Lower approach altitude using Wide Area Augmentation System

Regional and general aviation flights will not be endangered by the Federal Aviation Administration's (FAA) decision earlier this month to lower the altitude for allowable approaches using the Wide Area Augmentation System (WAAS) from 250 ft. to 200 ft. above an airport's surface, the agency says. The decision was made in full consideration of the safety concerns.

The change will allow an operational capability similar to a standard Category 1 instrument landing system (ILS) where suitable airport conditions exist. WAAS is a satellite-based navigation system that improves the accuracy, availability and integrity of Global Positioning System (GPS) signals needed in low-visibility flight operations.

A knowledgeable observer of FAA's actions through the years, who has a strong desire to remain anonymous, believes the agency waived its own safety standards in lowering the WAAS minimum. The FAA has a lot invested so far in WAAS development, this source explains, and the agency not only wants to expand its use, but have WAAS actually replace the use of instrument landing system (ILS) approaches at more airports.

FAA commissioned WAAS in 2003 to improve the signal accuracy provided by GPS satellites and ground stations. WAAS is generally thought to improve accuracy of the GPS signals by a factor of five - down to about three meters.

Initially, it was intended for use in all civilian craft, but commercial airlines today are mostly using ILS to make Category 1 (200 ft.) approaches. This leaves WAAS as the precision approach method of choice primarily for regional carriers and general aviation pilots, who often have no other option at smaller airports with no ILS capabilities. There are approximately 3,000 WAAS- equipped aviation users operating in the national airspace system today.

"WAAS-equipped commercial operators will gain access to Category 1 equivalent approach services at qualifying airports where there are no instrument landing systems. This will result in improved safety, including enhanced approach and landing operations in marginal weather," the agency says.

"This is a significant milestone, moving us closer to our ultimate goal of a satellite-based airspace system," FAA Administrator Marion Blakey adds.

Not wishing to "respond directly to Mr. Anonymous," Dan Salvano, the FAA's director of navigation services, told Regional Aviation News. "This is not something we did to meet some users' expectations."

For one thing, after looking at about 1.8 billion data points over the last couple of years, the FAA concluded that WAAS signals met or exceeded its requirements for their designed use. Also, in flight tests, WAAS was just as satisfactory as ILS. Taking into account such things as the variations in pilot skills and the range of altimeter readings, FAA's analysis showed that WAAS still met the criteria of the ILS collision risk model.

Another important consideration is that the new decision does not mean that 200 ft. is now okay in all situations. If planes have the proper avionics, pilots have the proper training, the surrounding airspace and terrain supports its use, and airports have the right infrastructure -- particularly runways that are long enough and wide enough -- then the new minimum is acceptable, Salvano explains.

Furthermore, the FAA is working on even more system improvements. The current WAAS software, for example, operates a bit too conservatively because it's too ready to back off from using a precision approach, Salvano says. By way of illustration, he cites a solar storm that occurred on Halloween in 2003. The software's algorithms saw that its parameters for safe use of a precision approach were exceeded. However, the agency also saw that there had been no loss of the signal or of signal accuracy. The algorithms are now being worked on so that the WAAS software will still allow a precision approach in such conditions.

Also, in certain places in the United States, such as the northeast, southwest, and Alaska's north slope, "the availability of the WAAS signal is not as high as we wanted it," Salvano says. So, the agency its expanding the number of WAAS ground reference stations in Canada and Mexico. When this expansion is completed, there should be a true "North American WAAS system."

The FAA also says that when it first commissioned WAAS, it "was approved to provide vertical guidance down to 350 feet." Then, "localizer performance with vertical guidance procedures down to 250 feet was later developed to take advantage of the increased performance provided by WAAS."

Our anonymous source says he's "not entirely surprised" that WAAS has now been declared safe at 200 ft. That level has a certain status among pilots, partly because it allows Category 1 precision approaches. He also tells Regional Aviation News that he doesn't think the FAA has necessarily made the wrong decision from a safety viewpoint. It's just that there some other considerations. There likely was some ego involved in ensuring that WAAS could keep up with other technologies.

Thus, it's even conceivable the agency might take some additional steps that force more pilots and operators to buy WAAS equipment, he adds. But such actions could create a strong backlash in the aviation community.

The FAA says the first procedures that will allow WAAS operations down to 200 ft. will be published in 2007. The FAA currently has more than 300 vertical- guidance procedures based on WAAS, and is expecting to publish 300 additional ones this year.

Over the past two years, WAAS has provided coverage to roughly 99 percent of the continental U.S. and has been available 99.87 percent of the time, according to the FAA.