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The NTSB report about the crash of American Airlines Flight 1420 discusses whether an EMAS would have mitigated the severity of the accident, but concludes, based on testimony from a representative of an EMAS manufacturer, that

...the benefit of an EMAS in this accident would have been limited by the airplane traveling partly outside the runway edges; thus, the airplane would not have been able to use the full length of the EMAS. The report also concluded that an EMAS would have reduced the speed of the airplane by 15 knots but would not have enabled the airplane to stop within runway 4R’s runway safety area. [report page 64/PDF page 78]

What I’m not seeing is any reason why an EMAS would have to be confined to the extended track of the runway; extending it from the ends up along the sides of the runway would provide additional protection for aircraft sliding off, or partly off, the sides of the runway (such as AA1420):

An EMAS only past the end of the runway

An EMAS that extends up the sides of the runway

(Aircraft tracks shown in these images are merely examples of possible runway-excursion trajectories, and are not intended to represent any specific flights in particular.)

Am I missing something here?

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    $\begingroup$ There are often taxi ways in that area. $\endgroup$
    – Dave
    Commented Feb 13, 2019 at 22:37
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    $\begingroup$ Think about what would happen to a plane hitting an EMAS at an oblique angle. $\endgroup$
    – StephenS
    Commented Feb 13, 2019 at 23:19
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    $\begingroup$ They already do this. Check out the end of runway 8 at KBUR. Southwest demonstrated how this works a few months ago. $\endgroup$
    – user71659
    Commented Feb 13, 2019 at 23:30
  • $\begingroup$ @user71659: And that's what I get for relying on a seventeen-year-old report... :-S $\endgroup$
    – Vikki
    Commented Feb 13, 2019 at 23:35

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I can think of a couple reasons why you might not want to expand EMAS all around the edges of the runway. One is simply cost vs benefit: If it is more likely that an airplane will run off the end of the runway than the edges it would make more sense to put the material there.

EMAS would seem most beneficial in stopping forward speed if the wheels were to make contact uniformly. An aircraft departing the side of the runway already has directional control problems. The situation would be amplified if the wheel on the side “pulling” the aircraft off the edge was to be dramatically decelerated. It could even cause a ground loop.

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  • $\begingroup$ Also, I am pretty sure that EMAS requires the plane to not be too fast, otherwise either the plane will rip the EMAS to shreds (well, that's the point of EMAS, but I mean without actually doing anything useful) or the other way around, the EMAS will rip the undercarriage to shreds. So, it's designed for a plane that is almost stopping but running out of runway, not stopping a fast-moving plane at the beginning of the landing roll / end of the take-off roll. $\endgroup$
    – Jörg W Mittag
    Commented Feb 16, 2019 at 10:36
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I can think of two general reasons not to place EMAS all around the runway. First, it would massively increase the amount of space required for a runway. Some airports might have that space to spare, others might not.

Second, it would cost a lot of extra money. Maybe there are studies on this but even if the money's available, would it be most useful to spend that money on EMAS or on something else? Again, that probably depends heavily on the situation at each individual airport.

In the end, it's another safety cost/benefit analysis. How many aircraft run off the sides of runways? Would EMAS have been helpful in those cases? What are the costs of installing EMAS? Are other measure more useful (and cheaper)? What would be the unintended consequences (like causing ground loops, as Michael mentioned)? Etc.

(There are also possible issues with all the other stuff usually found close to runway edges, like lights, signs and taxiways. Maybe they could be moved somehow, but that would presumably require more space.)

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