1. Introduction
Large passenger planes generally fly at high altitudes to minimize aerodynamic drag and maximize fuel efficiency. Unfortunately, humans cannot withstand prolonged exposure to low atmospheric pressure and low oxygen conditions above 3000 m altitude. Hence, to provide a comfortable environment and satisfy the physiological needs of the crew on board, a continuous supply of air is routed to the cabin, which keeps the cabin under a pressure corresponding to 2500–3000 m altitude [
1]. If an opening appears or the pressurization system abruptly fails when the plane is cruising at high altitude, the air in the cabin will suddenly flow out. Then, the inter-compartment pressure imbalance induces the extra decompression load, which is likely to lead to serious accidents. In 2018, an A319 airliner belonging to Sichuan Airlines experienced an accidental windshield crack while cruising. The pressure in the cockpit dropped abruptly, causing instrument damage and copilot injury. The tragedy was avoided by a successful forced landing [
2]. Prior to that, a DC-10 belonging to Turkish Airlines (1974) [
3] and a Boeing-747 belonging to Japan Airlines (1985) [
4] had airplane disasters induced by explosive decompression.
In order to ensure safety, 14-CFR 25.365(e) [
5] of the United States stipulates that: Any structure, component or part, inside or outside a pressurized compartment, the failure of which could interfere with continued safe flight and landing, must be designed to withstand the effects of a sudden decompression through an opening in any compartment at any operating altitude resulting from each of the listed conditions. Therefore, the analysis of decompression load is an important part of the strength evaluation of the preliminary design scheme of the cabin structure.
The 3-D simulation of the rapid decompression process is one of the most direct and accurate methods for decompression load analysis. Breard et al. [
6] used commercial 3-D CFD code to simulate the decompression process for an aircraft with the cockpit windshield off. Bai et al. [
7] investigated the influence of throttling and cabin volume on the explosive decompression process by 3-D numerical simulation. Due to the complexity of compartment structures and the transient and compressible characteristics of the decompression process, the 3-D simulation method is complex and time-consuming. Therefore, it is not suitable for the engineering application of aircraft structural design. Schroll and Tibbals et al. [
8] proposed the 0-D model based on the simple isentropic flow assumption and the gas state equation, which achieved a fast simulation of the cockpit decompression process. Daidzic and Simones [
9] applied the 0-D model to simulate the decompression process for typical large transport aircraft with different compartment geometries, discharge coefficients, leakage areas, and pressures. Pagani et al. [
10] and Zhang [
11] also applied the 0-D model to aircraft decompression load analysis. However, in the 0-D model, the pressure, temperature, and other aerodynamic parameter distributions in each compartment are assumed to be uniform, and the viscosity of air is also ignored. All of these factors reduce the accuracy of the decompression load analysis and adversely affect the structural strength assessment of the plane.
Active decompression panels between compartments are applied to enhance air circulation and reduce decompression load. Pratt [
1] and Pagani [
10] studied the decompression process in pressurized compartments with decompression panels. Zhang et al. [
11] investigated the effects of different flight altitudes (environmental pressure) and the location of a leakage hole on the rapid decompression process of pressurized compartments, and the influence of the opening process of the decompression panel was considered. Their research also reported that the external surface pressure of the plane is not equal to the environmental pressure, and the effects of flight speed on pressure should be considered in the decompression load analysis. The accident report of Sichuan Airlines 3U8633 [
2] also pointed out that there is local high pressure on the windshield due to the aerodynamic appearance when the passenger plane is cruising, which further affects the decompression process. Furthermore, the volume and shape of the compartment, the shape and area of the leakage hole, environmental pressure, flight altitude, and other parameters also have significant influences on the decompression load. Daidzic and Simones [
9] applied a 0-D model to simulate the decompression process for a plane with an installed cockpit security door and studied the influence of compartments and environmental pressure, the shape and volume of the compartments, the leakage area, and the discharge coefficient of the leakage hole. Zhang et al. [
12] conducted rapid decompression load analysis in a model with a layout similar to that of a real commercial plane, and the influence of compartment volume, discharge coefficient of leakage hole, and flight altitude was researched. Nevertheless, systematic research on the influence factors of decompression load is still scarce, and there are no commonly accepted non-dimensional parameters to evaluate the decompression process.
In this paper, the 0-D model and the 1-D model [
13,
14] are proposed to simulate the decompression process for a cabin-cockpit system with windshield failure. The accuracy of these models is presented by comparing them with experiments and 3-D CFD simulations. Then, the 1-D transient model is used to simulate the decompression process of a cabin-cockpit model with a cracked windshield. The effects of cockpit and cabin volume, windshield and decompression panel area, compartment and environment pressure, and flight Mach number on decompression load are discussed. The non-dimensional time and non-dimensional decompression load are proposed to evaluate the decompression process, and the correlation equations are established. This work provides a new engineering method for decompression load analysis with high accuracy and low consumption.
5. Conclusions
In this paper, 0/1-D transient flow models are proposed, which enables a rapid simulation of the decompression process with high precision. The effects of several geometric parameters and aerodynamic parameters on the decompression process are investigated. The conclusions are as follows:
(1) The decompression process is divided into two stages: in the first stage, the decompression load rapidly increases to the maximum value; in the second stage, the decompression load drops slowly. In the decompression process, the pressure wave reflects after contacting the cabin wall, which causes decompression load fluctuation.
(2) Compared to the experimental result and the 3-D CFD simulation results, the decompression load predicted by the 0-D and 1-D models both experienced a rapid rise and then a slow decline, and the 1-D model accurately simulates the characteristics of decompression load fluctuation. The decompression load predicted by the 1-D model is more similar to that of 3-D CFD simulation, and the relative deviation of the maximum decompression load is only 1.54%. In addition, the calculation time of the 1-D model is only 0.05% of that of the 3-D CFD simulation.
(3) Effects of compartment pressure and back pressure of windshield: When the pressure ratio remains constant, with the compartment pressure rising, the decompression load increases by the same proportion, while the total decompression time does not have significant changes. When the pressure ratio increases, both the maximum decompression load and the total decompression time rise.
(4) Effects of cabin and cockpit volume: When the volume ratio remains constant, the decompression time is directly proportional to the total volume, while the maximum decompression load has no obvious changes. Within the range of typical volume ratio (10–20) of larger passenger planes, the maximum decompression load and total decompression time almost remain constant with the volume ratio increase.
(5) Effects of decompression panel and windshield area: When the area ratio remains constant, the decompression time is inversely proportional to the windshield area. As the windshield area decreases significantly, the fluctuation effect of the decompression load decreases, and the maximum decompression load decreases slightly. As the area ratio increases, the air circulation rate between cabin and cockpit increases, thus the maximum decompression load and total decompression time are greatly reduced.
(6) Non-dimensional parameters and correlation equations: The non-dimensional decompression load and non-dimensional decompression time are proposed based on the dimensional analysis method, and the correlation equations are established. The relative deviation between the results of the correlation equation fit and the results of the one-dimensional simulation are less than 3%.