Evacuation slide

An evacuation slide is an inflatable slide used to evacuate an aircraft quickly. An escape slide is required on all commercial (passenger carrying) aircraft where the door sill height is such that, in the event of an evacuation, passengers would be unable to "step down" from the door uninjured (Federal Aviation Administration requires slides on all aircraft doors where the floor is convert|6|ft|m or more above the ground).

Escape slides are packed and held within the door structure inside the "slide bustle", a protruding part of the inside of an aircraft door that varies in size depending on both the size of the aircraft and the size of the door.

Planes Without Slides

Airplanes such as the Embraer 145 family and the Bombardier CRJ family do not have escape slides because all exits are a safe distance from the ground (less than 6'). On the primary entrance door, 1L, some of these aircraft have stairs that are either connected to the door or drop down.

Window Exits

Some aircraft have escape slides on the main doors of the plane, but do not have slides over the wings due to the fact that, when the flaps are fully lowered, the wings are low enough to the ground that passengers can evacuate safely. Some of these aircraft are the Embraer 190, Boeing 707, 717, 727 and 737. However, other aircraft require the use of overwing slides for the window exits to ensure passengers are able to reach the ground quickly and safely. These include Boeing 767, Boeing 757, Airbus A320-series aircraft. Typically, overwing evacuation slides are not designed for use in ditching situations as they cannot be detached and will not operate, as the system is disabled by the aspirators on the slide taking in water.

Window exits usually come in two configurations:
*An "unhinged hatch type exit", where the hatch is unlocked from the inside and pulled into the cabin, whereupon it can be disposed. Some carriers recommend placing the hatch onto the adjacent seats, while others may recommend dropping it in the next seat row, or rotating the exit and throwing it outside the aircraft as far forward as possible. A manual inflation handle for the evacuation slide, if equipped, can be found in the window frame. Most aircraft overwing exits are of this type.
* A "hinged "self disposing" exit hatch", that opens automatically outward using a spring when the exit handle is pulled. This exit design was designed in response to research generated after the Manchester air disaster which indicated that unhinged hatch type exits were difficult to open by untrained passengers. This design is currently found only on Boeing 737 NG aircraft.

Window exits are usually equipped with "ditching" or "life" lines. These may be attached to the inside frame of the window exit, or located in a nearby storage locker. One end has a buckle to connect to attachments on the aircraft's wings.

History

The first aircraft evacuation slide was developed and produced by Air Cruisers, founded by James F. Boyle, inventor of the WWII life vest, the "Mae West". Prior to inflateables, some passenger aircraft utilized canvas type slides which required the crew to undertake an extensive rigging procedure. Canvas type slides are still found on some Russian aircraft.

Types of slides

There are two primary types of aircraft evacuation slides: slides and slide/rafts. A slide is for use only on land as a means of escape, although it has sufficient buoyancy to allow passengers to hold on to a lanyard running the length of it and use it as a buoyancy aid. A slide/raft is an evacuation slide that can be used both as a means of escape in a land evacuation and also as a life raft in a landing on water. Slide/rafts usually feature an erectable canopy, outer compartments to hold passengers and survival packs containing items such as leak stoppers, paddles and flares.

Slides can also be single or dual lane, depending on the width of the exit, a dual lane slide being capable of evacuating a greater number of people quickly in an evacuation.

Slides and slide/rafts can be detached from the girt bar, usually by a two or three step procedures. This may, for example, involve lifting up the flap on the girt bar, and pulling the detach handle. These procedures are usually placarded red on the slide, "For Ditching Use Only". Once the slide is separated, the slide remains attached to the aircraft by a mooring line. This line will break if the airframe submerges, or can be disconnected with a pre-supplied knife or disconnect handle.

A ramp slide is an evacuation slide that has a small platform between the exit and the slide itself, and is used mainly where the proximity of the exit to an engine requires the slide to be angled away from the engine to prevent damage. Airbus A310, Airbus 340-600, Airbus A380 and Boeing 747 aircraft have ramp slide attachments for their overwing evacuation slides. The overwing exits on the Airbus A320 series, Boeing 757-variant and Boeing 767-variant aircraft also utilize ramp slide attachments.

Another unique type of evacuation slide is found on certain DC-9, MD-80 and Boeing 717 aircraft. This type of slide is located in the aircraft's tailcone. This slide deploys after the tailcone is jettisoned by flight attendants, allowing for evacuation through the rear of the airframe. The procedure to use this exit may involve removing a plug-type pressure bulkhead, or a swing type door that leads directly to a walkway. At the end of the walkway is the slide pack and a manual tailcone jettison handle for use if the tailcone has not already been automatically jettisoned by opening the walkway entrance.

Certain evacuation slides do not use a slide bustle on the door as a container. Instead, the slide is "fuselage mounted" and is attached to a container located underneath or below the exit close to the aircraft exterior. This design of slide is found in the Airbus A321 aircraft at the emergency doors, and typically at all overwing evacuation slides other than the Boeing 747-400 series aircraft.

One of the newest development in evacuation slide technology can be found on the Airbus A380, which was developed by Goodrich Aircraft Interior Products. Certain slides on board the aircraft have the Tribrid Inflation System, which is connected to a sensing system within the door. If the door is opened in emergency mode at an abnormal attitude (e.g. nose up position due to the loss of landing gear), the slide will inflate normally but will also inflate several feet of additional slide to ensure the slide reaches the ground. This compares differently to the Boeing 747 as doors found on that aircraft have no such system; should the slide not reach the ground, the doors must be blocked to prevent passenger injury.

Operation

Prior to departure (usually before engine startup) all the aircraft doors are placed into the armed (or automatic) mode by the cabin crew. Methods of arming vary from aircraft to aircraft, but ultimately what is involved is the girt bar (a metal bar attached to the door end of the slide) being physically attached to brackets either in or adjacent to the door sill. On older aircraft, such as the Boeing 737, this is done physically by the cabin crew and on most other aircraft it involves pushing a lever on the door itself which arms the door internally.

If a rapid evacuation is required and the doors are opened while "armed", the opening of the door pulls the slide pack out of the bustle (because the girt bar as physically attached to the aircraft floor). Due to the weight of both the door and the slide, great effort is involved in pushing the door open sufficiently to free the slide from the bustle, thus on larger aircraft a "power assist" function kicks in to aid the opening, either electrically or from compressed gas. Once the slide is completely free it will fall under gravity and after travelling a certain distance a pin will be pulled from a squib containing compressed gas and the slide will inflate. Should this system fail, the slide can be manually inflated by the cabin crew by pulling a manual inflation handle at the top of the slide. Should this also fail, standard operating procedures require the cabin crew to send passengers away from the door and to one that has a functioning escape slide.

Some Russian-built aircraft like the Tupolev Tu-154 have a very complicated process to activate and deploy the slides. The slides are stored in cabinets usually beside the emergency exit inside the aircraft. There are usually about the same width and height as a seat. To activate the slide, one must pull the front cover to a 90 degree angle, then pull the slide out so it is lying flat on the floor or door sill, open the emergency exit and kick or push it out. Gravity will then pull the slide to the ground and it will inflate.

Aircraft safety cards and in-flight safety demonstrations show the passengers where the nearest emergency exits are and how to use the evacuation slides. Additionally, Flight Attendants receive extensive safety training that covers the use of evacuation slides.

Exiting Safely Using the Slide

In an article in TIME, Amanda Ripley and Dan Johnson, an aviation safety expert, compiled some tips on how to avoid injury and escape from a plane on an inflatable slide. Here are their suggestions. Number one: Have a Plan. Know where the exits are when you sit down on a plane. Two: Have a back-up plan. Passengers often have trouble opening the exit hatches and slides sometimes malfunction. Three: Get out fast. Don't even think about grabbing your overhead luggage. Four: Jump! Johnson says if everyone would jump the evacuation could go 50% faster. And faster is better. Five: to avoid burns keep your heels up and your arms crossed over your chest. Women should avoid wearing pantyhose and spiked heels when they fly. The hose can melt onto the skin in the heat of a plane fire. Six: When you reach the bottom, get out of the way. Pile-ups at the bottom of the slide can be brutal and slow down the exit for everyone. [ [http://www.time.com/time/nation/article/0,8599,1706188,00.html How to Escape Down an Airplane Slide - and Still Make your Connection!] Amanda Ripley. TIME. January 23, 2008.]

Inadvertent Slide Deployment

Inadvertent slide deployment occurs when the operator of the aircraft door attempts to open the door when it is in the armed position. This costs the industry millions in lost revenue every year, estimated at $20 million in North America by cabin crew alone. [cite web|url=http://www.iata.org/training/toolkit/slide.htm|title=Slide Deployment|author=IATA|accessdate=2007-07-24]

A device can be used to prevent this problem. It works by sounding an audible alert (voice) when the door operator, whether trained or not, is about to open the door in the armed position. It works as an independent system, requiring no action other than arming the door as per normal standard operating procedures. When the door is placed in the armed position, the device is armed. It can be installed as a stand alone unit or integrated into the aircraft systems and powered from aircraft power.

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 cu. in. cylinder can fill a slide with about convert|850|cuft|m3|abbr=on. of air to a pressure of about 3 psi in about 4 - 6 seconds.

In order 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 convert|3|ft|m|abbr=on. wide, convert|2.5|ft|m|abbr=on. wide and about 12" 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 convert|2|ft|m|abbr=on. 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.

In order 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 convert|25|kn|km/h|sing=on 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 in order that the slide will survive for a short while even if fire is nearby. Older slides are yellow and made of neoprene/nylon fabric.

References