914. SECTION 25.725 - LIMIT DROP TESTS.
a. Rule Text.
(a) If compliance with § 25.723(a) is shown by free drop tests, these tests must be made on the complete airplane, or on units consisting of a wheel, tire, and shock absorber, in their proper positions, from free drop heights not less than -
(1) 18.7 inches for the design landing weight conditions; and
(2) 6.7 inches for the design takeoff weight conditions.
(b) If airplane lift is simulated by air cylinders or by other mechanical means, the weight used for the drop must be equal to W. If the effect of airplane lift is represented in free drop tests by an equivalent reduced mass, the landing gear must be dropped with an effective mass equal to Øh (1 - L)dWe = W Œº + h + d ø œß where - We = the effective weight to be used in the drop test (lbs.);
h = specified free drop height (inches);
d = deflection under impact of the tire (at the approved inflation pressure) plus the vertical component of the axle travel relative to the drop mass (inches);
W = WM for main gear units (lbs.), equal to the static weight on that unit with the airplane in the level attitude (with the nose wheel clear in the case of nose wheel type airplanes);
W = WT for tail gear units (lbs.), equal to the static weight on the tail unit with the airplane in the tail-down attitude;
W = WN for nose wheel units (lbs.), equal to the vertical component of the static reaction that would exist at the nose wheel, assuming that the mass of the airplane acts at the center of gravity and exerts a force of 1.0g downward and 0.25g forward; and L = ratio of the assumed airplane lift to the airplane weight, but not more than
1.0
(c) The drop test attitude of the landing gear unit and the application of appropriate drag loads during the test must simulate the airplane landing conditions in a manner consistent with the development of a rational or conservative limit load factor value.
(d) The value of d used in the computation of We in paragraph (b) of this section may not exceed the value actually obtained in the drop test.
(e) The limit inertia load factor n must be determined from the free drop test in paragraph (b) of this section according to the following formula:
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ØWe øn = nj LŒ œ + ºW ß
where - nJ = the load factor developed in the drop test (that is, the acceleration of
dv/dt in g`s recorded in the drop test) plus 1.0; and
We, W, and L are the same as in the drop test computation.
(f) The value of n determined in paragraph (e) of this section may not be more than the limit inertia load factor used in the landing conditions in § 25.473.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the landing gear will absorb the landing impact energy without exceeding the limit loads in the landing gear and without exceeding the design load factors specified in the airplane design.
c. Background. This rule was carried forward from § 4b.332 of the Civil Air Regulations (CAR).
Amendment 25-23 made clarifying changes to the rule.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970. Compliance may be shown by drop tests. Drop tests are required on all newly type certificated airplanes and on amended type certificated airplanes with new landing gear configurations.
e. References. None.
915 - 921. [RESERVED]
922. SECTION 25.727 - RESERVE ENERGY ABSORPTION DROP TESTS.
a. Rule Text.
(a) If compliance with the reserve energy absorption condition specified in § 25.723(b) is shown by free drop tests, the drop height may not be less than 27 inches.
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(b) If airplane lift is simulated by air cylinders or by other mechanical means, the weight used for the drop must be equal to W. If the effect of airplane lift is represented in free drop tests by an equivalent reduced mass, the landing gear must be dropped with an effective mass, WhWe = h + d where the symbols and other details are the same as in § 25.725(b).
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the landing gear has sufficient reserve energy capability to withstand landing conditions outside the normal design limit envelope without complete failure.
c. Background. This rule was carried forward from § 4b.332 of the Civil Air Regulations (CAR).
Amendment 25-23 provided some clarification of the rule.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970. Drop tests are required on all newly type certificated airplanes and on amended type certificated airplanes with new landing gear configurations
e. References. None.
923 - 928. [RESERVED]
929. SECTION 25.729 - RETRACTING MECHANISM.
a. Rule Text.
(a) General. For airplanes with retractable landing gear, the following apply:
(1) The landing gear retracting mechanism, wheel well doors, and supporting structure, must be designed for -
(i) The loads occurring in the flight conditions when the gear is in the retracted position,
(ii) The combination of friction loads, inertia loads, brake torque loads, air loads, and gyroscopic loads resulting from the wheels rotating at a peripheral speed equal to 1.3 VS (with the flaps in takeoff position at design takeoff weight), 1012
occurring during retraction and extension at any airspeed up to 1.6 VS1 (with the flaps in the approach position at design landing weight), and
(iii) Any load factor up to those specified in § 25.345(a) for the flaps extended condition.
(2) Unless there are other means to decelerate the airplane in flight at this speed, the landing gear, the retracting mechanism, and the airplane structure (including wheel well doors) must be designed to withstand the flight loads occurring with the landing gear in the extended position at any speed up to 0.67 VC.
(3) Landing gear doors, their operating mechanism, and their supporting structures must be designed for the yawing maneuvers prescribed for the airplane in addition to the conditions of airspeed and load factor prescribed in paragraphs
(a)(1) and (2) of this section.
(b) Landing gear lock. There must be positive means to keep the landing gear extended, in flight and on the ground.
(c) Emergency operation. There must be an emergency means for extending the landing gear in the event of -
(1) Any reasonably probable failure in the normal retraction system; or
(2) The failure of any single source of hydraulic, electric, or equivalent energy supply.
(d) Operation test. The proper functioning of the retracting mechanism must be shown by operation tests.
(e) Position indicator and warning device. If a retractable landing gear is used, there must be a landing gear position indicator (as well as necessary switches to actuate the indicator) or other means to inform the pilot that the gear is secured in the extended (or retracted) position. This means must be designed as follows:
(1) If switches are used, they must be located and coupled to the landing gear mechanical systems in a manner that prevents an erroneous indication of “down and locked" if the landing gear is not in a fully extended position, or of “up and locked" if the landing gear is not in the fully retracted position. The switches may be located where they are operated by the actual landing gear locking latch or device.
(2) The flightcrew must be given an aural warning that functions continuously, or is periodically repeated, if a landing is attempted when the landing gear is not locked down.
3) The warning must be given in sufficient time to allow the landing gear to be locked down or a go-around to be made.
(4) There must not be a manual shut-off means readily available to the flightcrew for the warning required by paragraph (e)(2) of this section such that it could be operated instinctively, inadvertently, or by habitual reflexive action.
(5) The system used to generate the aural warning must be designed to eliminate false or inappropriate alerts.
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(6) Failures of systems used to inhibit the landing gear aural warning, that would prevent the warning system from operating, must be improbable.
(f) Protection of equipment in wheel wells. Equipment that is essential to safe operation of the airplane and that is located in wheel wells must be protected from the damaging effects of -
(1) A bursting tire, unless it is shown that a tire cannot burst from overheat; and
(2) A loose tire tread, unless it is shown that a loose tire tread cannot cause damage.
[Amdt. 25-23, 35 FR 5676, Apr., 1970; Amdt. 25-42, 43 FR 2323, Jan. 16, 1978;
Amdt. 25-72, 55 FR 29777, Jul. 20, 1990; Amdt. 25-75, 56 FR 63762, Dec. 5, 1991]
b. Intent of Rule. The purpose of this rule is to ensure that the landing gear and its supporting structure, including up-locks, down-locks, and landing gear doors, are able to withstand all loads imposed anywhere within the approved operating envelope. The rule also requires a position indicator and aural warning to prevent inadvertent landing if the landing gear is not down and locked.
c. Background. This regulation was carried forward from § 4b.334 of the Civil Air Regulations (CAR). Amendment 25-23 added the requirement to consider gyroscopic loads resulting from wheel rotation. Amendment 25-42 revised the aural warning requirements. Amendment 25-75 allowed a manual means of shutting off the aural warning system.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5676, April 8, 1970; Amendment 25-42, 43 FR 2323, January 16, 1978; Amendment 25-72, 55 FR 29777, July 20, 1990; and Amendment 25-75, 56 FR 63762, December 5, 1991.
e. References. Advisory Circular (AC) 25-XX, Mechanical Systems Handbook.
930 - 938. [RESERVED]
939. SECTION 25.731 WHEELS.
a. Rule Text.
(a) Each main and nose wheel must be approved.
(b) The maximum static load rating of each wheel may not be less than the corresponding static ground reaction with -
(1) Design maximum weight; and
(2) Critical center of gravity.
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(c) The maximum limit load rating of each wheel must equal or exceed the maximum radial limit load determined under the applicable ground load requirements of this part.
[Amdt. 25-72, 55 FR 29777, Jul. 20, 1990] NOTE: Refer to Advisory Circular (AC) 25-XX, Mechanical Systems Handbook, for guidance.
940 - 946. [RESERVED]
947. SECTION 25.733 TIRES.
a. Rule Text.
(a) When a landing gear axle is fitted with a single wheel and tire assembly, the wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions and with a load rating approved by the Administrator that is not exceeded under -
(1) The loads on the main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, and
(2) The loads corresponding to the ground reactions in paragraph (b) of this section, on the nose wheel tire, except as provided in paragraphs (b)(2) and (b)(3) of this section.
(b) The applicable ground reactions for nose wheel tires are as follows:
(1) The static ground reaction for the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position with a force of 1.0g acting downward at the center of gravity.
This load may not exceed the load rating of the tire.
(2) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum landing weight) and center of gravity position combined with forces of 1.0g downward and 0.31g forward acting at the center of gravity. The reactions in this case must be distributed to the nose and main wheels by the principles of statics with a drag reaction equal to
0.31 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(3) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position combined with forces of 1.0g downward and 0.20g forward 1015
acting at the center of gravity. The reactions in this case must be distributed to the nose and main wheels by the principles of statics with a drag reaction equal to
0.20 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(c) When a landing gear axle is fitted with more than one wheel and tire assembly, such as dual or dual-tandem, each wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions, and with a load rating approved by the Administrator that is not exceeded by -
(1) The loads on each main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, when multiplied by a factor of 1.07; and
(2) Loads specified in paragraphs (a)(2), (b)(1), (b)(2), and (b)(3) of this section on each nose wheel tire.
(d) Each tire installed on a retractable landing gear system must, at the maximum size of the tire type expected in service, have a clearance to surrounding structure and systems that is adequate to prevent unintended contact between the tire and any part of the structure or systems.
(e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels.
[Amdt. 25-48, 44 FR 68752, Nov. 29, 1979; Amdt. 25-72, 55 FR 29777, Jul. 20, 1990, as amended by Amdt. 25-78, 58 FR 11781, Feb. 26, 1993] NOTE: Refer to Advisory Circular (AC) 25-XX, Mechanical Systems Handbook, for guidance.
948 - 954. [RESERVED]
955. SECTION 25.735 BRAKES.
a. Rule Text.
(a) Each brake must be approved.
(b) The brake system and associated systems must be designed and constructed so that if any electrical, pneumatic, hydraulic, or mechanical connecting or 1016
transmitting element (excluding the operating pedal or handle) fails, or if any single source of hydraulic or other brake operating energy supply is lost, it is possible to bring the airplane to rest under conditions specified in § 25.125, with a mean deceleration during the landing roll of at least 50 percent of that obtained in determining the landing distance as prescribed in that section. Subcomponents within the brake assembly, such as brake drum, shoes, and actuators (or their equivalents), shall be considered as connecting or transmitting elements, unless it is shown that leakage of hydraulic fluid resulting from failure of the sealing elements in these subcomponents within the brake assembly would not reduce the braking effectiveness below that specified in this paragraph.
(c) Brake controls may not require excessive control force in their operation.
(d) The airplane must have a parking control that, when set by the pilot, will without further attention, prevent the airplane from rolling on a paved, level runway with takeoff power on the critical engine.
(e) If antiskid devices are installed, the devices and associated systems must be designed so that no single probable malfunction will result in a hazardous loss of braking ability or directional control of the airplane.
(f) The design landing brake kinetic energy capacity rating of each main wheelbrake assembly shall be used during qualification testing of the brake to the applicable Technical Standard Order (TSO) or an acceptable equivalent. This kinetic energy rating may not be less than the kinetic energy absorption requirements determined under either of the following methods:
(1) The brake kinetic energy absorption requirements must be based on a rational analysis of the sequence of events expected during operational landings at maximum landing weight. This analysis must include conservative values of airplane speed at which the brakes are applied, braking coefficient of friction between tires and runway, aerodynamic drag, propeller drag or power-plant forward thrust, and (if more critical) the most adverse single engine or propeller malfunction.
(2) Instead of a rational analysis, the kinetic energy absorption requirements for each main wheelbrake assembly may be derived from the following formula, which must be modified in cases of designed unequal braking distributions.
KE = 0.0443 WV2 N Where - KE = Kinetic energy per wheel (ft.-lb.);
W = Design landing weight (lb.);
V = Airplane speed in knots. V must be not less than V SO, the poweroff stalling speed of the airplane at sea level, at the design landing weight, and in the landing configuration; and N = Number of main wheels with brakes.
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The formula must be modified in cases of unequal braking distribution.
(g) The minimum stalling speed rating of each main wheel-brake assembly (that is, the initial speed used in the dynamometer tests) may not be more than the V used in the determination of kinetic energy in accordance with paragraph (f) of this section, assuming that the test procedures for wheel-brake assemblies involve a specified rate of deceleration, and, therefore, for the same amount of kinetic energy, the rate of energy absorption (the power absorbing ability of the brake) varies inversely with the initial speed.
(h) The rejected takeoff brake kinetic energy capacity rating of each main wheelbrake assembly that is at the fully worn limit of its allowable wear range shall be used during qualification testing of the brake to the applicable Technical Standard Order (TSO) or an acceptable equivalent. This kinetic energy rating may not be less than the kinetic energy absorption requirements determined under either of the following methods:
(1) The brake kinetic energy absorption requirements must be based on a rational analysis of the sequence of events expected during an accelerate-stop maneuver.
This analysis must include conservative values of airplane speed at which the brakes are applied, braking coefficient of friction between tires and runway, aerodynamic drag, propeller drag or powerplant forward thrust, and (if more critical) the most adverse single engine or propeller malfunction.
(2) Instead of a rational analysis, the kinetic energy absorption requirements for each main wheel brake assembly may be derived from the following formula, which must be modified in cases of designed unequal braking distributions:
0.0443WV 2 KE = N Where - KE = Kinetic energy per wheel (ft.-lb.);
W = Airplane weight (lb.);
V = Airplane speed (knots) N = Number of main wheels with brakes; and W and V are the most critical combination of takeoff weight and ground speed obtained in a rejected takeoff.
[Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-48, 44 FR 68742, Nov. 29, 1979; Amdt. 25-72, 55 FR 29777, Jul. 20, 1990; Amdt. 25-92, 63 FR 8298; Feb.
18, 1998] NOTE: Refer to Advisory Circular (AC) 25-XX, Mechanical Systems Handbook, for guidance.
1018
956 - 960. [RESERVED]
1019
961. SECTION 25.737 - SKIS.
a. Rule Text.
Each ski must be approved. The maximum limit load rating of each ski must equal or exceed the maximum limit load determined under the applicable ground load requirements of this part.
NOTE: No guidance is currently available on this subject.
962 - 966. [RESERVED] 1020
Section 5. FLOATS AND HULLS
967. SECTION 25.751 - MAIN FLOAT BUOYANCY.
a. Rule Text.
Each main float must have -
(a) A buoyancy of 80 percent in excess of that required to support the maximum weight of the seaplane or amphibian in fresh water; and
(b) Not less than five watertight compartments approximately equal in volume.
NOTE: There is no guidance currently available on this subject.
968 - 973. [RESERVED]
974. SECTION 25.753 MAIN FLOAT DESIGN.
a. Rule Text.
Each main float must be approved and must meet the requirements of § 25.521.
NOTE: There is no guidance currently available on this subject.
975 - 977. [RESERVED]
978. SECTION 25.755 - HULLS.
a. Rule Text.
(a) Each hull must have enough watertight compartments so that, with any two adjacent compartments flooded, the buoyancy of the hull and auxiliary floats (and wheel tires, if used) provides a margin of positive stability great enough to minimize the probability of capsizing in rough, fresh water.
(b) Bulkheads with watertight doors may be used for communication between compartments.
NOTE: There is no guidance currently available on this subject.
1093