Technical field of the invention

[0001] The present invention is directed to a method for testing multilayer aircraft seat cushions by the use of static tests on specimens, i.e. without full seat certification.

Background of the invention

[0002] Aircraft seat cushions must be periodically replaced due to wear and abuse during normal service. Most airlines acquire replacement cushions that are identical to those originally certified by the seat supplier. However, operators cannot always count on the availability of identical cushions since the foam materials used in the cushion fabrication may no longer be produced. If this is the case, operators are required to conduct full-scale dynamic seat tests, for the purpose of certifying a replacement cushion.

[0003] Recently the market of aircrafts cabin interiors retrofits market has experienced two important changes: a) new restrictive regulations (Federal Aviation Regulation, FAR) by the Federal Aviation Administration (FAA) and (Certification Specification, CS) by the European Aviation Safety Agency (EASA) in 2009, impose that all new airplanes must be certified as "l6g" as per FAR/CS 25.562; b) increase in demand for cushions that are more comfortable and healthier to passengers than current monolithic models, thus multilayer, and for new floating models that avoid the need for life-jackets.

[0004] The properties of aircraft bottom seat cushions are known to have a strong influence on the lumbar load performance of l6-g (acceleration of gravity) dynamically certified aircraft passenger seats.

[0005] These combined factors have generated a new market demand for l6g certified multilayer and floating cushions. This affects 65% of in-use airplanes seats, that must gradually replace 9g/l6g monolithic cushions with the new l6g multilayer, as well as the future retrofit of new airplanes.

[0006] A test for monolithic cushions was developed in 2005 by the Federal Aviation Administration (FAA). The methodology established criteria to evaluate replacement cushions, which ensures an equivalent or improved level of safety to the cushions originally certified on those seats. However, this methodology only applies to monolithic cushions.

[0007] Therefore, there is a strong need to define whether a multilayer replacement cushion is suitable for use in a l6g certified seat without incurring in the high expenses of a full seat certification. Summary of the invention

[0008] The present invention solves the above-mentioned problem by defining a method for testing multilayer cushions, which method allows to verify if the cushion has a

performance under stress which is at least equivalent to the originally certified multilayer cushion. The method could be used by aviation safety agencies to certify replacement cushions.

[0009] The present invention is directed to a method for testing multilayer cushions and determine if the cushion is suitable as a replacement of a certified cushion, which method comprises the following steps:

a. Subjecting cylindrical specimen of the replacement cushion and of the original cushion to a compression test to simulate the l4g down dynamic test required by FAR/CS 25.562.

b. Use the load/deflection curves obtained in a) as input to a mathematical model capable of calculating lumbar spine load.

c. If the value of lumbar load of the original cushion is lower than a prefixed value of 6670 N as per SAE 8049A, and the replacement cushion presents a lumbar spine load lower than the original cushion, then the replacement cushion can be used without performing full seat certification.

[0010] Thus, the method according to the invention represents an improvement over the test approved by FAA for monolayer cushions (DOT/FAA/AR-05/5,I). In fact, in this test, monolayer cushions are subjected to the compression test as defined in point a. above, and the results of the original cushion and the replacement cushion are compared. If the curve of the replacement cushion is always below the curve of the original cushion, then the replacement cushion is certified. However, this method not only is limited to monolayer cushions, but does not take into account the mechanical characteristics of the seat, which play a role in the calculation of the maximum spine load.

Detailed description of the invention

[0011] The present invention is directed to a method for testing l6-g certified multilayer aircraft seat cushions, which method allows to verify if the cushion has a performance under stress which is at least equivalent to the originally certified multilayer cushion. The method makes use of static tests and of a mathematical model to calculate the maximum lumbar spine load on specimens by taking into account both the properties of the cushion and of the seat. [0012] The mathematical model is preferably a model derived from MuSIaC (Multi Scale Impact and Crash), whose disclosure is herewith incorporated by reference, which is a multibody modeling technique for the study of the passive safety of helicopter structure in the condition of vertical impact. The model is published as a doctoral thesis at the web address https://www.politesi.polimi.it/haudle/10589/51382.

[0013] Multilayer cushions usually comprise three layers; a first layer of hard foam, placed in contact with the seat pan, and made of a foam having density preferably comprised between 65 and 70 kg/m ³ and having an indentation hardness preferably of about 6 kPa; a second intermediate layer made of“medium foams” having density preferably of about 55 kg/m ³ and indentation hardness of about 3-4 kPa; a third layer below the seat cover and in direct contact with the passenger made of“soft foam” having density of about 45-55 kg/m ³ , and having the scope of providing comfort to the passenger and preventing deep vein thrombosis.

[0014] Typical percentages of the three layers are 50-70 for the hard foam, 20-30 for the medium foam and 10-20 for the soft foam. Most preferred composition is 70% hard foam,

20% medium foam and 10% soft foam. However, the invention also applies to multilayer cushions having different compositions both in terms of characteristics of the foam and percentage of the different foams.

[0015] Cylindrical specimens of the cushions subjected to compression test are prepared by cutting a disk of about 0.20 m of diameter and gluing it with the next layer. For example, if a cushion consisted of 30 % of material A, 20 % of material B and 50 % of material C, the specimens are prepared by respecting these values. The different layers are glued with the glue used in the production of the cushions.

[0016] The compression test is performed on at least 3, preferably at least 4, more preferably on 5 specimens of each cushion. The test is performed by using a press having a speed preferably around 800 mm/s. The thickness of the cushion is representative of the thickness of the seat cushion area under the pelvis of the occupant.

[0017] The mathematical model to be used in step b) of the method can be any

mathematical model which is capable of defining in a reliable manner the relationship between the load/deflection curves obtained by the compression test and the lumbar spine load which would be obtained by the dynamic test. The applicant, starting from the modelling program MuSIaC, has developed a“3 degree of freedom model” (3-DoF). The model could be implemented with any programming language. [0018] A 3-degree-of- freedom model is built considering the 60° attitude nose-up which must be set-up in the experimental test, because the sled moves horizontally. The upper body includes thorax, upper arms, part of the forearms, necks and head; its degree of freedom is represented by the coordinate XT (Thorax) from the sled reference level.

[0019] The lower body includes the pelvis and part of the upper legs; its degree of freedom is represented by the coordinate xp (Pelvis) from the sled reference level.

[0020] The upper seat includes a hypothetical moveable mass fraction of the seat structure; its degree of freedom is represented by the coordinate xs (Seat) from the sled reference level.

[0021] The sled finally is a reference kinematic body with assigned motion, representing the experimental test sled acceleration. Its coordinate x _c (Carriage) is then obtained by double integration of the experimental acceleration.

[0022] The set of equations to solve the dynamics is as follows: m _T x _T = F _L — f _T — m _T g cos a

m _P x _P =—F _L + F _c — f _P — ni _p g cos a

m _s x _s =—F _c + F _s — m _s g cos a

[0023] where:

[0024] . hΐt = upper body mass

[0025] mp = lower body mass

[0026] . ms = seat moveable mass

[0027] XT = upper body position

[0028] xp = lower body position

[0029] . xs = seat position

[0030] xc = carriage position

[0031 ] . Fp = spine load

[0032] Fc = cushion load

[0033] Fs = seat load

[0034] . fr = upper body friction force along backrest

[0035] fp = lower body friction force along backrest

[0036] a = backrest attitude with respect to vertical plane in the 60° nose-up condition [0037] g = gravity [0038] At the end of the validation process, the model parameters for an economy class Geven Piuma seat are reported in tab. 1. Different types of seat will require the elaboration of different seat parameters.

Table 1

[0039] The lumbar spine load is calculated as a function of the spine deflection and rate of deflection using the biomechanical parameters of the multi-body MuSIaC ATD model as follows:

F _L = k _L (x _T - x _P ) + c _L (x _T - Xp)

[0040] The seat force is similarly calculated, but with an additional yielding point which was observed during the dynamic tests, as follows:

F _s = k _s (x _s - x _c ) + c _s (x _s - x _c ) if (xs-xc) £D _Y

F _s = F _Y + c _s (x _s — x _c ) if (xs-xc) ³ D _Y

[0041 ] . Where D _Y is the seat plastic deflection, computed as follows:

[0042] The carriage position xc and its derivative are obtained respectively as double and single integration from the EASA/FAA ideal acceleration pulse pattern, which for a CS-25 category aircraft is a triangle with peak acceleration 14 g’s (in down direction), time to peak 80 ms and total integrated speed 10.7 m/s.

[0043] Since in the laboratory set-up the seat and ATD are installed on the carriage with a 60° nose-up attitude, the ATD pelvis and chest are actually pressing the seatback with a considerable fraction of their weight. The friction forces, whose contribution may be of some interest, are then calculated as Coulomb reactions but with a smoothing effect for low relative speeds, to avoid numerical instability, as follows:

f _T = m _T g sina m _t tanh

f _p = m _P g sina m _R tanh

[0044] The cushion load, Fc, is calculated by linear interpolation in the load-deflection curve obtained in the foam experimental tests. As a matter of fact, the foam experimental tests are carried out compressing the specimen at low speed between a couple of flat surfaces; the actual pelvis-cushion interaction is more complex because the pelvis footprint changes during compression and the foam is strain rate sensitive.

[0045] The best results are obtained by using the following adaptation of the load- deflection curve obtained by the specimen tests:

F _c = 1.8 R (1 + tanh 0.6r _c )

[0046] Where R is the load-deflection curve obtained by the specimen tests and rc is the cushion rate of compression, computed as difference of speed between the pelvis and the seat.

[0044] . The third step of the method comprises two parts, a first part is a verification that the static tests were performed correctly. In fact, since the original cushion was certified, it derives that the spinal load obtained with the original cushion must be lower than the maximum

acceptable spinal load. The second part of the third step consists of a comparison between the spinal load of the original cushion as obtained by the mathematical method and the spinal load of the replacement cushion, always calculated by the same mathematical method. If the spinal load of the replacement cushion is lower than the spinal load of the original cushion, then the replacement cushion can be used without performing dynamic tests.