Flight attendant gives safety briefing onboard

How the design of cabin safety equipment works


Following Southwest Airlines Flight 1380, there has been active speculation and energetic discussion on social media about possible design improvements to oxygen masks. It’s understandable. When things don’t make sense we try to make sense out of them.

But these products have a long design history, and a firm foundation. They have been tested over decades, to specific parameters of performance based on many real-life emergency conditions.

They have evolved from post-accident findings of investigators like the NTSB and their global peers. Approval tests involve real people in lab conditions that simulate an accident which are recorded and provided as evidence of function to regulators.

All design is a balance of form and function, but in the design of cabin safety equipment form follows function very closely. There is little room for variation – if any. What variation exists is there to enhance function. All designs are intended to be sturdy, easy to maintain, and reliable.

Cabin safety equipment – like other cabin equipment – must prove conformity with Technical Standard Orders (TSOs) issued by regulators. These TSOs have evolved from MIL-STD or MIL SPECS (military standards or specifications) where design is informed by experience in the field. Others are based on SAE (Society of Automotive Engineers) standards where design is informed by learnings from real-life commercial use.

It might seem odd to use “automotive” engineers to develop safety equipment for commercial airlines, but there are many industries in the SAE, including a dedicated aviation group. When the SAE was established in 1905 the objective was to set consistent engineering and safety standards that would ensure the safe operation of automobiles. In 1916 the SAE branched out to support all “mobility solutions” including the developing field of aeronautics. Orville Wright himself supported the incorporation of aeronautics into the activities of the SAE.

Over the years, the standards have changed, been refined and improved. The TSOs are under constant scrutiny. When designers and manufacturers come up with ways to improve the product, revisions to standards ensure conformance among all manufacturers.

Changes to TSOs are deliberate and carefully considered to avoid introducing a new failure mode with changes. Engineers and regulators must agree that the new product will work better in a wide variety of accident scenarios than the previous product.

We can look at some of the suggestions that have come up in the case of oxygen masks in recent days against these principles of design.

  1. Reshaping the mask—The mask is round so that it works when passengers cannot judge which way is up. Donning the mask quickly is the first priority, because loss of consciousness can occur in seconds. The silicon mask is flexible so that it can be shaped into an oval that fits over the nose and mouth, no matter how the passenger grabs it.
  2. Placing better instructions on the mask—Both oxygen masks and life jackets are marked with illustrated instructions that anyone can interpret, even if they don’t know how to read. This is a core design requirement. The instructions on the proper way to don the mask are stamped indelibly on the attached plastic bag which helps deliver the oxygen. One of the images which have circulated on social media shows passengers wearing the mask wrong, with these instructions clearly in their line of sight, but their focus is elsewhere.
  3. Adding sensors which can alert the user if they have the mask on wrong—It is an appealing idea, but any power source used for the sensor and its form of notification could not be a source of potential ignition while the oxygen bag is in storage – anything that could ignite oxygen would pose a deadly hazard to an aircraft. Even if that can be resolved, a sound alert would need to be clearly discernible over the noise in the cabin during any emergency setting so that a passenger would hear it, but also not get confused by an alert directed at the passenger next to them. A light source indicator would face the same challenges.

The general principle is to keep cabin safety equipment as analogue as possible, and as intuitive as possible, so that it will work when everything else fails. One example is the signal light attached to life jackets. To ensure safety and reliability, it uses chemistry. When the inert materials in the battery source come in contact with water, they generate enough power to turn the light on. Otherwise, they are just there, waiting to work and posing no risks to flight safety.


But there’s always the human element. People do unexpected things. That is why a study of human factors is critical. There may be a way to ensure safety compliance, and address modern distraction, without breaking what works.

For example, some cities are experimenting with street crossing signals designed for people who walk while looking down at their phones. With Wi-Fi onboard, maybe airlines could push short safety instructions to passenger devices, before flight and during an actual emergency. It might not work in all situations, but the existing layers of safety design would still be in place.

Clearly, something is broken with the current procedures. The truth is that there are no easy answers and plenty of good questions. But the aviation industry has the resources to design solutions that work. It is time to study what is happening and fix what needs to be fixed.

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