Three U.S. airlines -- Southwest, American and Delta -- recently decided to make major investments to retrofit their aircraft, so they can fly the very precise approach paths enabled by required navigation performance (RNP).

The biggest of these investments will likely be made by Southwest, which has decided to upgrade its entire fleet of Boeing 737s to be RNP-capable. The airline's 272 737-700s are already equipped for RNP, and the retrofit would bring the remaining 219 aircraft up to the same standard.

While the internal decision has been made to proceed with the upgrade, timetable details are still being considered, a Southwest spokeswoman said. The airline is also not revealing what the cost would be. Southwest is the first U.S. airline to commit to making its entire fleet RNP-capable, FAA Associate Administrator for Safety Nicholas Sabatini believes.

Meanwhile, American's board recently approved $100 million to bring its entire 757 and 767 fleets up to RNP capability. The retrofit will involve about 100 aircraft and should be completed by 2011, American's VP-Flight Mark Hettermann said. Some of the carrier's 757s are already equipped for RNP.

By the time the 757/767 retrofit is finished, about 75% of the American fleet will have RNP capability, said Hettermann. The airline's 777s and 737s are already equipped, although American must still go through the FAA approval process to use RNP approaches. The carrier plans to win approval to use an RNP approach at a single airport -- probably Quito, Ecuador -- and would then have this approval expanded.

Delta will also upgrade its 757 and 767 avionics for RNP. The carrier is not releasing the value of its investment but expects to complete the retrofit in the next two years, said Delta's Executive VP-Operations Joe Kolshak. This would make 70% of Delta's fleet RNP-capable. The airline's 737-800s were certified to use RNP approaches May 6, and although the carrier's 777s are equipped, they are not yet certified.

RNP is regarded as one of the essential building blocks of the next-generation ATC system that is being developed, FAA Administrator Marion Blakey told reporters at an RNP briefing yesterday. She noted that Delta joins Alaska, Horizon and Continental on the list of airlines that have been approved for RNP approaches. The number of airlines making RNP investment decisions represents a "tipping point for performance-based navigation."

RNP capability lets aircraft use flight management systems and GPS to fly very precise approaches into certain airports. Area navigation (RNAV) routes are similar, although they don't require the same equipage level as RNP. RNAV approach procedures are more widespread, and a far greater proportion of the U.S. fleet is already capable of using them.

FAA has 37 RNP approaches in place at 17 airports, with another 34 expected to be in place by the end of the year, Blakey said. On May 10, FAA plans to publish 10 more RNP approaches for Atlanta and three more for Dallas/Fort Worth. The agency intends to publish at least 25 approaches next year.

Meanwhile, FAA has established 155 RNAV arrival and departure routes at 38 airports, with another 42 expected by yearend. Delta reports savings of $36 million a year from the RNAV routes at Atlanta, said Blakey.