AMC
25.723 Shock absorption tests
ED Decision 2006/005/R
1. PURPOSE. This AMC sets forth an acceptable means, but not the only means, of demonstrating compliance with the provisions of CS-25 related to the use of landing gear shock absorption tests and analyses to determine landing loads for large aeroplanes.
2. RELATED CERTIFICATION SPECIFICATIONS. CS 25.723 "Shock absorption tests" and CS 25.473 "Landing load conditions and assumptions."
3. SHOCK ABSORPTION TESTS.
a. Validation of the landing gear characteristics. Shock absorption tests are necessary to validate the analytical representation of the dynamic characteristics of the landing gear unit that will be used to determine the landing loads. A range of tests should be conducted to ensure that the analytical model is valid for all design conditions. In addition, consideration should be given to ensuring that the range of test configurations is sufficient for justifying the use of the analytical model for foreseeable future growth versions of the aeroplane.
b. Recommended test conditions for new landing gear units. The design takeoff weight and the design landing weight conditions should both be included as configurations subjected to energy absorption tests. However, in cases where the manufacturer has supporting data from previous experience in validating the analytical model using landing gear units of similar design concept, it may be sufficient to conduct tests of the new landing gear at only the condition associated with maximum energy. The landing gear used to provide the supporting data may be from another model aircraft but should be of approximately the same size with similar components.
c. Changes to type designs. CS 25.723(c) allows changes in previously approved design weights and minor changes in design to be substantiated by analyses based on tests of the same basic landing gear unit with similar energy absorption characteristics.
A landing gear unit would be considered to be of “the same basic landing gear system” when the design concept has not been changed. “Similar energy absorption characteristics” means that the changes to the landing gear unit, either taken individually or as a whole, would not have a significant effect on the validation of the analytical results for the modified aeroplane. Changes that may be acceptable without further energy absorption tests include minor changes and adjustments incorporated in the landing gear unit to maintain similar energy absorption characteristics with changes in design weight and landing speeds.
For example, the following changes may be acceptable without further tests:
(1) Minor changes in shock absorber details including pre-load, compression ratio, orifice sizes, metering pin profiles.
(2) Minor changes in tyre characteristics.
(3) Minor changes in unsprung mass (e.g. brakes).
(4) Local strengthening or minor sizing changes to the landing gear.
To allow justification by analysis for the reserve energy requirement, neither the shock strut nor the tyres should bottom during the reserve energy analysis or the tests upon which the analysis is correlated.
4. LIMIT FREE DROP TESTS.
a. Compliance with CS 25.723(a) may be shown by free drop tests, provided they are made on the complete aeroplane, or on units consisting of a wheel, tyre, and shock absorber, in their proper positions, from free drop heights not less than--
(1) 475 mm (18.7 inches) for the design landing weight conditions; and
(2) 170 mm (6.7 inches) for the design takeoff weight conditions.
b. If aeroplane 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 aeroplane lift is represented in free drop tests by a reduced weight, the landing gear must be dropped with an effective weight equal to
where:
We = the effective weight to be used in the drop test (kg);
h = specified free drop height (mm);
d = deflection under impact of the tyre (at the approved inflation pressure) plus the vertical component of the axle travel relative to the drop weight (mm);
W = WM for main gear units (kg), equal to the static weight on that unit with the aeroplane in the level attitude (with the nose wheel clear in the case of nose wheel type aeroplanes);
W = WT for tail gear units (kg), equal to the static weight on the tail unit with the aeroplane in the tail-down attitude;
W = WN for nose wheel units (kg), equal to the vertical component of the static reaction that would exist at the nose wheel, assuming that the mass of the aeroplane acts at the centre of gravity and exerts a force of 1.0 g downward and 0.25 g forward; and
L = ratio of the assumed aeroplane lift to the aeroplane 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 aeroplane landing conditions in a manner consistent with the development of rational or conservative limit loads.
d. The value of d used in the computation of We in paragraph 4.(b) of this AMC may not exceed the value actually obtained in the drop test.
5. RESERVE ENERGY FREE DROP TESTS.
a. Compliance with the reserve energy absorption condition specified in CS 25.723(b) may be shown by free drop tests provided the drop height is not less than 69 cm (27 inches).
b. If aeroplane 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 aeroplane lift is represented in free drop tests by an equivalent reduced weight, the landing gear must be dropped with an effective weight:
where the symbols and other details are the same as in paragraph 4 above.
[Amdt 25/2]