Evacuation Slide Functional Issues
From SKYbrary Wiki
History shows that even current-generation evacuation slides require ongoing risk mitigation of several known functional issues. Aircraft crews involved in problematic evacuations may use terms such as “slide malfunction” or “slide failure” to describe what they experienced, but the survival factors analyses done during accident investigation tend to be more complicated.
In particular, on-scene conditions in a number of commercial air transport accidents — with winds exceeding regulatory requirements for aeroplane slide systems — proved to be a significant causal factor when slides did not deploy, inflate or stay in position as expected.
Other functional issues affecting evacuation slides, which are known in regulations as approved assist means, have involved the door and/or escape slide mechanism, the inflation system, external damage to air compartments of slides, and internal or external obstructions that cause abnormal slide operation.
The SKYbrary article "Emergency Evacuation on Land" notes that problems in operating emergency exits and deploying emergency slides have delayed many evacuations, potentially compromising the success of the evacuation. Cabin crewmembers supervising exits therefore must secure the exit until the slide (if the exit is so equipped) inflates and “block the exit from use in the event of a slide malfunction.”
This article discusses several related issues, investigative findings and recommendations, and implications mentioned by expert groups.
An emergency evacuation means the urgent abandonment of an aircraft utilising all useable exits. Typical reasons why exits are blocked by flight attendants for being “unuseable” are fire, water or other danger — but also predominant can be the unexpected functional issue of a slide’s unsafe condition or position, and problems operating a door, overwing hatch or tailcone exit.
For aeroplanes, the umbrella term escape slide encompasses single lane and dual-lane emergency evacuation slides, slide/rafts, exit ramps and combination ramp/slides.
The evacuation slide is a device folded, packed and installed inside a cabin door or into an external fuselage compartment. The slide deploys automatically a few seconds after an armed door has been opened with electrically powered actuation or manually with a handle.
Automatic inflation occurs until the erection of the air compartments stops (in a maximum 10 seconds). If functioning normally, the evacuation slide at this point enables safe descent from a floor-level exit or from an aircraft wing. (A manual-inflation system serves as an optional backup to the automation.)
The slide/raft functions similarly as a slide for an emergency landing on the ground and adds capabilities for evacuation after a water landing (ditching). The device’s mooring line, which fastens the slide/raft to a point near its exit door, can be cut by occupants with the supplied emergency knife. Otherwise, the line’s fail-safe breakpoint will release the slide/raft due to forces exerted by the sinking aircraft.
After separation, the slide/raft functions as a typical life raft with lanyards, heaving line, protective canopy and a fitting to accept a hand-operated air pump. Survival kit equipment, the hand pump, lights, raft-boarding station and emergency-locator devices support the rescue of survivors. Flight attendants carry equipment to slide/rafts and life rafts, such as emergency locator transmitters, megaphones, first aid kits and flashlights.
An exit ramp is an inflatable platform designed to help evacuees to safely descend from certain overwing exits to the wing.
A combination ramp/slide, also called a slide/ramp, is an inflated platform (ramp) extending horizontally at sill height from certain overwing exits, leading to an evacuation slide for descent from the wing to the ground.
Functional Issue Threats
Initial and recurrent training of cabin crews cover how functional issues might affect any type of escape slide. Slide deployment, inflation and stable positioning can be disrupted as described in the following examples.
- A prominent functional issue appearing in recent studies is strong wind shifting/lifting the slide into an unuseable condition or position in relation to the aircraft cabin. The applicable part of the relevant European regulation (EASA CS 25.810) states: “The assist means must have the capability to deploy in 46 kilometres/hour (25-knot) winds directed at it from the most critical angle with any engine(s) simultaneously running at ground idle, and to remain useable after full deployment to evacuate passengers and crew to the ground.”
- Another potential functional issue is the scenario in which the flight attendant makes errors in safety procedures while disarming a door and actuating the exit-opening mechanism. The door opens at high speed with extreme force. Residual pressurisation may pull the flight attendant out the door and deploy the slide, possibly damaging the slide enough to render it unuseable, seriously injuring the flight attendant and striking others outside the aircraft.
- Another potential functional issue is inadvertently allowing cabin waste items or excessive debris to interfere with the mechanisms used to arm/disarm and close/open the exit door, interfering with normal slide deployment and inflation.
- Another potential functional issue is mishandling or throwing carry-on baggage through a door — typically when passengers have disregarded crewmembers’ evacuation instructions to leave carry-on baggage on the aircraft — also damaging the slide enough to render it unuseable.
Defences and Solutions
The following are examples of how regulatory standards and guidance materials may help reduce the vulnerability of evacuation slides to functional issues during emergencies:
- A 2003 European Aviation Safety Agency (EASA) standard states: “The device [escape slide] must be capable of resisting puncture and tear of the sliding and walking surfaces and supporting structure from objects normally carried or worn by passengers that could result in collapse of the device, prevent the device from performing its intended function, or both.”
- EASA regulations require that the inflation system must be connected to the escape slide and ready for instant use; the inflation mechanism must minimise leakage due to back pressure after inflation; and the system design must prevent ingestion of small foreign objects to prevent an inflation failure or malfunction. Pressure-relief valves, if installed, must be unrestricted (i.e., not blocked by debris) to prevent damage to air chambers by overpressure.
- The EASA standard also states: “[The evacuation slide, slide/raft and combination ramp/slide] must be so constructed as to permit their use with ground personnel assistance as a non-inflatable device in the event of puncture or tear which may render the device incapable of holding air and sustaining inflation. … If the device is of a multiple-inflatable compartment construction, loss of any one of these compartments must not render the device totally unuseable.”
- In its 2018 summary of advances relevant to functional issues with slides, the Royal Aeronautical Society and the Honourable Company of Air Pilots, stated: “Aeroplane evacuation facilities have developed significantly over the years and many improvements have been made to emergency exits, including the installation of ‘power-assist’ systems to make exit operation easier and faster. This is especially the case for floor level exits on larger aeroplanes, many of which have large and heavy exits and some that need significant force to open and to deploy [automatically inflated] evacuation slides.”
Researchers at the Transport Safety Board of Canada in 2013 re-examined accidents involving commercial passenger transport aeroplanes in 1978–1992 and others from 1993 and 1994 (see References below). Their report described several recurrent functional issues — calling them “slide failures” — and stated: “Slides were deployed in 15 of the 21 evacuations examined. In seven of the evacuations where slides were used, there were problems related to their deployment or to their angle of inclination. The study report also contained the following event descriptions:
- “Wind had an adverse effect on the use of escape slides. In two evacuations where slides were used, the wind blew them up against the sides of the aircraft, thereby preventing their use until someone was able to exit the aircraft via another exit, reposition the affected slide, and hold it in place. Other exits were unusable for the entire evacuation. Wind velocity was recorded as southeast 17 kt gusting to 22 kt in one of these occurrences and at approximately 18 kt gusting to 28 kt in the second.
- "There were two occurrences where the slides did not deploy automatically. In both occurrences, they were deployed manually. However, on deployment, one slide went straight down into the ground and had to be repositioned from the outside before the exit was useable.
- “At Wabush, neither of the rear slides deployed properly. The slides were twisted, tangled, and curled back, almost under the aircraft, and they were only partially inflated. Both exits were temporarily blocked while fire fighters repositioned the slides. … Finally, there was one occurrence where the R4 slide would not deploy either automatically or manually. …
- “There does not appear to be a simple explanation why some slides did not deploy automatically or properly. In one instance, the problem was traced to excessive clearance between the bar on the door and the aft latch on the floor, which allowed the bar to pull free. … The Board recommends that: The Department of Transport, in concert with industry, re-evaluate the performance of escape slides on all large passenger-carrying aircraft registered in Canada, to confirm that they can be functionally deployed in accordance with the criteria of the Airworthiness Standard.”
Researchers at the National Aerospace Laboratory (NLR)–Netherlands, in a study based on accident/incident data and data from other sources, in the early 2000s, found that the most significant slide problem was failure to inflate. Examination of NLR’s study sample of 150 survivable aircraft accidents in which slide were used during 1970-2003 showed that in 81 of those accidents one or more slides did not function properly. The most significant slide problems involved inflation, aircraft attitude, wind, fire, incorrect rigging, and rips.
Accidents & Incidents
- B772, San Francisco CA USA, 2013: On 6 July 2013, an Asiana Boeing 777-200 descended below the visual glidepath on short finals at San Francisco after the pilots failed to notice that their actions had reduced thrust to idle. Upon late recognition that the aircraft was too low and slow, they were unable to recover before the aircraft hit the sea wall and the tail detached. Control was lost and the fuselage eventually hit the ground. A few occupants were ejected at impact but most managed to evacuate subsequently and before fire took hold. The Probable Cause of the accident was determined to be the mismanagement of the aircraft by the pilots.
- “Emergency Evacuation Slides, Ramps, Ramp/Slides and Slide/Rafts,” European Technical Standard Order (ETSO) C69c, by European Aviation Safety Agency, October 24, 2003.
- Cabin Operations Safety Best Practices Guide, 3rd Edition, by International Air Transport Association, 2017.
- “Emergency Evacuation on Land” by SKYbrary, August 11, 2018.
- A Safety Study of Evacuations of Large Passenger-carrying Aircraft by the Transportation Safety Board of Canada, Report No. SA9501, last modified October 11, 2013.
- “Analysis of Evacuation-slide Problems Calls Attention to Recurrent Issues,” by Gerard van Es and Hans Post, Flight Safety Foundation Cabin Crew Safety, May-June 2005.
- Emergency Evacuation of Commercial Passenger Aeroplanes by Royal Aeronautical Society and the Honourable Company of Air Pilots, 27 April 2018.
- Manual on the Investigation of Cabin Safety Aspects in Accidents and Incidents, by International Civil Aviation Organisation, Document No. 10062, First Edition 2017.
- “Survival Factors: NTSB hearing on Asiana Airlines Flight 214 expands insights into occupant protection, escape and rescue” by Wayne Rosenkrans, Flight Safety Foundation AeroSafety World, March 17, 2014.