Thales recently won an FAA award for up to 300 ILS systems, to be installed between 2007 and 2013. Many will replace older equipment, but at least 100 systems will provide precision approaches at new and currently unequipped runways, the company estimates. Options allow FAA to purchase up to 612 ILS. At some currently equipped runways, installed systems will be upgraded to higher category standards to support approaches to lower visibility levels. The new contract follows previous FAA awards, which have added more than 300 ILS installations to the U.S. inventory over the last three years. By 2013 FAA could be operating as many as 1,800 ILS across the country.
For aircraft operators, an increase in ILS installations is good news. But are 1,800 enough? Not really, especially for corporate pilots, who routinely operate under instrument meteorological conditions (IMC) into most of the more than 5,000 airfields in the United States. A majority of the airfields have no ILS and are unlikely to ever have systems installed. Also, installation of 1,800 future systems does not mean that 1,800 airports will have an ILS approach by 2013. Almost all airports used by the commercial airlines have several ILS, each serving different runways. Dallas/Fort Worth and Denver International have 12 systems each. Chicago/O’Hare has 11, and Atlanta has eight.
Non-mainline airports could wait a long time for a new ILS. Yet the raison d’etre of corporate aircraft is to take company personnel swiftly and efficiently to places where the airlines either don’t offer frequent service or, more often, don’t fly at all. Executives relying on commercial air transport would face a two- or three-day, multichange, marathon vs. a one-day trip. Hence the incongruous but common sight today of a sleek business jet or turboprop perched among a small flock of Pipers and Cessnas at remote, one-runway, sans-towered airfields.
Unlike many private owners, however, the corporate aircraft’s passengers usually don’t have the luxury of changing their plans if the destination weather turns sour. And while most smaller airfields have some form of instrument approach guidance system–using signals from a local VOR, NDB or, more recently, a basic GPS procedure–none provides the accuracy of ILS nor its lower, bad-weather decision heights (DH) above the ground. (The DH is the altitude at which the pilot must be able to clearly see the runway or perform a missed approach.)
Typically, an ILS will support a DH of 200 feet on the final approach to the runway, whereas DHs for VOR, NDB and basic GPS approaches are often 400 feet or higher. And that often can be the critical difference between landing and diverting to another airfield.
While ILS provides a continuous steady descent path, the VOR and NDB methods employ a series of descending, then level, downward steps to arrive at the DH. While intrinsically safe, these procedures–officially known as non-precision approaches (NPAs) but called "dive and drive" approaches by pilots–can sometimes be demanding to fly accurately. Failure to constantly monitor altitude during their execution has caused a number of accidents.
Statistically, the ILS’ much narrower and more precise approach guidance beams and its continuous descent path are reported to be more than five times safer than the NPA technique. On the other hand, there are many locations, such as at airports in mountainous areas, where none of the NPAs works well, and even the ILS is impractical due to signal reflections from the surrounding terrain.
Two Solutions Considered
Corporate aircraft operators are considering two solutions to more safely access airfields with NPAs and airfields without them. The regional airline community also is considering them. The two solutions–the GPS wide area augmentation system (WAAS)-based localizer performance with vertical guidance (LPV) and the required navigation performance (RNP) techniques–employ quite different methodologies. And they are generally regarded as lending themselves to two different classes of aircraft.
Many view WAAS-LPV as being more economical for piston, turboprop and smaller jets, and RNP as more appropriate for larger jets equipped with more advanced avionics. But both have the potential to provide safe, positive guidance along continuous, steady descent paths down to "near ILS" DH values, as low as 250 feet above the ground.
What About LAAS?
Where does the GPS local area augmentation system (LAAS) fit into this picture? LAAS was intended to replace ILS and enter nationwide service as the future precision approach guidance aid. Despite its promise, the system hasn’t been able to overcome its technical difficulties, and FAA was forced to reassign it to research status.
That could change. FAA recently granted Honeywell a $4.5-million modification to its category I LAAS contract. It calls for hardware and software upgrades, incorporating a new set of system integrity algorithms into the company’s SLS-3000 Beta-LAAS ground systems in Memphis International Airport. Installation plus flight testing is to be accomplished over an 18-month period.
"Our goal is to restore confidence in LAAS," says John Oelschlaeger, director of Honeywell’s satellite landing systems business. In addition, the Department of Defense continues its development of a military version of LAAS, and some feel that it could reemerge as a civil system by the end of the decade.
For now and in the near future, however, the choice remains between RNP and LPV.
LPV takes advantage of the very high performance of the GPS signals when corrected by WAAS. Typically, these can yield horizontal and vertical accuracies of less than 16.5 feet (5 m). And, when processed by remotely installed C-146 or panel-mounted C-145 GPS receivers meeting the industry’s "Gamma III" specifications, WAAS can provide ILS-like lateral and vertical approach guidance to a near-ILS DH, as low as 250 feet.
Furthermore, because LPV guidance accuracy is similar to that of ILS, the certification criteria used to define the lateral and vertical safety boundaries within which the aircraft will approach the runway are those used for ILS. That is, they are very much narrower, or "tighter," than those used for non-precision approaches. In turn, this means that many obstacles which fall within the much wider V-shaped splay of an NPA–and which may therefore require a higher DH to avoid them–will fall outside the narrower LPV approach path. And this allows corporate aircraft pilots to use a lower DH.
Which to Choose?
Meanwhile, RNP achieves similar results by using the advanced avionics of newer, larger aircraft. Sometimes called a "performance-based" navigation solution, RNP can use several different position sensor inputs, including GPS, VOR, DME/DME, inertial systems, baro-VNav and other sources. A fairly sophisticated flight management computer (FMC) integrates all of the sensor inputs to yield a very accurate position. A flight crew also uses weather radar ground mapping and a terrain awareness warning system (TAWS) to monitor navigation progress. Depending on the sensor mix, accuracies of less than 0.10 mile, described as RNP 0.1, can be achieved. This means that the aircraft’s horizontal position will remain within 0.1 mile of its intended track for 95 percent of the time.
For additional safety a further buffer of width equal to the RNP value is added to either side of the track. This produces a closely defined path, including turns, somewhat analogous to a railroad track, within which the aircraft is continuously contained. In this case, however, special FMC circuits monitor the aircraft’s actual position and alert the crew if it leaves the RNP corridor.
Unlike LPV, an RNP aircraft can, if necessary, actually "snake" along a curving path, under full FMC and autopilot control, between obstacles on its approach to the runway. Even more important, perhaps, is that, in the event of a missed approach, the aircraft can fly a predetermined curving path safely away from the airport, avoiding the high ground that may lie ahead.
So is RNP the better solution? In some ways, yes, and in other ways, no. RNP offers more approach path flexibility in an "obstacle rich" environment, but the lowest RNP accuracy certified to date, in Boeing’s latest 737NGs, is 0.1 mile, while LPV’s accuracy is 0.02 mile.
So there can be a bit of a tradeoff. The narrow, ILS-like, V-shaped signal coverage of LPV converges towards the runway and lies well inside the RNP’s "railroad track" near the runway threshold. This means that a close-in obstacle could raise the RNP’s DH while not penetrating the LPV’s coverage and raising its DH. Conversely, an obstacle farther out on the approach could penetrate the wider LPV coverage and raise its DH while falling outside the RNP corridor and not affecting the RNP DH.
Obviously, in a comparison, strong cases can be made for both approach techniques.
Proponents of RNP can argue that LPV is purely an approach aid, while the world’s overland and oceanic airspace will eventually be defined by routes of varying RNP values, depending on traffic density. This process is already under way in Europe, where RNP 5 routes called B-RNav (for basic) and RNP 1 routes called P-RNav (for precision) are in daily use. RNP backers also can point out that GPS and WAAS signals are more susceptible to external interference–although this is quite rare–compared with other terrestrial navaids used in the RNP solution, while the RNP’s inertial systems are totally immune.
Conversely, WAAS supporters can argue that their system, on which LPV is based, can support the full range of RNP route values. Besides its LPV runway approach application, WAAS is an element of the future worldwide performance-based navigation portfolio. WAAS and its coming overseas equivalents in Europe, India and Japan–known as satellite based augmentation systems (SBAS)–can, with appropriate track "containment" monitoring and alerting, also meet almost all international RNP requirements.
The bottom line in virtually all cases is that both LPV and RNP will always be more than adequate to reach the runway safely from the DH point. Perhaps the key question, therefore, is how soon will we see widespread RNP and LPV procedures in the national airspace system (NAS)? Unfortunately, RNP procedure development and certification for specific airport approaches is an exacting task. So far FAA has approved relatively few RNP approaches. Most of them are to meet airline demands at airports with difficult access, and most mandate special aircraft and aircrew authorization required (SAAR) approval.
Some observers feel that providing certified non-SAAR RNP public approaches for a significant number of remote airports not used by airlines could take several years. And these probably will be limited to RNP 0.3 and above. In comparison, FAA already is publishing LPV approach procedures. One-hundred and fifty runway ends are expected to be covered by the end of 2005, and as many as 300 per year thereafter. European authorities also are actively pursuing the potential of LPV in conjunction with Eurocontrol’s European geostationary navigation overlay system (EGNOS), their equivalent to WAAS.
The procedures expert for the National Business Aviation Association (NBAA), Steve Bergner, offered the association’s broader view on the need for the two technologies. "NBAA members," he states, "are significant stakeholders in both RNP and WAAS, and want to see increased use of both to enhance flight safety.
"Clearly, RNP is the only solution at locations like Palm Springs, Calif., but WAAS/LPV can do an excellent job in less challenging environments. We’re just anxious to ensure that FAA considers the needs of corporate aviation as they prioritize future procedure developments." Bergner, incidentally, captains a corporate Dassault Falcon 900 EX, whose EASy flight deck is more advanced than any commercial airliner.
But the critical bottom line for operators is cost. Purchase of a current mid- to high-end corporate jet pretty well guarantees that its avionics will provide RNP capability. And, to date, none of the manufacturers of those aircraft offers WAAS and its LPV capability as an option. In fact, one industry expert suggests that the factory cost of custom integrating and certifying LPV into a high-end corporate jet’s complex avionics for a single customer could be high, and a post-delivery retrofit possibly even higher. (In the big airplane business, things are different. Boeing doesn’t offer WAAS and its LPV application, and only provides advanced RNP capability, while Airbus includes RNP but plans to offer WAAS/SBAS as a customer option.) But for the operator of a small business jet or turboprop, the cost of upgrading with RNP avionics today could also be high, whereas WAAS with LPV could be a more economical approach solution.
Overall, industry opinion appears to be that after two new WAAS geostationary Earth orbit satellites commence North American service by early to mid-2006, and international SBAS networks follow later, both WAAS and RNP eventually will come to be regarded as comparable, and often interdependent, solutions to tomorrow’s performance-based nav requirements. This will be true both for approach guidance and for en route operations. As always in aviation, the future looks brighter down the road.