Evacuation Slide - Inflation Systems

Inflation Systems

Both slides and slide/rafts use a non-explosive, inert gas inflation systems. The FAA requires evacuation of the entire aircraft in 90 seconds using 50% of the available evacuation exits. To meet this, all evacuation units need to deploy in less than 10 seconds. For large, wide body aircraft such as A300's and B747's a successful deployment is complete in about 5–7 seconds, depending on conditions (such as cold and winds).

The inflation system usually consists of a pressurized cylinder, a regulating valve, two high pressure hoses and two Aspirators. The cylinder can be from 100 to about 1000 cubic inches, filled to about 3000 psi with either gaseous Nitrogen, or a mixture of gaseous CO₂ and Nitrogen. Once made of steel, most cylinders now are made of aluminum or alloy cores wrapped with fiberglass, or other lightweight, fuel saving materials. The CO₂ is used to slow down the rate at which the valve expends the gases.

The valve is used to mechanically meter out the gas at a rate of roughly 3 - 600 psi and 4 CFM. Typically there are two high pressure hoses attached to the valve, which are connected at the other end to 'Aspirators'. These are usually cylindrical, hollow aluminum tubes with sliding cylindrical or internal 'flapper' doors that open when high pressure gas is applied, and close when the gas stream subsides and the internal slide back pressure reaches about 2.5 - 3.0 psi. They work on the 'Venturi' principle, and draw outside air into the evacuation unit at a rate of about 500:1. A 750 in3 (0.43 ft3) cylinder can fill a slide with about 850 cu ft (24 m3). of air to a pressure of about 3 psi in about 4 – 6 seconds.

For the slide to deploy correctly, it is packed in a manner that has the aspirators directly under the outer cover. The entire, self-contained "slide pack" is approximately 3 ft (0.91 m) wide, 2.5 ft (0.76 m) long and about 1 ft (0.30 m) high, depending on aircraft type. In the center, forward part of the pack, a multi-layered piece of heavy urethane or neoprene/nylon fabric, called the 'girt', is left hanging out to a length of about 2 ft (0.61 m). When installed in the aircraft, a 'girt bar' is put through the center, outside end of the girt and attached to the interior floor, just inside and in front of the exit door. On the face of the girt are instructions in large red lettering, and a handle with the word 'PULL' on it.

This is rarely used however, because the lanyard attached to the handle runs through the girt to the valve, which is several inches too short when the girt is extended fully. When the slide is in the 'armed' position and the door is opened, the slide pack falls free of the door bustle (a semi-rigid outer container) and the weight and momentum of the slide pulls the lanyard from the valve, initiating the flow of gas. At about the same time, a metal pin that holds the center of the Valise closed is also pulled, releasing a 'daisy chain' and the two halves of the cover. When the cover is released and the inflation system activated, the two aspirators come shooting out of the pack, gulping vast quantities of air and restrained only by the fabric tubes to which they are securely fastened.

To compensate for any wind, new evacuation slides contain internal baffles, which cause the ends nearest the aircraft to inflate first, which are constructed to come out like four elbows and press against the fuselage of the aircraft to the forward and aft sides of the exit door. There are also 'half-tie' restraints which keep the inflating slide from drooping or blowing under the aircraft. These restraints are constructed so that when the slide becomes fairly rigid, around 1.5 - 2.0 psi, they detach very quickly (there are usually two), and since the header tubes are already against the fuselage, the slide 'pops' almost horizontally out from the door, then drops relatively gently to the ground. Tests in 25-knot (46 km/h) cross winds have shown these deployment systems to be very effective.

Independent of the inflation system, all slides are equipped with at least one pressure relief device per inflation chamber. This protects the chamber from catastrophic failure due to over pressurizing. (Typically, modern slides are made of at least 2 inflation chambers, and should be able evacuate an aircraft even when one chamber loses all pressure.)

All new evacuation slides are tested on a mock-up of an aircraft exit door and filmed prior to being certified as 'airworthy' and delivered to a customer. Also, new units are usually constructed of urethane materials and impregnated or coated with an aluminized coating so that the slide will survive for a short while even if fire is nearby. Older slides are yellow and made of neoprene/nylon fabric.