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EMAS -- Emergency Material Arresting System -- is a form of foamed concrete used to slow down and stop aircraft that have overshot the end of runways, safely. According to the US Federal Aviation Administration, [the] material deforms readily and reliably under the weight of an overrunning aircraft and the resulting drag forces decelerate the aircraft to a safe stop. Under test since 1999, EMAS has gone through a major design revision in order to better withstand the "jet blast" from aircraft successfully taking off. Now installed in 14 locations in the US, EMAS has had numerous successful uses:
In May 1999, a commuter aircraft overran the runway into the EMAS at JFK International Airport. In December 2002, EMAS safely arrested an overrunning Pacific Jet Gulfstream II at Burbank Airport in California. And in May 2003, an overrunning Gemini Cargo MD-11 was safely decelerated at JFK International Airport. In all cases, serious damage to the aircraft was averted and there were no injuries to passengers on board.
A Washington Post article this week mentions additional examples of EMAS in action, including the safe stop of a 747 cargo plane at JFK earlier this year.
So why isn't EMAS more widely used? Cost, primarily. Installation can run upwards from $5 million, and annual maintenance expenses -- including special vehicles to clear snow without breaking the concrete -- can be expensive, as well (an air force base in Arkansas, estimated $12-20 million over 20 years (PDF), although the use there would be potentially greater than at a civilian location). A big part of the expense also comes from the time (nearly two weeks) and equipment necessary to repair the EMAS if an "arrestment" happens. The Post article notes, however, that both the National Transportation Safety Board and the airline pilots' union are considering calling on EMAS to be mandatory.
Right now, EMAS is made primarily by one company, ESCO. But recent innovations in the world of concrete suggest that we might see new players in this arena. The flexible concrete that Jeremy posted about, for example, might provide an alternative form of deformable concrete that could slow the runaway jet without crumbling; such a material would be far easier to maintain, as a result. Another possibility might be a variation of "grancrete," an extremely inexpensive sprayable concrete material that hardens quickly in the air.
A cheaper version of EMAS would allow its use to spread rapidly around the globe; it's not just planes in North America that are liable to run off the end of the runway.