
(1) Landing gear dynamic characteristics.
(2) Spin-up and springback.
(3) Rigid body response.
(4) Structural dynamic response of the airframe, if significant.
(d) The limit inertia load factors corresponding to the required limit descent velocities must be validated by tests as defined in § 25.723(a).
(e) The coefficient of friction between the tires and the ground may be established by considering the effects of skidding velocity and tire pressure. However, this coefficient of friction need not be more than 0.8.
[Amdt. 25-23, 35 FR 5673, 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 airplane, landing gear and its supporting structure are able to withstand any load distributions likely to occur in service.