b. Intent of Rule. The purpose of this rule is to provide a method for calculating the water reaction load factor distribution for use in calculating hull loads.
c. Background. This rule was carried forward from § 4b.253 of the Civil Air Regulations (CAR).
Amendment 25-23 made minor editorial changes.
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.
561 - 568. [RESERVED]
569. SECTION 25.529 - HULL AND MAIN FLOAT LANDING CONDITIONS.
a. Rule Text.
(a) Symmetrical step, bow, and stern landing. For symmetrical step, bow, and stern landings, the limit water reaction load factors are those computed under § 25.527. In addition -
(1) For symmetrical step landings, the resultant water load must be applied at the keel, through the center of gravity, and must be directed perpendicularly to the keel line;
(2) For symmetrical bow landings, the resultant water load must be applied at the keel, one-fifth of the longitudinal distance from the bow to the step, and must be directed perpendicularly to the keel line; and
(3) For symmetrical stern landings, the resultant water load must be applied at the keel, at a point 85 percent of the longitudinal distance from the step to the stern post, and must be directed perpendicularly to the keel line.
(b) Unsymmetrical landing for hull and single float seaplanes. Unsymmetrical step, bow, and stern landing conditions must be investigated. In addition -
(1) The loading for each condition consists of an upward component and a side component equal, respectively, to 0.75 and 0.25 tan ß times the resultant load in the corresponding symmetrical landing condition; and
(2) The point of application and direction of the upward component of the load is the same as that in the symmetrical condition, and the point of application of the side component is at the same longitudinal station as the upward component but is directed inward perpendicularly to the plane of symmetry at a point midway between the keel and chine lines.
(c) Unsymmetrical landing; twin float seaplanes. The unsymmetrical loading consists of an upward load at the step of each float of 0.75 and a side load of 0.25 458
tan ß at one float times the step landing load reached under § 25.527. The side load is directed inboard, perpendicularly to the plane of symmetry midway between the keel and chine lines of the float, at the same longitudinal station as the upward load.
b. Intent of Rule. The purpose of this rule is to define the types of landing conditions to be considered for structural design.
c. Background. This rule was carried forward from § 4b.254 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.
570 - 575. [RESERVED]
576. SECTION 25.531 - HULL AND MAIN FLOAT TAKEOFF CONDITION.
a. Rule Text.
For the wing and its attachment to the hull or main float
(a) The aerodynamic wing lift is assumed to be zero; and
(b) A downward inertia load, corresponding to a load factor computed from the following formula, must be applied:
C V SI 2 TOn = 2 /3 1/ 3 tan( b W ) where - n = inertia load factor;
CTO = empirical seaplane operations factor equal to 0.004;
VSI = seaplane stalling speed (knots) at the design takeoff weight with the flaps
extended in the appropriate takeoff position;
b = angle of dead rise at the main step (degrees); and
W = design water takeoff weight in pounds.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the wing and its attachment to the hull or main float are able to withstand the inertia loads likely to be encountered in service.
c. Background. This rule was carried forward from § 4b.255 of the Civil Air Regulations (CAR).
Amendment 25-23 made only minor editorial changes.
d. Acceptable Compliance Methods. For guidance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5673, April 8, 1970.
e. References. None.
577 - 583. [RESERVED]
584. SECTION 25.533 - HULL AND MAIN FLOAT BOTTOM PRESSURES.
a. Rule Text.
(a) General. The hull and main float structure, including frames and bulkheads, stringers, and bottom plating, must be designed under this section.
(b) Local pressures. For the design of the bottom plating and stringers and their attachments to the supporting structure, the following pressure distributions must be applied:
(1) For an unflared bottom, the pressure at the chine is 0.75 times the pressure at the keel, and the pressures between the keel and chine vary linearly, in accordance with figure 3 of Appendix B. The pressure at the keel (psi) is computed as follows:
K V SI 2 P = C 2 k 2 tan b k where - Pk = pressure (psi) at the keel;
C2 = 0.00213;
K2 = hull station weighing factor, in accordance with figure 2 of Appendix B;
VSI = seaplane stalling speed (knots) at the design water takeoff weight with flaps
extended in the appropriate takeoff position; and b k = angle of dead rise at keel, in accordance with figure 1 of Appendix B.
(2) For a flared bottom, the pressure at the beginning of the flare is the same as that for an unflared bottom, and the pressure between the chine and the beginning of the flare varies linearly, in accordance with figure 3 of Appendix B.
The pressure distribution is the same as that prescribed in paragraph (b)(1) of this 460
section for an unflared bottom except that the pressure at the chine is computed as follows:
K V 2 2 SIPch = C3 tanb where - Pch = pressure (psi) at the chine;
C3 = 0.0016;
K2 = hull station weighing factor, in accordance with figure 2 of Appendix B;
VS I = seaplane stalling speed at the design water takeoff weight with flaps
extended in the appropriate takeoff position; and b = angle of dead rise at appropriate station.
The area over which these pressures are applied must simulate pressures occurring during high localized impacts on the hull or float, but need not extend over an area that would induce critical stresses in the frames or in the overall structure.
(c) Distributed pressures. For the design of the frames, keel, and chine structure, the following pressure distributions apply:
(1) Symmetrical pressures are computed as follows:
K V SO2 P = C 2 4 tanb where - P = pressure (psi);
C4 = 0.078 C1 (with C1 computed under § 25.527);
K2 = hull station weighing factor, determined in accordance with figure 2 of Appendix B;
Vso = seaplane stalling speed (knots) with landing flaps extended in the appropriate position and with no slipstream effect; and b = angle of dead rise at appropriate station.
(2) The unsymmetrical pressure distribution consists of the pressures prescribed in paragraph (c)(1) of this section on one side of the hull or main float centerline and one-half of that pressure on the other side of the hull or main float centerline, in accordance with figure 3 of Appendix B.
These pressures are uniform and must be applied simultaneously over the entire hull or main float bottom. The loads obtained must be carried into the sidewall structure of the hull proper, but need not be transmitted in a fore and aft direction as shear and bending loads.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the bottom structures of the hull and floats are able to withstand likely pressure loads developed during takeoff and landing.
c. Background. This rule was carried forward § 4b.256 of the Civil Air Regulations (CAR).
Amendment 25-23 made minor editorial changes.
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.
585 - 589. [RESERVED]
590. SECTION 25.535 - AUXILIARY FLOAT LOADS.
a. Rule Text.
(a) General. Auxiliary floats and their attachments and supporting structures must be designed for the conditions prescribed in this section. In the cases specified in paragraphs (b) through (e) of this section, the prescribed water loads may be distributed over the float bottom to avoid excessive local loads, using bottom pressures not less than those prescribed in paragraph (g) of this section.
(b) Step loading. The resultant water load must be applied in the plane of symmetry of the float at a point three-fourths of the distance from the bow to the step and must be perpendicular to the keel. The resultant limit load is computed as follows, except that the value of L need not exceed three times the weight of the displaced water when the float is completely submerged:
2 2/ 3 VSO WL = C5 2 /3 2/3 2 tan b s (1 + ry ) where L = limit load (lbs.);
C5 = 0.0053;
VSo= seaplane stalling speed (knots) with landing flaps extended in the
appropriate position and with no slipstream effect;
W = seaplane design landing weight in pounds;
bs = angle of dead rise at a station 3/4 of the distance from the bow to the step, but need not be less than 15 degrees; and 462
ry = ratio of the lateral distance between the center of gravity and the plane of symmetry of the float to the radius of gyration in roll.
(c) Bow loading. The resultant limit load must be applied in the plane of symmetry of the float at a point one-fourth of the distance from the bow to the step and must be perpendicular to the tangent to the keel line at that point. The magnitude of the resultant load is that specified in paragraph (b) of this section.
(d) Unsymmetrical step loading. The resultant water load consists of a component equal to 0.75 times the load specified in paragraph (a) of this section and a side component equal to 3.25 tan ß times the load specified in paragraph (b) of this section. The side load must be applied perpendicularly to the plane of symmetry of the float at a point midway between the keel and the chine.
NOTE: The 3.25 factor is an error, it should be 0.25.
(e) Unsymmetrical bow loading. The resultant water load consists of a component equal to 0.75 times the load specified in paragraph (b) of this section and a side component equal to 0.25 tan ß times the load specified in paragraph (c) of this section. The side load must be applied perpendicularly to the plane of symmetry at a point midway between the keel and the chine.
(f) Immersed float condition. The resultant load must be applied at the centroid of the cross section of the float at a point one-third of the distance from the bow to the step. The limit load components are as follows:
vertical = rgV r 2 aft = Cx V2 /3 (KVSO)2
r 2 / 3 2side = Cy V (KVSO )2 where r = mass density of water (slugs/ft.)2;
V = volume of float (ft.) 2;
Cx = coefficient of drag force, equal to 0.133;
Cy = coefficient of side force, equal to 0.106;
K = 0.8, except that lower values may be used if it is shown that the floats are incapable of submerging at a speed of 0.8 VSO in normal operations;
VSo = seaplane stalling speed (knots) with landing flaps extended in the appropriate position and with no slipstream effect; and g = acceleration due to gravity (ft/sec2)
(g) Float bottom pressures. The float bottom pressures must be established under § 25.533, except that the value of K 2 in the formulae may be taken as 1.0.
The angle of dead rise to be used in determining the float bottom pressures is set forth in paragraph (b) of this section.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to establish structural design requirements for auxiliary floats and their attachments.
c. Background. This rule was carried forward from § 4b.257 of the Civil Air Regulations (CAR).
Amendment 25-23 made minor editorial changes.
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.
NOTE: In paragraph (d), the 3.25 factor is an error and should be 0.25. The error was corrected by FAA policy letter dated September 23, 1993.
e. References. None.
591 - 597. [RESERVED]
598. SECTION 25.537 - SEAWING LOADS.
a. Rule Text.
Seawing design loads must be based on applicable test data.
NOTE: This rule also applies to hydrofoils.
b. Intent of Rule. The purpose of this rule is to ensure that seawings and hydrofoils are able to withstand any likely loads encountered in service.
c. Background. This rule was carried forward from § 4b.537 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.
599 - 602. [RESERVED] 464
Section 8. EMERGENCY LANDING CONDITIONS
603. SECTION 25.561.- GENERAL.
a. Rule Text.
(a) The airplane, although it may be damaged in emergency landing conditions on land or water, must be designed as prescribed in this section to protect each occupant under those conditions.
(b) The structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing when -
(1) Proper use is made of seats, belts, and all other safety design provisions;
(2) The wheels are retracted (where applicable); and
(3) The occupant experiences the following ultimate inertia forces acting separately relative to the surrounding structure:
(i) upward, 3.0g
(ii) Forward, 9.0g
(iii) Sideward, 3.0g on the airframe; and 4.0g on the seats and their attachments.
(iv) Downward, 6.0g
(v) Rearward, 1.5g
(c) For equipment, cargo in the passenger compartments and any other large masses, the following apply:
(1) Except as provided in paragraph (c)(2) of this section, these items must be positioned so that if they break loose they will be unlikely to
(i) Cause direct injury to occupants;
(ii) Penetrate fuel tanks or lines or cause fire or explosion hazard by damage to adjacent systems; or
(iii) Nullify any of the escape facilities provided for use after an emergency landing.
(2) When such positioning is not practical (e.g. fuselage mounted engines or auxiliary power units) each such item of mass shall be restrained under all loads up to those specified in paragraph (b)(3) of this section. The local attachments for these items should be designed to withstand 1.33 times the specified loads if these items are subject to severe wear and tear through frequent removal (e.g.
quick change interior items).
(d) Seats and items of mass (and their supporting structure) must not deform under any loads up to those specified in paragraph (b)(3) of this section in any manner that would impede subsequent rapid evacuation of occupants.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970; Amdt. 25-64, 53 FR 17646, May 17, 1988; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997] 539
b. Intent of Rule. The purpose of this rule is to ensure that occupants will have a reasonable chance of escaping serious injury after an emergency landing on either land or water.
c. Background. This rule was carried forward from § 4b.260 of the Civil Air Regulations (CAR).
Amendment 25-23 made minor editorial changes for clarification. Amendment 25-64 increased the inertia load factors in the upward, sideward, and downward directions and added a rearward load factor. Amendment 25-64 also removed the reference to the five f.p.s. ultimate descent velocity.
Amendment 25-91 provided a factor of 1.33 for frequently removed items.
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; Amendment 25-64, 53 FR 17646, May 17, 1988; and Amendment 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
604 - 610. [RESERVED]
611. SECTION 25.562 - EMERGENCY LANDING DYNAMIC CONDITIONS.
a. Rule Text.
(a) The seat and restraint system in the airplane must be designed as prescribed in this section to protect each occupant during an emergency landing condition when -
(1) Proper use is made of seats, safety belts, and shoulder harnesses provided for in the design; and
(2) The occupant is exposed to loads resulting from the conditions prescribed in this section.
(b) Each seat type design approved for crew or passenger occupancy during takeoff and landing must successfully complete dynamic tests or be demonstrated by rational analysis based on dynamic tests of a similar type seat, in accordance with each of the following emergency landing conditions. The tests must be conducted with an occupant simulated by a 170-pound anthropomorphic test dummy, as defined by 49 CFR Part 572, Subpart B, or its equivalent, sitting in the normal upright position.
(1) A change in downward vertical velocity (DV ) of not less than 35 feet per second, with the airplane's longitudinal axis canted downward 30 degrees with respect to the horizontal plane and with the wings level. Peak floor deceleration must occur in not more than 0.08 seconds after impact and must reach a minimum of 14g.
(2) A change in forward longitudinal velocity (DV ) of not less than 44 feet per second, with the airplane's longitudinal axis horizontal and yawed 10 degrees 540
�� � ��
�� � �� �
9/1/99 AC 25-21 either right or left, whichever would cause the greatest likelihood of the upper torso restraint system (where installed) moving off the occupant's shoulder, and with the wings level. Peak floor deceleration must occur in not more than 0.09 seconds after impact and must reach a minimum of 16g. Where floor rails or floor fittings are used to attach the seating devices to the test fixture, the rails or fittings must be misaligned with respect to the adjacent set of rails or fittings by at least 10 degrees vertically (i.e., out of Parallel) with one rolled 10 degrees.
(c) The following performance measures must not be exceeded during the dynamic tests conducted in accordance with paragraph (b) of this section:
(1) Where upper torso straps are used for crewmembers, tension loads in individual straps must not exceed 1,750 pounds. If dual straps are used for restraining the upper torso, the total strap tension loads must not exceed 2,000 pounds.
(2) The maximum compressive load measured between the pelvis and the lumbar column of the anthropomorphic dummy must not exceed 1,500 pounds.
(3) The upper torso restraint straps (where installed) must remain on the occupant's shoulder during the impact.
(4) The lap safety belt must remain on the occupant's pelvis during the impact.
(5) Each occupant must be protected from serious head injury under the conditions prescribed in paragraph (b) of this section. Where head contact with seats or other structure can occur, protection must be provided so that the head impact does not exceed a Head Injury Criterion (HIC) of 1,000 units. The level of HIC is defined by the equation:
2.5 Ø 1 ø 2t a( )t dt ( t( t t where:
t1 is the initial integration time,
t2 is the final integration time, and
a(t) is the total acceleration vs. time curve for the head strike, and where
(t) is in seconds, and (a) is in units of gravity (g).
(6) Where leg injuries may result from contact with seats or other structure, protection must be provided to prevent axially compressive loads exceeding 2,250 pounds in each femur.
(7) The seat must remain attached at all points of attachment, although the structure may have yielded.
(8) Seats must not yield under the tests specified in paragraphs (b)(1) and (b)(2) of this section to the extent they would impede rapid evacuation of the airplane occupants.
[Amdt. 25-64, 53 FR 17646, May 17, 1988] HIC t1 = - μ)2 ) ϧ t1 - 1 2 MAX 541
b. Intent of Rule. The purpose of this rule is to provide protection for each occupant during emergency landing conditions.
c. Background. This rule was made effective by Amendment 25-64 on May 17, 1988. Prior to this amendment, there was no requirement to dynamically test seats or restraint systems.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-64, 53 FR 17646, May 17, 1988. Advisory Circular (AC) 25.562 1A, or latest revision, provides guidance information regarding compliance with the provisions of part 25 applicable to dynamic testing of seats intended for use in transport category airplanes.
e. References. Advisory Circular 25.562-1A, or latest revision, Dynamic Evaluation of Seat Restraint Systems & Occupant Protection on Transport Airplanes.
612 - 620. [RESERVED]
621. SECTION 25.563 - STRUCTURAL DITCHING PROVISIONS.
a. Rule Text.
Structural strength considerations of ditching provisions must be in accordance with § 25.801(e).
b. Intent of Rule. The purpose of this rule is to ensure that for aircraft certificated for ditching, the structural integrity of doors and windows will be maintained during ditching.
c. Background. This rule was brought forward from § 4b.261 of the Civil Air Regulations (CAR) with a minor editorial change.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
622 - 628. [RESERVED] 542
Section 9. FATIGUE EVALUATION
629. SECTION 25.571 - DAMAGE-TOLERANCE AND FATIGUE EVALUATION OF STRUCTURE.
a. Rule Text.
(a) General. An evaluation of the strength, detail design, and fabrication must show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane. This evaluation must be conducted in accordance with the provisions of paragraphs (b) and (e) of this section, except as specified in paragraph(c) of this section, for each part of the structure which could contribute to a catastrophic failure (such as wing, empennage, control surfaces and their systems, the fuselage, engine mounting, landing gear, and their related primary attachments).
For turbojet powered airplanes, those parts that could contribute to a catastrophic failure must also be evaluated under paragraph (d) of this section.
In addition, the following apply:
(1) Each evaluation required by this section must include -
(i) The typical loading spectra, temperatures, and humidities expected in service;
(ii) The identification of principal structural elements and detail design points, the failure of which could cause catastrophic failure of the airplane; and
(iii) An analysis, supported by test evidence, of the principal structural elements and detail design points identified in paragraph (a)(1)(ii) of this section.
(2) The service history of airplanes of similar structural design, taking due account of differences in operating conditions and procedures, may be used in the evaluations required by this section.
(3) Based on the evaluations required by this section, inspections or other procedures must be established as necessary to prevent catastrophic failure, and must be included in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by § 25.1529. Inspection thresholds for the following types of structure must be established based on crack growth analyses and/or tests, assuming the structure contains an initial flaw of the maximum probable size that could exist as a result of manufacturing or service-induced damage:
(i) Single load path structure, and
(ii) Multiple load path "fail-safe" structure and crack arrest "fail-safe" structure, where it cannot be demonstrated that load path failure, partial failure, or crack arrest will be detected and repaired during normal maintenance, inspection, or operation of an airplane prior to failure of the remaining structure.