Research status and future development of crashworthiness of civil aircraft fuselage structures: An overview
Introduction
Crashworthiness is the ability of civil aircraft structure and internal systems to provide maximum occupant protection in a crash or emergency landing event, and to enable the occupants to successfully evacuate the aircraft [1,2]. The crash impacts of civil aircraft are inevitable, which can seriously threaten the survivability of occupants. About 40% of civil aviation accidents occur in the take-off or landing phase, and most of them belong to the category of survival crash accidents [3]. How to ensure the occupants’ safety during the crash process is an important issue of crashworthy design.
To ensure civil aviation safety, the airworthiness standards of civil aircraft were given in the FAR 25/CCAR 25 [4,5]. There are no specific rules for crashworthiness of the fuselage sections, but the crashworthy requirements of the fuselage sections are included in more than 40 relevant rules [[6], [7], [8]]. For example, Section 25.561 specifies that the fuselage structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash-landing event when seats, belts and all other safety design provisions are properly used. Section 25.562 specifies the requirements of emergency landing dynamic conditions.
In order to meet the airworthiness requirements of civil aircraft, a large number of crashworthy tests have been carried out on the fuselage sections and full-scale aircraft. During a crash event, the impact load is transmitted along the sub-cargo floor support struts, cargo cross beam, fuselage frame, cabin floor support struts and passenger cross beam, and finally transmitted to occupants through the seats, as shown in Fig. 1 [9]. The crashworthy performance can be immensely improved with well-designed energy absorption materials and structures, and the crashworthiness of civil aircraft is dominated by the impact response characteristics of the fuselage section [10]. The impact response of the fuselage section can be evaluated to ensure that the impact energy can be absorbed by the primary structures (frames, stringers, skins, cabin floors and support struts, cargo floors and support struts) with the goals of managing the fuselage deformation and reducing the impact load. It is difficult to evaluate the crashworthiness of civil aircraft by conducting theoretical analysis, because of the high degree of nonlinearity in materials and geometry during a crash event. The crashworthiness of civil aircraft is evaluated early by conducting impact tests of the fuselage sections, but there are still disadvantages in conducting impact tests, such as high costs, long preparation period, etc. Numerical simulation technology can be used to analyze and evaluate the crashworthy behavior, which can greatly compensate for the disadvantages of impact tests. So, the combination of methods (numerical simulation and impact test) can be used to evaluate the crashworthiness of civil aircraft.
In recent years, composite materials have been widely used in civil aircraft fuselage structures, and are gradually applied to the main load bearing structures. The Boeing company has increased composite material usage to 50% for Boeing 787, and the Airbus company has increased composite material usage to 52% for A350-900. Due to the significant difference of failure and energy absorption characteristics between metal structures and composite structures, the massive use of composite materials has brought great challenges to the crashworthy design, verification and certification of civil aircraft. For composite aircraft, no specific rules address the impact response characteristics for a survivable crash condition. Therefore, the crashworthy requirements of composite fuselage structure was put forward in FAA AC 20–107B [11], and the Special Conditions (SC) for Boeing 787-8 (25-362-SC) and A350-900 (25-537-SC) had also been issued to ensure the crash safety level equivalent to that provided by FAR 25/CCAR 25 [12,13]. Boeing 787–8 and A350-900 must meet the following criteria over a range of vertical descent velocities from 0 to 9.14 m/s (30 ft/s): 1. Maintenance of a survivable volume; 2. Maintenance of acceptable acceleration and loads experienced by the occupants; 3. Retention of items of mass; 4. Maintenance of occupant emergency egress paths. The crashworthy design, verification, and certification of composite fuselage structure has been and will continue to be the main concern.
The research on the crashworthiness of civil aircraft structure mainly involves fuselage structure [14,15], seat system [16], landing gear system [17], and overhead stowage bins [18], etc. This paper mainly focuses on the crashworthy design, verification, and certification of civil aircraft fuselage structure, and a building block approach is recommended with several levels, i.e. coupons - elements - details - subcomponents - components - full-scale aircraft. Based on the different levels of building block approach, the impact tests and numerical simulations are described, and the design methods for improving the crashworthiness of an aircraft fuselage section are also introduced. Then the current crashworthy verification and evaluation processes for civil aircraft are highlighted. Finally, the future development of crashworthy design, verification, and certification of civil aircraft fuselage structure is proposed.
Section snippets
Building block approach of crashworthiness
The MoC (Means of Compliance) using the building block approach was put forward in FAA AC 20–107B, and a series of impact tests and numerical simulation could be conducted with several levels, i.e. coupons - elements - details - sub-components - components, as shown in Fig. 2(a). To support the crashworthy verification and certification of Boeing 787 and A350, the building block approach of crashworthiness are recommended with several levels, as shown in Fig. 2(b) and (c) [19,20].
For the civil
Materials
The materials performance is extremely important for the crashworthy design and numerical simulation of civil aircraft fuselage structures, such as tensile/compression/shear performance, in-layer/interlayer performance, and so on [21,22]. The mechanical properties can be obtained to support the crashworthy design by conducting material performance tests under quasi-static and dynamic conditions [23,24], and the macroscopic failure modes and micro failure mechanisms can be analyzed to better
Energy absorption columns
Thin-walled columns are widely used as energy absorption structures, because a large amount of impact energy can be absorbed due to the longer axial crushing distance. The specific energy absorption (Es) is commonly used to evaluate the energy absorption characteristics, which is calculated as follows:where EA is the total energy absorption, m is the mass of the crushed failure part of the structure, F is the crushing force, ρ is the density, A is the effective section
Details
The failure modes, damage sequences, and energy absorption characteristics can be well predicted by conducting the impact tests for the details, such as sub-floor webs, sub-floor energy absorption columns and fuselage panel, which are extremely important for crashworthy verification and evaluation of the civil aviation airframe.
Sub-components: sub-cargo fuselage section
For the civil aircraft fuselage section, the energy absorption structures, such as composite C-channels [19,20], semi-circular channels [94] and composite corrugated plates [127,128], can be arranged under the sub-cargo fuselage section, which is the main energy absorption area. Dynamic impact tests are conducted to evaluate the failure mode, failure mechanism, failure sequence and energy absorption capability.
Drop tests
Many drop tests have been conducted to research the crashworthiness of civil aircraft fuselage section. Since the 1970s, the FAA and NASA (National Aeronautics and Space Administration) begun to study the crash dynamics and crash safety of GA aircraft and helicopters. In the 1980s, NASA and FAA launched the CID (Controlled Impact Demonstration) project, and a crash test of a Boeing 720 full-scale aircraft was conducted to obtain the structural response data, which could be used to validate the
Crashworthy verification and evaluation
The FE simulation analysis, which must be verified by drop test results, has been widely used in the crashworthy verification and evaluation of civil aircraft fuselage structures. The crashworthy compliance of the A340 is demonstrated through the FE simulation, which are validated and verified by the drop test results of the A320 fuselage section. For the A350XWB composite fuselage section, the crashworthy compliance is demonstrated to show a similar crash safety level as the A340 metal
Conclusions
Crashworthiness is an important issue in aircraft design, and it is also a necessary condition for obtaining an airworthiness certificate. Especially for composite aircraft and wide-body aircraft, the similar crash safety levels must be guaranteed as those of certified metal aircraft of the same size, and must meet the following criteria over a range of vertical descent velocities from 0 to 9.14 m/s (30 ft/s): 1. Maintenance of a survivable volume; 2. Maintenance of acceptable acceleration and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the supports from Aeronautical Science Foundation of China (2017ZD67002), Tianjin Municipal Education Commission Scientific Research Project (2019KJ135), Central Universities Foundation of Civil Aviation University of China (3122019162).
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