Departure routes from a runway must diverge by a minimum angle to ensure safe separation between departures. Equivalent Lateral Spacing Operations (ELSO) refers to the reduction of this minimum made possible by more precise aircraft navigation, which can create opportunities to add diverging departure routes without reducing safety. Since less separation is often required for successive departures that diverge, controllers can sequence departures so as to reduce the time between takeoffs. This can increase the number of takeoffs that a runway can accommodate during busy periods, and, by extension, reduce the time that aircraft spend in line waiting to depart.

Performance Based Navigation

On October 20, 2011, two new departure routes enabled by Equivalent Lateral Spacing Operations (ELSO) were implemented at Hartsfield-Jackson Atlanta International Airport (ATL) through a waiver of the national standard—one each for use during the airport's east and west-flow configurations. The revised departure route structure allows Atlanta controllers to segregate departure flows more efficiently from the three departure runways used in each configuration. Controllers can line-up departures from runways 08R (in east operation) and 27R (west) in order to take advantage of the less restrictive separation standards, which could be applied to roughly 25 percent of departures at ATL at the time of implementation. For affected flights taking off from these runways, this means less time waiting for a departure clearance when positioned for takeoff. This allows ATL controllers to process flights at a faster rate, thereby increasing the airport's capacity.

The addition of Equivalent Lateral Spacing Operations (ELSO) enabled routes had a significant positive impact on departure operations at Hartsfield-Jackson Atlanta International Airport (ATL). Following implementation, the FAA observed:

• A reduction in the time between departures on the two affected runways, from 62 to 46 seconds, as measured by the most typical value. This translates to an increase in departure throughput.
• A resulting increase in ATL's departure capacity as set by the facility, from 101 to 105 departures per hour.
• A reduced reliance on the third, and most distant, departure runway, from 3.1 percent to 1.2 percent of all departures. Affected flights saved about six minutes taxiing for departure.

The increased departure rate and reduced reliance on the most distant departure runway translated to an average saving of 2.5 minutes taxiing out per flight, based on an FAA analysis of ATL taxi out times using FAA's Aviation System Performance Metrics (ASPM) data.

Click here for a full description of the NextGen Operational Performance Assessment.

The FAA estimates that the reduction in taxi-out time for Hartsfield-Jackson Atlanta International Airport (ATL) departures translated to about $90 million in cost savings to aircraft operators between 2012 and 2014 (expressed in 2015 dollars). In terms of the benefit to passengers, these time reductions were valued at nearly $250 million over the same period. These estimates are based on aircraft operating cost and passenger load factors that reflect ATL's traffic mix.

As of this publication date, Equivalent Lateral Spacing Operations (ELSO) has not been implemented at other locations. However, in June 2015 the FAA reduced the pre-existing divergence standard that applied to the National Airspace System, making ELSO available for consideration at additional sites.

Time Based Flow Management (TBFM) is a capability used to manage traffic flows by metering, or sequencing aircraft to their arrival airport. Through TBFM, an automation system uses a schedule of runway assignments and landing times to sequence inbound flights, and allocates delays to various segments of each flight in order to meet the assigned schedule. TBFM is administered by traffic managers at the Air Route Traffic Control Center (ARTCC) or Terminal Radar Approach Control (TRACON) of the arrival airport. For some airports, TBFM is used routinely, while at others it is used as needed.

TBFM provides four time-based metering functions:

• Arrival management/situational awareness to inform traffic managers of projected arrival demand
• Airborne metering to sequence flights and provide controllers with allocated delay assignments for each flight to meet the proposed schedule
• Departure scheduling to provide increased management of arrival demand by assigning delays to flights at their origin airports
• En route departure capability (EDC) to efficiently integrate departures into overhead en route streams

TBFM enables the more efficient use of available capacity by tailoring the allocation of delays to individual flights, thereby reducing the need for less efficient "one-size-fits-all" techniques such as Miles-in-Trail (MIT) restrictions. In turn, this can reduce total aircraft delays, and transfer delays to more fuel-efficient phases of flight, such as on the ground or at higher altitudes. Importantly, the transfer of delays out of the terminal approach area positions inbound flights to take advantage of Optimized Profile Descent (OPD) procedures, where these have been implemented. The use of TBFM varies significantly by location, reflecting differences in operating environments and air traffic management strategies.

A predecessor of TBFM called Traffic Management Advisor (TMA) was developed and implemented in the 1990s. TMA was deployed at all 20 ARTCCs by 2007, and was modernized as TBFM in 2013 as a result of a major system re-architecture.

Time Based Flow Management

At Hartsfield-Jackson Atlanta International Airport (ATL), airborne metering and departure scheduling are routinely used to manage arrivals. Until recently, Time Based Flow Management (TBFM) was used to meter arrivals to Atlanta Air Route Traffic Control Center (ARTCC) (ZTL), after which these flights were managed with distance-based, miles-in-trail restrictions. This combination of airborne metering to the ARTCC boundary and Miles-in-Trail (MIT) restrictions to the airport was unique among facilities that use TBFM. Since August 2015, however, ZTL has been metering arrivals within its boundaries due to enhancements to TBFM software, and in preparation for the Atlanta Metroplex implementation in 2016.

The use of time-based metering with TMA at ATL predates the NextGen program. However, the deployment of the modernized TBFM system in 2013 provided technical enhancements and a platform for additional operational benefits at ATL and other airports.

The FAA conducted an operational assessment of two Time Based Flow Management (TBFM) functions, airborne metering and departure scheduling, at eight airports (four per function) where these are used widely, including Hartsfield-Jackson Atlanta International Airport, due to its routine use of airborne metering. The locations were selected based on how frequently each function is used alone and in combination, so the impact of each function could be measured. For this reason, results cannot be interpreted to be representative of impacts at other locations.

For each function, the assessment looked at the impact on both arrival delays and airborne delays as indicators of how efficiently available capacity was used. The FAA's assessment of flights between July 2011 and December 2013 found that:

• For three of four airports studied that use Departure Scheduling, arrivals tended to experience 1.0 to 1.3 minutes less of arrival delay when the facility was using TBFM to schedule departures
• With few exceptions, departure scheduling also significantly reduced the variability of delays
• There is an 8 to 10-minute difference between the average arrival delay for metered flights and those subject to Miles-in-Trail (MIT) restrictions alone, for the four airports studied. It is unclear, however, how much of this difference can be attributed exclusively to TBFM because much of the difference occurs on the ground, beyond the immediate scope of TBFM's influence
• Metered flights also experienced about a minute less airborne delay than those subject to MIT restrictions, as well as less extreme and more predictable airborne delays

Click here for a full description of the NextGen Operational Performance Assessment.

The FAA estimates that the combined reductions in aircraft delays for the eight airports evaluated translated to about $640 million in savings between 2011 and 2014 (expressed in 2015 dollars). These savings reflect reduced operating costs to airlines of $209 million, and time-savings to passengers valued at over $430 million. The estimate applies the average observed per-flight delay savings to the base of arrivals managed by airborne metering or departure scheduling, in accordance with the FAA's performance assessment.

Time Based Flow Management (TBFM) is deployed at 93 facilities across the National Airspace System (NAS), including 20 en route, 28 terminal and 45 tower facilities. It is deployed at all but two of the Core 30 airports, with Tampa International Airport (TPA) and Honolulu International Airport (HNL) as the two exceptions.

Read how Time Based Flow Management (TBFM) is used at other locations in the National Airspace System.