Evolution of Modern Aircraft Cabin Safety

DALLAS — Every time you step into an aircraft, you are entering an environment that is highly controlled to be as safe as possible. From the design of the doors and seats to the safety briefing, everything is highly regulated. However, many of these regulations are sometimes considered to be "written in blood."

What this means is that in aviation, cabin safety has evolved from what it used to be decades ago. Unfortunately, a lot of what was learned has come from when things go wrong, i.e., when aircraft end up in an accident.

Many times, it is accidents where not all passengers survive that give a lot of information to investigators and designers. This is logical. When all passengers survive, that means everything goes right; in many accidents where all passengers perish, the "container" or fuselage has catastrophic damage.

Recent reports have surfaced stating three people were hurt during an American Airlines (AA) aircraft evacuation at San Francisco International Airport (SFO) when smoke started to fill the cabin while others were still boarding.

The back left cabin door of the AA Airbus A321 aircraft seemed to have been opened in an effort to evacuate the passengers, but the slide did not go down. This was most likely due to the fact that the aircraft was still at the gate and the slide was not armed.

In light of this incident, this article will briefly step through how an investigation works under ICAO Annex 13; then, what a recommendation is; and finally, it will look at case studies on how investigations have shaped modern cabin safety.

What Exactly Is an Investigation?

When reading about aviation accident investigations, the term ICAO Annex 13 gets thrown around a lot. That is since Annex 13 is a guideline for all countries on how an aviation accident should be investigated.

There are different parts of the Annex 13 guidelines, from how the accident should be registered and who should take part to how to write the final report.

When an accident or incident occurs, the first step is that it needs to be officially reported. This starts a whole chain of events based on the severity of the accident or incident.

For the actual investigation, the country of occurrence in most cases will run the investigation, and they will invite the countries where the aircraft and engines are manufactured. Other parties invited are usually the operator and its country, plus the countries of the citizens on board.

The first steps will be to collect facts and data from the site, as well as to secure the black boxes. This is a priority to be done in the first hours and days, depending on external factors such as weather or the need to reopen an airport. The site will not be able to be preserved.

Beyond the traditional wreckage gathering and interviews, there are also novel investigation methods being used. One common one is to use drones to map the site; another is to use a laser scanner device for the same purpose.

Fire damage can give a lot of clues to the investigators, but it can also hide evidence and require in-depth analysis.

Out of every accident investigation, there are always recommendations to the aviation industry, or specific players, on how to improve safety for the traveling public.

What Is a Recommendation?

At the end of every accident report is a section titled “Recommendations.” These recommendations can be for any part of aviation and are based on the findings of the investigation.

Investigators have looked at the whole sequence of events and usually identified areas where practices can be changed to make aviation safer.

The regulatory authorities accepted some of these recommendations before the final report was even released. Others, like the flight deck video recorder, appear repeatedly in many different reports.

As the goal of the investigation is only about safety and not about blame or financial reasoning, sometimes there is a lot of pushback on some recommendations. The most current example is the flight deck video recorder, which, while invaluable to the investigators, has massive privacy concerns for the Pilots themselves.

It is highly unlikely, therefore, that a video recording will ever make it into the flight deck.

How Did We Get Here?

Aviation cabin safety is a special sector of safety since the normal traveling passenger is not in any way trained regarding aircraft. Beyond that, the cabin needs to be designed for people of all sizes, from the seat sizes to the seat pitch and the size of the doors.

What does this mean in a practical sense? It means that things are designed for an "average"-sized person, trying to take into account the averages for both female and male builds. So people who are on the lower or higher end of the size range might have trouble using parts of the cabin.

There is also a lack of accessible design for disabled passengers, even with the strides made in parts such as the toilets.

The regulation of cabin design is something that has come up in the last few years, as some of the regulations are quite old. However, changing these older regulations can be challenging as it usually requires testing and research, which can be expensive and extensive.

What this can mean is, as stated above, that changes to regulations usually only happen after something goes wrong. Unfortunately, that means that, usually for cabin safety, people might be injured or even die before the regulation comes under scrutiny.

Case Studies

The next sections will cover some specific case studies and their impact on current-day cabin safety. All information related to the accidents themselves will be taken from their respective accident reports, written and released by the corresponding accident investigation organization.

Evacuation

Evacuation has become more of a hot topic in the last few years, as there is only a limited amount of time that the people on board can safely exit. The current regulation is listed at 90 seconds for a full aircraft of people to exit with only half of the doors in use.

Both the time and the limited exits are based on what could be expected in actual crash conditions. The time is also based on what could be expected due to fire and smoke, the usual primary hazards after a crash occurs.

There have been many cases in recent years where evacuation was successful, even if the plane was subsequently destroyed by fire. Speed is of the essence before the fuselage is penetrated by fire or the smoke overtakes the people on board.

As in the case of Pacific Western Airlines (PWA) 501, delays can occur due to confusion and not enough information. While everyone on board survived, minutes were lost due to miscommunication.

These delays mean as well that there might not be 90 seconds anymore for the people on board to evacuate. In the case of PWA 501, the investigators stated that the last passengers evacuated at the last possible moment.

A big point that has been at the forefront of evacuation safety is the fact that passengers are taking their cabin baggage with them when they evacuate. Combined with airlines charging for checked bags, the amount of luggage carried in the cabin is ever-increasing.

The luggage taken on board includes not just small personal items, but also large rolling bags, which are more of a concern. Since passengers have been taking photos and videos of evacuations, there is evidence that around 50% of passengers are taking their bags with them when evacuating the aircraft.

While so far there have been no injuries or deaths related to baggage taken by passengers during the evacuation, it might become an issue in the future. Taking bags cuts into the 90-second window and makes it harder to maneuver within the aircraft. In addition, the bags themselves can puncture or damage slides.

The RAeS states in its Emergency Evacuation of Commercial Passenger Aeroplanes, 2nd Ed. 2020, that it believes that certification of evacuation for aircraft does not take these larger pieces into account. The baggage used in the certification runs is usually lightweight and not representative of the bags people are bringing.

Lockable overhead bins are perhaps an option, but the hazards might outweigh the benefits. If it is the case that passengers try to break into the overhead bins, it could use up valuable time.

At best, an aircraft evacuation is a highly stressful and fearful experience. With a lack of clear communication between the flight deck and cabin, as well as hazardous conditions such as engines still running and fuel spills, an evacuation can become a hazard within itself.

Laudamotion had an incident in 2019 where an evacuation was initiated by the cabin Crew without the Flight Crew being aware. The passengers exited the aircraft in the path of the running engine, and if they had moved into the danger area, they could have been seriously injured.

Dynamic International Airways had a similar event where a passenger was seriously injured by passing behind the running engine.

Fire

Fire could be considered one of the biggest hazards to commercial aviation. In an accident, the fire might be one of the starting hazards, or it might be a hazard once the aircraft has reached the ground.

For a fire to exist, it requires three things: fuel, oxygen, and heat. Fuel can be anything from cabin materials and cargo to the aircraft fuel itself.

The addition of oxygen can be hazardous when opening doors during an evacuation, as the fresh air can cause a flashover within the cabin. Or, in the ValuJet 592 case, the oxygen can be from a different source. In this accident, a shipment of expired oxygen generators was packed improperly in the cargo hold.

Oxygen generators create oxygen through an exothermic reaction, meaning that the canisters themselves become very hot. This heat, plus the oxygen emitted and further fuel within the cargo hold, allowed the fire to quickly get out of control.

The investigation into ValuJet 592 led to the recommendation to have smoke detection and fire suppression systems in the cargo hold.

Heat sources can be due to the engines and electric shorts within the aircraft wiring system. The engines are important when there are fuel spills. While aviation fuel is very stable in its liquid form, once it is heated past a flashpoint, it will explode.

According to the final report for the TWA 800 accident, the heated fuel vapors were ignited by a wiring short that caused catastrophic damage.

While TWA 800 was a catastrophic accident, this aviation fuel-heat source interaction is one of the big reasons why evacuation is conducted as fast as possible.  Beyond just exiting the aircraft, passengers need to get away from the plane itself, especially if firefighting services are not present and spray foam.

For the cabin, Swissair 111 is an example of flammable and non-flame-resistant materials being used in aircraft with catastrophic results. The Transportation Safety Board of Canada found that the oxygen system in the aircraft could fail in a fire, thereby releasing oxygen to further feed the blaze.

The hazard of Swissair 111 in terms of cabin safety is that the fire was inaccessible and was not able to be accurately analyzed. The investigation into this accident found that even though there were already regulations on materials used within aircraft, items were still found to be flammable.

The outcome of the investigation was a major push to clarify testing methods and ensure that improperly flammable materials were no longer used within aircraft and cabins.

Air Canada 797 had a hidden fire somewhere in the rear of the aircraft (the exact location was not determined). The recommendations from this investigation have led to the inclusion of fire detectors in lavatories, automatic fire extinguishing systems in waste bins, and the inclusion of modern halon fire extinguishers.

For cabin safety, the goal is to not have a fire start. The fires that are occurring now are also usually different than the ones that happened in the past. Now, we are hearing more and more about personal electronic devices catching fire, many times due to battery punctures. A battery puncture can occur when the device is jammed into an aircraft seat.

This shift in fire causes might mean that the regulations will again need to change. We have already seen it in the flight safety briefing at the beginning of a flight. Furthermore, on some aircraft, safety devices are carried by Crew to isolate these passenger electronic devices if they catch fire.

Smoke

Where there is fire, there is smoke, and in some cases, the smoke can be more hazardous. In their base forms, many materials used in aircraft are quite flammable, but the materials have been adjusted to fulfill the certification requirements either with self-extinguishing technology, slower-than-normal burn materials, or other requirements.

However, even with these newer materials, they will still break down and emit toxic smoke at some point. Ideally, this breakdown occurs once an evacuation has been completed and all passengers and Crew are moved to safety.

The aforementioned Air Canada 797 incident also affected the regulations relating to smoke. The addition of emergency lighting was mandated, as was the addition of smoke hoods for the Crew.

When the acrid smoke gathers in the cabin, it first fills the upper part. This smoke is impossible to see through, which is why emergency lighting is on the floor and seat areas.  For AC797, the investigators found that passengers had gone past the overwing exits and succumbed in the rear of the cabin.

Since smoke is directly related to fire, tacking on the fire will also remove the smoke hazard. There have been changes to materials to prevent them from emitting acrid gases for as long as possible, but that is only a stopgap measure until full material breakdown occurs.

That means that fire prevention, timely landing, and quick evacuation will be the first lines of defense against a smoke hazard. Moving the passengers to less smoke-impacted areas can also help with passenger health as well as evacuation times.

Brace Position

The brace position, as we know it, is relatively new. British Midlands 92 was the accident that spawned a lot of research on the topic. The reason why this accident was the key is relatively morbid, but it was because not all passengers died, and some escaped without serious injury.

I was lucky enough to be present at a lecture by Professor Angus Wallace on this topic while at Cranfield University. Professor Wallace treated several passengers in the accident and then did further research into what a better brace position could be.

British Midlands 92 occurred in 1989, and in the ’90s, research was conducted into what would be the best passenger brace position. However, this type of research was very expensive, as it needed to be conducted with live testing with dummies. In the decades following this research, other groups worldwide conducted further research.  However, as could be expected, the outcomes of the studies did not all agree.

The goal of the brace position is to prevent injury to passengers so that they can subsequently exit from the aircraft in an evacuation under their power.

As passenger seatbelts are only lap belts, they require bracing of the lower and upper body to prevent impact with objects around the passenger, such as the seat in front. As seat pitches can vary and passengers are of varying sizes, the testing needs to be exhaustive to determine the most effective brace position.

As of 2016, the IBRACE group was formed, which is supported by the ICAO cabin safety group. The goal of this group is to plan and conduct the research needed to make a final recommendation on what the proper brace position should be.

Summary

The research and testing required for cabin safety items can be extremely expensive or raise ethical concerns when considering evacuation research.

This can mean that the investigation into accidents and incidents is perhaps critical to further safety in aviation. At the very least, it can help direct what aspects require further research.

“Written in blood?” Perhaps. But what kind of engineers would we be if we did not intend to learn from the aforementioned events to try and make aviation safer?