Tighter Air Control

tighter air control

A sophisticated operating system, combining currently available technologies, may be a key step toward greater safety in the skies.

By John A. Andersen, Stephen D. Fulton, and John H. Andersen

You have been settled into your airline seat for quite some time, working away, and you hear the familiar message from the captain that you are nearing your destination, there will be some turbulence, and be sure that your seat belt is fastened. The cabin is cleared and secured; your laptop is stowed.

You take more notice of the flight: outside the window, no apparent visibility beyond the wingtip; there's some bouncing, and the control surfaces on the wing are moving. An announcement tells you the landing will be delayed, because of weather and traffic. You detect changes in engine speed, more movement of the wing's control surfaces, banking and turning, speeding up, slowing down, changing course, changing cabin pressure. After quite a while, there is the reassuring thump of the wheels on the runway, which has appeared beneath the aircraft.

In the cockpit, the two pilots have been busy. A small and crowded paper chart, about 5 by 8 inches, called an approach plate, has been clipped up on the control column. Radio transmissions from the approach controller are practically unremitting; only occasionally is one addressed to your aircraft, but all must be listened to attentively, and immediate reply and action are required when the message is for your airplane.

Multiple radios have been set to the proper receiving frequencies, both for voice communication with controllers and for navigation based on radio transmitters located strategically on the ground. Interpretation of these radio signals defines imaginary control points in the sky.

The cockpit of an aircraft flying under the RNP system replaces paper charts with real-time monitoring of navigational performance.

The approach plate shows the navigation points to be flown (lateral nav) and a side view of the altitudes required over each point (vertical nav) and the glideslope angle to be flown between the points. There is no three-dimensional presentation. A large amount of other vital information is also squeezed in at the top and bottom of the small chart.

The airplane must be flown in the pattern directed by the controller on the ground or as otherwise indicated on the approach plate, noting the ground track over the imaginary points by inference from the movement and alignment of needles on gauges or on flat panel displays. Power settings, control surfaces, and trim wheels must be changed constantly to maintain the proper flight configuration.

Many aircraft systems must be monitored simultaneously, including the airspeed, attitude, engines, fuel management, navigation indicators, and weather radar; the controllers must be heard; the visual scan of the instruments must be interspersed with looking all around outside. Only some of this work can be set into the autopilot; much or all must be hand flown, by a pilot reacting to subtle changes in the alignment of needles on gauges or of pictorial displays on screens. Situational awareness is vital, integrated in the pilots' minds from all of these visual and audible clues.

This compressed description of cockpit activity is based on a precision approach and landing at a controlled field—a major airport with navigation aids, including radar, and sets of human controllers for the enroute segment, the approach phase, the landing phase, and ground control.

The cockpit workload is quite different at an uncontrolled field, which may have few or no navigation aids, or radar or controllers. Night and bad weather compound the challenge.

This is called a nonprecision approach. Here, the pilots may be alone with a single nondirectional radio beacon, perhaps quite far from the field. At times, a circling approach is called for while descending, looking for the ground to show up through the weather. Both the precision and nonprecision approaches are typical of controlled flight today, called instrument flight rules, or IFR.

It has been known for a long time that there can be a better way, an engineered system that uses the advantages of available modern technology, including Global Positioning System satellites, inertial reference systems, flight management systems (specialized computers), and autopilots.

One of these better ways is in service today.

The national and international airways and airports are congested. The recent wave of aviation terrorism is adding complexities and challenges. Thinking and planning have been ongoing for some time to modernize and improve this complex system, and many new ideas have emerged since the day of infamy on Sept. 11, 2001. Technology and the combined wills of industry and government can address these challenges.

More than a decade ago, 85 member states of the International Civil Aviation Organization endorsed a global Communications, Navigation, Surveillance, and Automated Traffic Management concept. This concept, called Future Air Navigation System II, advocates a change from terrestrial-based technology to space-based technology and digital communication. Extensive use is made of satellites for both navigation and communication. In 1995, the first-generation system was placed in use over the Pacific, where aircraft were out of range of the older radio control systems for lengthy time periods.

The RNP-guided route to Reagan National avoids flight into Washington's forbidden air spaces and reduces the need for visual contact with the Potomac to reach the airport.

In recent years, the U.S. Federal Aviation Administration proposed a broad-based approach to a modernized air traffic control system, called Free Flight. This is a multistage and multiyear government program that takes into account numerous viewpoints and stakeholders. Free Flight gradually introduces new segmental elements on a trial basis. One of the first, placed into limited trial use, is increased accuracy of the approach to landing, using high-precision GPS.

In contrast with the decades-long international and FAA processes, a much shorter-term, FAA-approved solution called RNP, or Required Navigation Performance, was rapidly developed and certified, and placed into use in a critical environment. It has been in commercial airline service on 50 or more airliners at about 10 airports for several years, and is now on the brink of expansion.

When RNP is applied to the critical approach phase of flight, the cockpit activity and workload are quite different from our opening scene. The aircraft is placed on a highway in the sky, and navigation and control are accomplished automatically under full surveillance of the flight crew. The controllers simply give the aircraft permission for an RNP approach. Attention turns from physical execution of many details to an overview and monitoring of the results.

Design and development of the present system was initiated by a small group of engineer/pilots and avionics technicians at Alaska Airlines in December 1994, with support from avionics experts from Smiths Industries, and aircraft integration support from Boeing Co. Operational evaluations were flown in June 1995, and the FAA-approved system was placed in revenue service in Alaska in May 1996. This is an unprecedentedly short time for a major development in modern aviation under government oversight.

The system first went into service at the state capital of Juneau, which has only air and water transportation and involves overflight of high terrain from all directions. The weather is often challenging, restricting visibility. RNP was designed for Sitka, Ketchikan, and Wrangell, where the airfields are perched on small flat areas at the edge of water and are closely surrounded by high mountains. The fields are often obscured by clouds, rain, snow, and ice.

RNP has often been the difference between making this scheduled service, which is the only public transportation besides ships or ferries, or not landing or departing, sometimes for days. RNP uses space-based and autonomous (that is, within the aircraft) guidance systems, and numerous modern flight safety enhancement subsystems, in an integrated approach for safer, more efficient flight. RNP is based on an accurately defined containment, as required by the flight path, and not on a nominal assumption of performance provided by an external navigation system. The flight crew inputs the specific navigation requirement for the intended route to the flight computers, by means of a highly checked and verified flight program.

The Gastineau Channel to Juneau is a tight path through mountains. Flying by GPS, inertial reference, and other control systems can keep the way open in weather that de-feats conventional flight rules.

A data input device accepts a floppy disk that contains the specific flight management and navigation information for the selected routes and destinations and all possible alternates. This information is checked again on the ground, immediately prior to takeoff.

This total flight management assures that a carefully engineered pathway in the sky, and appropriate aircraft configuration settings (airspeed, altitude, attitude, power settings, and flight control surfaces) are maintained autonomously, by the flight management computers and the autopilot, to an extent previously not achieved. This workload assumption allows the flight crew to assume a monitoring role, thereby freeing up valuable time for increased situational awareness. Actual navigation performance is constantly computed by the system and compared to the required navigation performance, so that an alert can be provided if the actual position of the aircraft is outside the intended bounds. Clear instructions are provided as to where and how to fly if this alert occurs.

In addition, actual performance is constantly displayed so that the pilots can be prepared to take over the flight, if they need or choose to do so.

The present system of instrument flight rules is based on determining vertical position by a barometer (called an altimeter) in the aircraft and lateral position from radio navigation beacons on the ground. The advent of radar improved this from the perspective of ground monitoring by controllers. Relatively large flight containment margins are regulated for safe flight.

Avoiding Overflight Automatically

RNP provides a far more precise containment and, thus, supports more efficient use of airspace. RNP can be used to automatically avoid overflight of critical assets on the ground, such as buildings or other infrastructure of strategic importance. This is especially vital to Washington, where RNP routes have been designed and demonstrated to the FAA. And the implications for increased safety in New York City are now obvious.

When allowances are made for the dynamics of the vehicle and the associated minor deviations of the flight path from the intended (programmed) path, the minimum authorized RNP lateral control is currently set at 0.11 nautical miles. Conventional instrument flight rules may have to allow for an error of three statute miles for the same airspace.

In a given situation, the actual navigation performance, the product of the on-board equipment, typically delivers an accuracy of 0.06 nautical miles, including provisions for integrity. Vertical clearances are set by other considerations, such as the altitude being flown, air traffic, and terrain.

RNP also has the potential to avoid a prominent cause of aviation accidents, known as "controlled flight into terrain." This type of accident, the crash of a properly operating aircraft that may run into a mountain or miss a runway, is virtually eliminated when RNP is used for the approach to landing. The system copes with hostile terrain and bad weather to make flight both safer and more reliable; much lower weather minimums (vertical and lateral visibility) are needed for safe landings.

When applied to long-range phases of flight, more aircraft may be safely placed into the finite space available. With improved safety in landing and with better use of parallel runways at major airports, especially in bad weather, RNP can be a major factor in resolving critical airspace problems in developed nations.

Service by suitably equipped and crewed aircraft becomes available when others are grounded. RNP approaches and departures are easier to fly, require fewer steps in the vertical flight path, and may be designed to cover abnormal situations, such as losing thrust on an engine during approach to landing, or circling and returning to the field in the case of a missed landing. These situations can be especially taxing on a flight crew, but can be provided for in advance by RNP flight programming.

An additional benefit is more direct routing with less time in the air, yielding fuel savings, air pollution decreases, and payload increases.

Much of the necessary RNP equipment is considered to be standard for a modern air transport category aircraft. In fact, every new production aircraft made by Boeing and Airbus is 100 percent equipped and certified with everything it needs for RNP.

The RNP system with dual flight management computers is also compatible with an enhanced ground proximity warning system and distance measuring equipment and very-high-frequency omnidirectional radios.

What is new about the system is the synergistic use of all of these assets in an integrated form, using specifically engineered flight paths, as well as landing and departure routes for specific aircraft. All of this is programmed into the aircraft in a tested and carefully benchmarked and reviewed manner. The full system is checked prior to every takeoff and is constantly monitored in flight. All critical systems are redundant.

Preventing terrorist use of commercial aircraft as weapons of mass destruction became apparent on Sept. 11, 2001.
RNP assures a defined and controlled ground track as the aircraft flies. This is important in areas that have a critical infrastructure and have ground assets of high strategic importance, as in New York, Washington, and many other cities.

The RNP equipment can be enhanced to include automatic reporting of any deviation from the prescribed flight path. Totally automated, this would alert designated authorities of an abnormal circumstance. Being automated, it requires no overt action by the flight crew, such as resetting the aircraft transponder (a radio broadcasting device that transmits an encoded message, identifying a specific aircraft and its altitude). In the September 11 takeovers, terrorists knew how to shut off the transponders, making it more difficult for FAA controllers to notice the deviation from the assigned flight path.

In the future, control could be locked in to the installed RNP route by coded permission from the authorized flight crew. In these special situations, the aircraft flight path could not be diverted or reprogrammed. The aircraft would proceed along its designated path unless destroyed in flight. Although a massive assault from within the aircraft could obviously have tragic results, the aircraft could not be controlled by an outsider and turned into a weapon of mass destruction against an alternate point.

Automated landing can be added to the final segment of the present RNP guidance. In this case, RNP would be integrated with present landing guidance at major airports and would link to the latest GPS local area augmentation, a segment of the FAA's Free Flight.

RNP technology in its most advanced form as an arrival and departure technology is currently in limited use—in one region by one airline. It was voluntarily developed by industry.

In view of recent tragic events in the United States, it is receiving new and broader attention. A plan has been designed specifically by a cooperative industry and government partnership for the Ronald Reagan Washington National Airport, and is close to being implemented. The enhancements of positive ground track and automatic reporting of any deviation from the programmed accurate flight path could be implemented rapidly.

Given approval and appropriate manufacturer and aircraft operator support, the coded access and locked-in automated flight would be possible within a short development time, for some aircraft. Regulatory processes have a significant impact and, in some cases, exceed the time of the engineering process. Careful knowledge sharing, cooperation, and familiarization by air traffic controllers are vital to a broad and rapid implementation and acceptance of the technology.

Perhaps this evolution in aviation technology has parallels in the past, when ASME codes for safe boilers and pressure vessels, and elevators and escalators were voluntarily adopted and, eventually, legislated into practice. Aviation is a vital national and international service. Problems of safety and efficient use of assets require solution. And now, aviation terrorism must be dealt with. RNP addresses these issues.

John A. Andersen, P.E., is chairman of AFlightTech Inc. in Edgewood, N.M., and is a Life Fellow of ASME. Stephen D. Fulton, a captain with Alaska Airlines, and a test pilot and FAA-designated engineering representative, is also president of FANEC Inc. in Federal Way, Wash. John H. (Hal) Andersen is a captain and technical pilot with Alaska Airlines and is president of AFlightTech, based in Tacoma, Wash.