In this blog post, we’ll examine the structure and operating principles of emergency evacuation slides, which help passengers evacuate quickly in critical situations such as plane crashes, as well as the technology that rapidly inflates these massive slides using a small amount of compressed gas.
Even if a car or ship breaks down while in operation, it can, depending on the situation, stop, undergo repairs, and resume operation. However, an aircraft flying in the sky cannot simply stop mid-flight to be repaired if a problem arises. For this reason, aircraft are subject to strict safety management systems, including pre- and post-flight inspections and regular maintenance, and are equipped with various safety devices to protect passengers and crew in emergency situations. The emergency evacuation slide is one such device, designed to help passengers quickly and safely exit the aircraft and reach the ground in an emergency. It resembles a slide made of a large inflatable tube. Depending on the type of aircraft and equipment, there is also a “slide/raft” configuration in which the emergency evacuation slide can be detached from the aircraft and used as a life raft on water. Emergency evacuation slides are normally folded into a very compact size and stored in compartments near the exits, but when deployed in an emergency, they expand into a massive structure in a very short time using compressed gas and outside air. Today, the requirements for emergency evacuation devices used on large aircraft vary depending on the type of exit and the device’s design; however, typical slides for floor-level exits are designed to fully deploy within a few seconds after activation. In particular, standard regulations require that devices for floor-level exits be deployed and ready for use within 6 seconds. Although such a massive slide must be deployed in just a few seconds, aircraft are subject to strict space and weight constraints; therefore, simply storing compressed air equivalent to the slide’s total volume in gas cylinders is not an efficient solution. Therefore, engineers have developed a method that uses a relatively small amount of stored gas while drawing in surrounding air to rapidly inflate the slide. This method utilizes the principle of a jet pump or aspirator, in which a rapidly flowing fluid draws in surrounding fluid.
A rapidly moving stream of air can draw in surrounding air. While this is often explained simply by Bernoulli’s principle—that “as the speed of air increases, pressure decreases”—it is more accurate to understand the inflation mechanism of an emergency escape slide as the action of a jet pump, in which a high-speed stream of gas draws in surrounding air and carries it along. When compressed gas passes through a narrow passage at very high speed, a strong jet flow is created, and this flow draws in surrounding air, carrying it along into the slide. In everyday life, you can also experience the phenomenon where airflow changes significantly around fast-moving objects. For example, when a train passes by at high speed, you can feel the strong airflow it generates if you are nearby. However, it is inaccurate to simply describe this phenomenon as the “train sucking people in.” In reality, the train’s movement causes complex changes in the pressure and velocity of the surrounding air, resulting in strong turbulence. Therefore, on the platform, you must stay behind the safety line and maintain a sufficient distance from a rapidly passing train.
The sensation of your ears popping when a train passes through a tunnel is also related to changes in air pressure. However, it is difficult to explain this simply as the air around the train slowing down or speeding up and thereby drawing in air from the passenger compartment. When a high-speed train enters a tunnel, the air in front of the train is rapidly compressed within the narrow tunnel, generating a pressure wave. This pressure wave travels along the tunnel and reflects off its walls, causing changes in the external pressure relative to the passenger compartment as the train enters and travels through the tunnel. Modern high-speed trains mitigate these changes by improving cabin airtightness and using pressure regulation systems; however, when external pressure changes are transmitted to the cabin to some extent, a temporary pressure difference can occur between the inner and outer parts of the ear. This exerts force on the eardrum, causing the ear to feel stuffed up or uncomfortable. This is fundamentally similar to the reason why ears feel stuffed when an airplane ascends or descends. Therefore, the ear discomfort experienced when a train passes through a tunnel is not so much a phenomenon directly identical to the operating principle of the suction device used in emergency evacuation slides, but rather another example illustrating how airflow and pressure changes—caused by a rapidly moving object in a confined space—manifest in our daily lives.
An emergency evacuation slide consists primarily of a structure made of an air tube that inflates significantly and a device that rapidly inflates it. Under normal conditions, the slide is stored in a dedicated compartment near the aircraft door, tightly folded. In a situation requiring an emergency evacuation, the flight crew sets the door to emergency mode and opens it; as the slide unfolds outward, the inflation mechanism activates. However, the slide does not inflate automatically simply by opening the emergency exit in every situation. Aircraft doors are equipped with a mechanism that distinguishes between the normal boarding and deplaning mode and the emergency evacuation mode; during normal operations on the ground, this mechanism is disengaged to prevent the slide from deploying when the door is opened. Conversely, when the slide is set to activate for emergency evacuation, it is designed to automatically deploy and inflate upon opening the door. Measures are also in place to allow flight attendants to manually activate the inflation mechanism in case automatic inflation fails.
The mechanism that inflates the slide uses gas stored under high pressure, and the specific type and composition of the gas may vary depending on the design of the aircraft and the slide system. It is important to note that the stored gas alone does not fill the entire slide. When the compressed gas is released, it passes through a narrow passage at a very high speed, creating a powerful jet flow. As this flow passes through a suction device or jet pump, it draws in the surrounding atmosphere into the system. The external air drawn in this way enters the massive air tube along with the stored gas. As a result, the amount of high-pressure gas that the aircraft must carry on board is reduced, while still securing a much larger volume of gas required for slide inflation. This method has the advantage of reducing the volume and weight of the emergency evacuation device while enabling the rapid supply of a large volume of air when needed.
This principle is one of the key technologies that enables the emergency evacuation slide to be stored within the confined space of an aircraft while still deploying rapidly to its full size in an emergency. Since aircraft must minimize weight as much as possible, it is not practical to store and transport all the air needed to fill a massive slide in heavy high-pressure cylinders. Instead, only the necessary amount of high-pressure gas is stored, and the high-velocity flow created when that gas is released draws in surrounding air to inflate most of the slide. This suction method has been a key technology since the early days of aircraft emergency evacuation slide development, and as the devices have evolved, the inflation time has been significantly reduced. Today’s emergency evacuation devices must do more than simply inflate quickly. Once fully deployed, they must maintain their shape and pressure even as passengers descend one after another, and they must remain functional for a certain period under various conditions expected in emergency situations, such as radiant heat from a fire and wind.
The role of an emergency evacuation slide goes beyond that of a simple slide. In the event of an aircraft accident, if there is a possibility of fire or smoke after the aircraft has come to a stop, how quickly passengers and crew can evacuate the aircraft can be a critical factor in survival. For this reason, the aircraft emergency evacuation system operates as a single integrated system, working in conjunction with exits, emergency lighting, floor-level guide lights, and the crew’s emergency procedures. In an actual emergency evacuation, passengers must follow the crew’s instructions and proceed to the nearest “safe” exit; however, even a nearby exit may be unusable if there is a fire or other danger outside. Furthermore, stopping to retrieve carry-on luggage not only blocks the aisle and delays others’ evacuation but also risks damaging the slide or injuring other passengers. Therefore, when an emergency evacuation order is given, it is crucial to leave your belongings behind, move immediately, and follow the cabin crew’s instructions.
The process by which a compact, folded device transforms into a massive structure capable of carrying a person down in just a few seconds involves more sophisticated engineering than one might expect. The suction technology—which rapidly releases a limited amount of stored gas and draws in surrounding air to secure the necessary volume—is a prime example of engineering that achieves a significant effect with a small device. To create a safety device that can be used in actual emergencies, factors such as the slide’s strength, heat resistance, expansion speed, attachment to the aircraft, wind stability, emergency lighting, and passenger evacuation routes must all be taken into account. Although emergency evacuation slides are folded away near the doors and largely out of sight during normal operations, they become vital life-saving devices that allow passengers and crew to quickly evacuate the aircraft in an emergency.
