Because runway overruns often end badly, and many runway ends (especially older ones) don’t have enough clear space available beyond the runway end for a full-size runway safety area (RSA), EMAS has come into wide use (at least in the U.S.) for quickly stopping overrunning aircraft within a smaller-than-normal RSA. An EMAS system consists of a bed of crushable pavement blocks (usually cellular concrete or foamed silica) which collapse when an aircraft runs over them, causing the aircraft’s landing gear to sink into the EMAS bed and drag through it, slowing the aircraft quickly to a stop.
However, EMAS is far from the only conceivable system for slowing and stopping overrunning aircraft:
- One option would be a simple bed of sand or gravel (like the runaway truck ramps ubiquitous on steeply-inclined highways), which would slow an aircraft down through exerting drag on its landing gear; although not as effective as a full-blown EMAS, it would be capable of handling small aircraft that wouldn’t be heavy enough to crush the paving material of an EMAS, and would also be far less expensive, making it ideal for smaller general-aviation airports.1
- In a similar vein to the sand/gravel arrestor bed, a thick bed of tall, stiff bristles would also slow aircraft fairly quickly through landing-gear drag (like how it takes a lot more effort to walk through waist-deep grass than on bare dirt).
- Another possibility would be a large retractable net, similar to the nets aircraft carriers carry to stop aircraft if need be. Although aircraft-carrier nets are, admittedly, usually used for aircraft considerably smaller than your average airliner, the aircraft more than make up for this by landing at very high speeds compared to an airliner and by travelling a much shorter distance (and, thus, having much less time in which to slow the aircraft by braking) between touching down and hitting the net than an airliner at a land-based runway would, and the aircraft do have the ability to handle the considerably-larger aircraft that sometimes land on aircraft carriers; additionally, nets were formerly used at emergency landing sites for the space shuttle orbiters, which were about the size of a DC-9, but much heavier and having landing speeds over twice as fast, strongly suggesting that similar nets would be capable of handling even large airliners.
So why is the crushable-block EMAS the only system widely used for stopping overrunning aircraft?
1: One airport does have sand-filled arrestor beds at two of its four runway ends; however, this airport serves aircraft large enough that it should have a full EMAS instead, the sand beds being woefully inadequate for stopping aircraft of these sizes, as was spectacularly demonstrated in 2010. Sand/gravel beds are far better suited to stopping smaller general-aviation aircraft that aren’t heavy enough for an EMAS to be effective.