Numerous regulatory agencies and yet-to-be-determined technological hardware will shape the future NAS. Pilots will play a larger interactive role in ATC management and separation.
Aircraft OEMs have been installing electronic flight information system (EFIS) and flight management system (FMS) equipment for nearly 3 decades. The equipment has given the industry safer nonprecision approaches (NPAs) by eliminating inefficient level-offs during approaches. A continuous descent approach (CDA) has been possible for most of this time, using FMS on the intermediate and final approach sectors, but the current term—advanced CDA—includes an uninterrupted idle-power descent from altitude.
Integrating WAAS with current GPS navigation
At present, advanced CDA, as described by ICAO and JAR OPS 1 (commercial operators) procedures, involves idle-power descents from top of cruise to decision altitude (height)—otherwise DA(H)—with provisions for stabilizing the approach with power at either 1000 ft agl (in IMC) or 500 ft agl (in VMC). These approaches are already in use in many parts of the world but are only now gaining acceptance and limited use in the US.
This transition from a “dive-and-drive” procedure to advanced CDA will eventually replace nearly all approaches requiring an intermediate level-off segment. The cooperation of ATC and FAA in the effective implementation of CDA is essential to its widespread acceptance.
At first, approaches will be divided into those conducted by aircraft that are CDA/Wide Area Augmentation System (WAAS) capable and those that are noncapable. Currently, only a handful of airlines are authorized to conduct approaches using area navigation (RNAV)/required navigation performance (RNP). Most notable of these is Alaska Airlines, which pioneered the RNP concept in the Alaskan wilderness and has been integral to providing FAA with significant user data for implementation in the lower 48. The first corporate Part 91 aircraft was certified for RNP Special Aircraft and Aircrew Authorization Required (SAAAR) approach procedures in 2008.
LAAS and WAAS implementation
WAAS and Local Area Augmentation System (LAAS) are simply augmented signals to aircraft receivers. These are space-based and ground-based, respectively. The GPS constellation provides the navigation architecture. In addition, for US-based operators, 2 additional GPS satellites with WAAS capabilities have been placed in geosynchronous orbit on an equatorial plane. These dedicated WAAS-capable units transmit 3 additional types of information critical to the WAAS function, specifically:
Acting FAA Administrator Bobby Sturgell is a staunch proponent of NextGen and WAAS technology. His background in the cockpit and as a manager will help overcome the many obstacles on the way to a digital NAS. Sturgell should be championed as a supporter and advocate of corporate aviation.
• Corrected clock error from the other GPS satellites. WAAS satellites correct for the nanosecond errors that exist in all time transmissions. Clock error updating—which is not to be confused with vertical and horizontal dilution of position (VDOP and HDOP), which are satellite geometry errors—allows a more accurate level of time signal, and thus better position accuracy, from available in-view satellites.
• Ephemeris. WAAS technology corrects for the position error of other GPS satellites and provides better navigation solutions using trilateration.
• Ionospheric correction (the biggest factor). The electronic envelope that surrounds the Earth produces significant variations in the way the satellites transmit data to GPS receivers. The WAAS-capable satellites correct this variation and provide a higher degree of accuracy. A good example is provided by the aurora borealis. As charged particles from the Sun pass through Earth’s ionosphere, they change direction and speed, resembling a flowing curtain. Thus, in a similar fashion, the data transmitted from a GPS satellite gets slightly distorted, and WAAS corrects for this.
Many regions are investing in the WAAS concept, among them Asia, with its Multifunctional Satellite Augmentation System (MSAS), and Europe, with the Euro Geostationary Navigation Overlay Service (EGNOS). This will allow the rest of the world to begin the transition away from ground-based navigation for individual airports. Some regions, such as the European Union, will eventually place into service their own satellite constellations and reduce their dependence on the biggest single source provider—US-controlled GPS. In Europe, the much-publicized Galileo constellation—with similar capabilities to GPS—is scheduled to begin service in 2011.
Integrity and WAAS approaches
Current 12-channel GPS receivers are fully capable of WAAS approaches right now, but the WAAS/ LAAS concept provides integrity—a critical backup component in equipment reliability both throughout the approach segment and beyond. For example, suppose a clock error occurs within one of the satellites being used during an RNAV/RNP approach. Without WAAS, the time lag between detection and notification to the pilot (by “degrade” light or similar) is unacceptably long. WAAS, through integrity, alerts the pilot to a degrade situation. A decision can then be made near-instantaneously (< 6 seconds) whether to continue or miss.
RNP approaches in the AFM—staying legal and safe
Many aircraft flight manuals (AFMs) have notations that include RNP-0.3 approach criteria. These may be misleading and could result in confusion during an approach. The key to staying legal is reading the type of approach and notes—in most modern FMS-equipped cockpits, RNAV (GPS) approaches are authorized, but the RNAV/RNP approach will always have an SAAAR notation. Thus, for Part 91 operations you may not fly the RNP approaches without an LOA.
Honeywell, Rockwell Collins and Universal have introduced new avionics suites to accommodate the growing demand for the new approaches, which are expected to arrive during 2008. FlightSafety Intl, SimuFlite and other providers have already begun the training processes and developed syllabuses for gaining the LOA and training crews for RNP operations.
CDA operations will indeed help our current crowded terminal airspace. With precise control from top of descent to a predetermined point on arrival, including curved approaches, ATC can control the flow of traffic into and out of busy terminal areas more efficiently. The key lies with aircraft equipage and crew training. So far, the only approved operators are airlines—particularly Alaska Airlines. Many more, including Southwest and American, have invested heavily—as have a score of foreign operators. There are more than a dozen RNP approaches and departures in Alaska, and some are now entering the continental US. At the first of these—DCA (National, Washington DC)—a customized form of CDA is used to follow the river on the south approach to Runway 18.
Drawbacks and limitations
High-tech WAAS/LPV approaches are gaining widespread acceptance, and more are in the approval process. This point-in-space approach will save lives for EMS crews and victims alike.
The primary drawback to incorporating WAAS/LAAS into an approach environment is the vertical component. Lateral, or “along track,” flightpath accuracy is exceptional—all modern 12-channel GPS receivers coupled to autoflight can maintain RNP-0.3 standards. The problem is that the vertical element still depends on barometric VNAV, which is subject to large changes due to temperature. In fact, all RNP-based approaches provide for minimum and maximum temperatures permissible before commencing the approach. Until a precise satellite-based VNAV/DA(H) solution can be implemented, pilots will have to be keenly aware of the limitations imposed by the use of barometric altitude reference.
A unique downside to LAAS is the “single point of failure.” For example, we can consider a large airport like ATL (Hartsfield, Atlanta GA) or ORD (O’Hare, Chicago IL), which have at least 8 ILS approaches and associated components for each runway. LAAS could replace all 8 ILS ground components, but a terror event or system failure could effectively close the entire airport during IMC. Redundancy, both within the NAS and in the cockpit, will be required before FAA approves a single-source navigation aid.
RNP pioneers
Naverus Corp, founded by Steve Fulton, has pioneered and implemented RNP approaches for numerous countries and airlines. It is the leading source of knowledge for US and global OEMs, regulatory agencies and avionics manufacturers for data about WAAS, LAAS and RNP requirements.
Fulton sees future US RNP approaches and applications moving slowly, due to the extensive network of ground-based navaids already in place. Considerable expense will be required to re-equip the entire US commercial fleet before the government can begin to decommission ground-based devices.
This scenario may change if the airlines are forced to adopt a CDA-type descent coupled with a preferential routing from ATC. This assumes that ATC and private industry will collaborate on solutions to move aircraft more efficiently than the current system.
For this to occur, a great deal of effort from FAA is vital. As a bonus, airlines could operate more efficiently and reduce CO2 emissions considerably without the great cost of new engine or fuel technology. Clearly, CDAs are attractive and viable from both an economic and environmental standpoint.
Elsewhere—ie, outside the US—countries with little infrastructure can adapt to WAAS and LAAS navigation easily once the respective satellite architecture (EGNOS, Galileo, MSAS) is placed into service. For US flag carriers, this could hasten their transition to RNP-capable aircraft to compete globally. Boeing and Airbus both confirm that their latest offerings are delivered to RNP specifications—once an individual carrier applies for the appropriate LOA and completes aircrew training, it will be ready to go. Corporate OEMs, including Bombardier and Gulfstream, will be among the first to be put RNP-capable aircraft into service.
Environmental effects and conservation
One benefit of CDAs is reduced carbon emissions and fuel savings through idle-power descents. This benefit can enhance and bolster aviation’s claim to be a proactive and dedicated group, focused on maximizing efficiency and improving green standards through technology and application.
Illustration of a “green” RNP approach, showing some of the benefits of a curved CDA approach (constant trajectory descent in the European ATM). IMC flightpath routings away from populated areas will address and perhaps appease both ATC and neighborhood noise concerns.
Environmentally speaking, aviation contributes less than 2% of global CO2 emissions and greenhouse gases. This means that if all the aircraft stopped flying today, the result would be a reduction of less than 2%. This small yet politically significant sum, further enhanced by OEM efforts toward greener production practices, ensures that aviation remains proactive in environmental issues and sets a good example for all industry to follow.
Emerging countries will continue to expand their carbon emission footprints and aviation’s current global 2% stake will seem tiny as India, China and other up-and-coming states embrace capitalism and compete globally.
The environmental summit in Bali, Indonesia which concluded in Dec 2007 established a “road map” for these countries to contain and control emissions. Like the Kyoto Treaty, little was accomplished that could be regulated, but the consensus was that progress had been made. And again, like Kyoto, aviation played a small and insignificant part in the overall plan. The cost of alternative fuels and aircraft design will remain prohibitive until the price of Jet A1 rises to unacceptable levels, and certainly above its current rate.
Summary
Curved continuous descents and approaches without the need for 5 to 7-mile finals are the future of WAAS and CDA/RNP approaches. The many problems associated with airport noise corridors and surrounding community sensitivities can be reduced, if not eliminated. Reduced greenhouse gas emissions and a cleaner environment are other significant byproducts of this technology, and the benefits will improve aviation’s image as a good global citizen.
Look for the first RNP-approved corporate aircraft soon, and advanced CDA descents in the near future
David Bjellos is the aviation manager for a private corporation whose flight department was the first in south Florida to achieve IS-BAO certification. The company operates a Gulfstream IVSP, a Dassault Falcon 2000, 2 Bell 407s and a Eurocopter EC120.
