At press time, the FAA’s GPS local-area augmentation system (LAAS) appeared to be hanging in the balance while agency officials were attempting to determine whether there really was a firm industry need for the system. But at the same time, and coincidentally, the agency announced that it expected this month to approve the use of required navigation performance (RNP) techniques to allow IMC approaches to the closely spaced parallel runways at San Francisco International Airport (SFO).
Informed sources cite three main factors that could have contributed to the uncertainty about LAAS. First, cash-strapped airlines are understood to be reluctant to purchase, install and certify Cat I LAAS units to achieve no better limits than today’s ILS, for which they are already equipped.
Second, the predicted ability of the FAA’s wide-area augmentation system (WAAS) to provide “near-Cat I” precision to decision heights as low as 250 ft has attracted renewed industry attention, particularly following its regained credibility after successfully completing a 60-day continuous performance test. It failed a similar test in early 2000, triggering a three-year delay.
Third, it has been suggested that FAA Administrator Marion Blakey may be understandably cautious about signing off on such a major program–currently estimated to be nearing the $1 billion mark on completion–so soon after taking office.
RNP, on the other hand, is moving positively ahead, and with Blakey’s endorsement. Speaking at an aviation summit meeting on October 10, she said, “Within a month we will approve the special approach procedures for equipped aircraft to use RNP for San Francisco International Airport.” Here, she was referring to the proposal by Alaska Airlines to fly its RNP-certified Boeing 737s on converging approaches in IMC to visual landings at SFO’s Runway 28R, which is parallel to 28L, but separated by only 750 ft.
Runway 28R is usually closed to operations when the ceiling and visibility fall below 3,500 ft and six miles, generally due to morning stratus, when 28R traffic can no longer maintain safe separation from that on ILS approaches to 28L as they converge on their separate parallel runways. From that point on, all landing traffic is sequenced onto 28L, decreasing the landing rate to 30 per hour, from 60 per hour when both runways are open, and resulting in delays and diversions.
The RNP technique, supported by the rapidly scanning, radar-based precision runway monitor (PRM) and an offset ILS operating as a localizer directional aid (LDA), will allow converging simultaneous offset instrument approaches to 28R down to a ceiling of 1,600 ft, but with a DA(H) of around 1,100 ft for aircraft with RNP-0.11 capability.
This 500-ft difference gives the converging crew about 30 sec to visually acquire the runway and adjacent traffic on the 28L ILS before reaching the missed approach point, located 3,000 ft perpendicular from the 28L centerline. With the runway and traffic in sight, the aircraft continues its descent to gently roll out visually onto the 28R runway heading. If either the runway or the 28L traffic are not acquired by the MAP, the aircraft turns to 280 deg and executes a missed approach.
RNP will therefore confer a significant economic benefit for certified operators that frequently fly into SFO during the morning hours. But the price of admission is high: purchase, installation, certification and crew training involved in the RNP-0.11 inertial/GPS/FMS package aboard Alaska’s 737s reportedly cost the airline close to $1 million per airplane.
Also, while the ceiling and visibility limits cancel 28R operations for around 25 percent of the time, the actual RNP “window” between 3,500 ft and 1,600 ft lasts for around only 10 percent of the time, with the 15-percent balance being below 1,600 ft, forcing all traffic, however well equipped, to join the 28L “daisy chain.”
Eventually, SFO airport authorities hope to build a third parallel runway, but environmental and other groups have so far made this almost impossible. Until this wish materializes, Alaska’s initiative is likely to prompt its competitors to bite the RNP bullet too.