Two of the four runways at San Francisco International Airport will temporarily close so that their runway safety areas can be upgraded. As part of the project, zones comprising engineered-material arresting systems will be added to all four ends of the runways, shown in green, and the taxiways, shown in yellow, will be realigned. San Francisco International Airport
Two runways at San Francisco International Airport are set to temporarily close so that work on upgrading their safety zones can be completed.
April 22, 2014—Flying into and out of San Francisco International Airport is a memorable experience—one that more than 45 million people experienced in 2013 alone. With four runways that jut out into the San Francisco Bay, many of the arriving and departing passengers get an up-close and personal look at the bay directly from their airline seats. Two of those four runways are slated to close on May 17 for a precisely scheduled four-month upgrade of the runway safety areas located at the end of the runways.
While the Federal Aviation Administration (FAA) typically requires a runway safety area to be 500 ft wide and to extend 1,000 ft beyond the end of runways, the agency recognizes that because this requirement was established 20 years ago it is not physically possible at some airports that predate the requirement. San Francisco International Airport, commonly known by its airport code SFO, is one of those airports—in this case because of its location on a tight site comprising man-made ground reclaimed from the San Francisco Bay. The FAA provides an alternative manner of constructing runway safety areas in airports with limited space, but even this required more space than that available at SFO. As a result, the design team at the airport had to work carefully to add the necessary safety zones without extending the airport’s footprint into the bay.
Runways are named on the basis of the heading of a plane taking off from it in relation to magnetic north. As such, SFO’s two main runways, which have headings of 283 degrees with respect to magnetic north, are called Runway 28L (for left) and Runway 28R (for right). Because the runways can be used in either direction, these are also known as Runways 10R and 10L. The secondary runways, which will be closing next month, are the airport’s Runways 1L and 1R, also known as 19R and 19L.
As part of the runway safety area project, engineered material arresting systems (EMAS) zones will be added to both ends of the runways to provide a safe method of stopping aircraft that overrun, or undershoot, the runways. The EMAS elements are beds of crushed and “crushable” concrete blocks designed to absorb energy, according to Daniel Lee, P.E., the airport’s lead design engineer for the project. “They’re going to be kind of similar to truck sand pits,” he says, which are used to prevent accidents when a truck overruns a roadway or loses its brakes.
The blocks are FAA-approved and were developed by a team comprising experts from the University of Dayton, the Port Authority of New York and New Jersey, and the Engineered Arresting Systems Corporation (ESCO) of Logan Township, New Jersey, according to the FAA.
“We’re trying to design and provide safety areas at the end of the runways as required by the FAA, but yet we need to …maintain the usable runway length for airport operations,” says Jimmy Chiu, P.E., M.ASCE, the airport’s program manager for the runway project. “I think that’s a challenge right there—you want to design for safety features, but still be able to maintain the useable runway.”
Because of the tight site at SFO—the runways’ lengths limited on one side by the bay and on the other by U.S. Highway 101—the airport design team decided to use nonstandard EMAS elements. This allowed the zones to be added to the existing site without reclaiming additional land from the bay.
Each of the zones will measure 227 ft in width. At the northern ends of Runways 1L and 1R the beds will measure 413 ft in length. At the southern ends, the beds will measure 438 ft and 373 ft in length, respectively. (A standard EMAS bed extends 600 ft in length, according to the FAA.)
The EMAS beds themselves slope upward in half-inch increments from 8 in. to 26 in. deep so that the aircraft can be brought to as gradual a stop as possible and jet blasts from departing planes don’t displace the blocks, according to Lee. “[The slope] helps when an aircraft overshoots, so it doesn’t just hit a wall,” he says. “It also helps in terms of the jet blasts when an aircraft takes off, so that the velocity of the jet blast will be able to shoot over the blocks.”
Runways 1L and 1R will be extended farther south, where two open retention basins are being covered and converted to underground drainage systems. The extension will enable sections of the runways’ northern ends that are currently part of the runways to be converted to EMAS zones. Taxiways will be realigned to abut the new northern runway ends. To the southh, the taxiways loop around the terminus of the runways and these will be reconfigured to extend around the EMAS beds.
In addition to adding the EMAS zones and realigning taxiways, the project also includes relocating airfield lights and FAA navigational systems. “We’re probably digging up a third of the airfield in terms of how much construction we’re doing,” says Chiu.
The design of the EMAS beds also had to take into consideration the fact that the airport sits on a silty-sand fill. This fill is located atop a thick layer of clay, which tops the bay mud, Lee explains. “We run into a lot of variations in terms of our subsurface material,” Lee says. As a result, each time a runway is repaved—which typically occurs every 8 to 10 years, even though the pavements meet FAA standards for a 20-year lifetime—geotechnical studies are conducted to ascertain the precise characteristics of the runway’s subsurface.
The runways can measure up to 40 in. thick and use a typical asphalt-concrete hybrid over a cement-treated base layer to create a flexible pavement that can withstand the ongoing settlement that the airport experiences, Lee says. Settlement is a continuing aspect of maintaining the airfield; while the terminals are located atop piers, the runways are not.
“I’ve seen as much [settlement] as an inch every 8 years, to even 12 inches,” Lee says. “Because of the settlement, building portland-cement concrete (PCC) runways and taxiways would just cause a lot of pavement failures and cracks all throughout, so by designing a flexible pavement, we were able to accommodate the movement and the variations in terms of the settlement throughout the airport.”
Despite losing 50 percent of its runway space this summer to the construction work, the impact on flight times will be minimal. Since the runways at the airport crisscross one another, air traffic controllers already have to coordinate arrivals and departures to create the necessary space between flights, says Doug Yakel, the airport’s spokesperson.
“On a normal day, we operate about 100 flight operations per hour, and that is roughly about 50 arrivals and 50 departures per hour,” Yakel says. “When we close the two runways, that capacity will be reduced to about 85 per hour, so it’s about a 15 percent reduction in capacity.” However, by working with the airlines to adjust their schedule times throughout the day, the airport will be able to complete the necessary runway work without creating heavy delays or limiting the availability of flights, Yakel says.
And the airport is already familiar with operating only two runways. “Last year, because of wind, we operated on only two runways a total of 101 days,” Yakel says. To avoid a repeat of the long lines of planes that would form waiting for departure time slots due to wind closures, however, the airport has begun to use a departure metering system to help schedule aircraft during the construction period. The system works much like the metering system that is used at highway on-ramps, according to Yakel. “It will essentially assign each departing flight a time for it to push back from the gate, and this metering process will help us ensure that we really don’t have a long line of aircraft waiting to take off at the end of the runway,” he says.
The runway work at SFO is part of a federally mandated and funded upgrade that is being overseen by the FAA and is anticipated to cost a total of $214 million for work on all four of the airport’s runways. The two main runways at the airport have already been upgraded, and preparatory work that could be completed without the closure of Runways 1L and 1R has been under way.
The FAA mandate requires that runway upgrades be complete nationwide by 2015. The summer months were chosen for the current work at SFO because they are best suited to round-the-clock construction and minimal weather delays. Work will be completed in two 10-hour shifts each day, scheduled from Monday through Saturday, according to Lee.
Currently EMAS systems are installed at 74 runway ends at 47 airports in the United States; 14 systems at 8 additional airports are currently in the planning and installation stage, according to the FAA.
As of April 4, 2014, there have been nine incidents at airports in which EMAS beds have safely stopped overrunning aircraft, according to the FAA. A total of 243 crew and passengers were aboard those successfully stopped flights, which occurred between 1999 and 2013.