FAA readies regs for GPS transition

October 11, 2007, 5:33 AM

The FAA is proposing numerous revisions to instrument flight rules and procedures to reflect technological advances intended to “facilitate the transition from ground-based navigation to new reference sources,” principally GPS and enhanced vision systems. The notice of proposed rulemaking (NPRM) cuts a 60-page swath through the Federal Register, affecting Parts 1, 71, 91, 95, 97, 121, 125, 129 and 135.
The changes proposed in this NPRM would “facilitate the use of Rnav throughout all phases of flight (departure, en route and approach), which is a goal of the Free Flight program,” the agency said. Free Flight is designed to enhance the safety and efficiency of the National Airspace System by moving it from a centralized command-and-control system between pilots and ATC to a system that allows pilots, whenever practical, to choose their own routes and file flight plans that follow the most efficient and economical routes. Adopting the proposed changes, the FAA maintains, would result in “greater flexibility in air traffic routing, instrument approach procedure design and airspace use than is now possible under a ground-based system structure.”

This proposal is also part of a continuing effort to recognize the advent of new technologies and international efforts to create a seamless ATC system by making the terms used in the FAA’s regulations consistent with those used by the International Civil Aviation Organization (ICAO).

One of the first items listed under the proposed revisions is to formally drop the middle marker as a required component of ILS. It has been more than 10 years since regulations required landing minimums to increase when middle markers were out of service.

References to “decision height” and “DH” are being replaced with references to “decision altitude” and “DA,” respectively, where minimums are based upon barometric altitude, which is expressed in feet msl. DA would be used only with Category I approaches. In contrast, where minimums are based upon agl the term decision height (DH) is used and would apply only to Category II and III approaches. These changes are being proposed to make the FAA’s regulations consistent with ICAO terminology and to “more accurately describe when the decision to continue the approach below the authorized minima or make a missed approach is made.”

As a result of this proposed change and others, many current references to DH in Part 91, 135 and 121 will be changed to “DA/DH.” Similarly, all references to HAT would change from “height above touchdown” to “height above threshold.” Also, references to “ground” and “radio” would be addressed because using these terms restricts the type of navigation and communication systems pilots can use. The amendments would either replace those words with less restrictive language or remove them entirely, which would allow for the expanded use of Rnav systems and permit operators to take advantage of future changes in technology.

The FAA is proposing new terms–“Air Traffic Service (ATS) route” and “area navigation (Rnav) route.” An ATS route would be used to describe the U.S. en route structure. ATS routes would include federal airways, jet routes and area navigation routes in the U.S.

Rnav routes would refer to ATS routes established for the use of aircraft capable of using area navigation. But the FAA noted that not all Rnav-capable aircraft are suitably equipped to operate on all Rnav routes. The agency would determine the means to qualify aircraft for various area navigation operations and the method for promulgating the requirements to operate on Rnav routes. The definition of area navigation would be broadened by removing the words “station-referenced navigation signals,” which refer to ground-based signals, and adding the words “flight path” to cover operations in both the lateral and vertical planes.

An “approach procedure with vertical guidance” (APV) is a new term for an instrument approach procedure based on a lateral path and glide path. This approach is flown to a DA. Although these procedures include glide-path information, they may not meet the requirements for precision approaches, the FAA cautioned. An example of an APV approach is the Lnav/Vnav approach minima currently published on Rnav approach plates.

The term “Category I operation” commonly has been used in the conduct of precision approaches and in the preambles of FAA regulatory documents for years, but operators may find it surprising that it has never been defined in the FARs themselves. The agency is therefore proposing to add a definition of this term. The proposed definition of a Cat I operation is a “precision approach with a decision altitude that is not lower than 200 feet above the threshold and with either a visibility of not less than one-half statute mile or a runway visual range of not less than 1,800 feet.”

Cat II, III and IIIa definitions would be revised to incorporate the concept of precision Rnav. In each of these definitions, the terms “ILS approach” or “ILS instrument approach” would be replaced with the terms “precision approach” and “precision instrument approach,” respectively. The new definitions would also be compatible with JAA terminology.

More Restrictive ‘Night’ Definition
The FAA is proposing to revise the definition of the term “night” to reflect that “local night” may differ from the times published in the American Air Almanac. This concept of local night could limit operations at a particular location when the agency determines it to be necessary for safety for example, when terrain causes sunset significantly earlier than the almanac indicates. This proposal would essentially adopt a recommendation from the NTSB.

Last April the Safety Board asked the FAA to revise restrictions that currently refer to night operations at airports in mountainous terrain to account for the “entire period of insufficient ambient light conditions” rather than the 30 minutes after sunset currently used. The recommendation emerged from the Safety Board’s investigation into the fatal crash of a Gulfstream III on March 29, 2001, while on short final to Sardy Field in Aspen, Colo.

The accident, which killed all 18 aboard, occurred about 30 minutes after official sunset. But the Safety Board determined that the sun would have set below the mountainous terrain about 25 minutes before official sunset. Witnesses reported that it was “very dark” at the time of the crash. Visibility was 10 miles in light snow. Although the pilot told ATC that he had the runway in sight, that “would not ensure that he could also have seen intervening unlighted terrain,” NTSB said.

Under the NPRM, the term “electronic glideslope” would be eliminated from approach procedures. The term would apply to navigation systems that provide lateral (but not vertical) path deviation guidance. The new term, “precision final approach fix” (PFAF), would indicate that it is associated with a precision approach or an approach procedure with vertical guidance. Also, the definition of “route segment” would be revised to mean a portion of a route bounded on each end by a fix or navaid.

The FAA is proposing to revise FAR 91.131(c)(1), “Operation in Class B Airspace,” by adding the words “suitable Rnav system” to provide another option for meeting the communications and navigation equipment requirement. This change would be consistent with the proposed definition of Rnav. Currently, FAR 91.131(c)(1) allows only a VOR or Tacan receiver.

FAR 91.177, “Minimum altitudes for IFR operations,” now specifies that an aircraft can operate down to the minimum obstruction clearance altitude (MOCA) only within 22 nm of a tuned-in VOR. A proposed change under the NPRM would clarify that an aircraft could fly at the MOCA for the full route segment if the pilot is using GPS-based Rnav.

If the proposal is adopted, operators will also see many references to “two-way radio communications system and navigation equipment appropriate to the ground facilities to be used” replaced with “two-way communication and navigation systems suitable for the route to be flown.” This revision will facilitate future developments in communications, the FAA said.

Aircraft operating at or above FL180 would have to be equipped with DME, under the proposed revisions. The current rule sets the limit at FL240. This proposed change would make FAR 91.205 (equipment requirements) consistent with the current floor of Class A airspace. While most affected aircraft already meet these standards, 91.205 would be amended to include suitable Rnav systems as an alternative to DME, since modern area-nav equipment includes a distance readout.

Helicopter operations haven’t been forgotten in the NPRM. For example, a new technical term–“helipoint crossing height” (HCH), used in the construction of helicopter instrument approach procedures–would be added to heliport approach charts. The HCH, the computed height of the vertical guidance path above the helipad or heliport elevation, “affects the size of the obstacle evaluation area for the instrument approach and is another means of providing a margin of safety to the operator,” the FAA said.

This proposal would also add the term “helipoint,” which is normally the center point of the touchdown and lift-off area. It is usually a designated arrival and departure point located in the center of an obstacle-free area, 150-feet square, overlying an approved landing area, where the approach may be terminated in a hover or touchdown.

Some of the proposed requirements are directed only at airline operations, but the FAA’s justification for one new requirement in particular provides insight into the agency’s view on the vulnerability of GPS. A proposed revision to Part 121 would replace the requirement for two independent navigation receivers with a requirement for two independent navigation systems. There would be no requirement for the two systems to be identical, so that a single VOR and a single Rnav system would satisfy this requirement on a Victor airway.

The intent of this rule is to “ensure that there is no single point of failure or event affecting aircraft navigation systems.” The change is also “intended to address the vulnerability of GPS, which uses very weak signals that are susceptible to interference.” For example, two GPS (or other satellite navigation) receivers may not be considered “independent,” since both are considered by the FAA to be “so vulnerable to interference.”

However, the proposed rule would be “performance based” rather than prescriptive; thus, “it is possible that two GPS receivers with an anti-jam capability could be considered independent, since they would not be so vulnerable to interference.” Systems are considered independent, to the FAA, if there is no probable failure or event that could affect both systems.

The proposed wording would allow for the future evolution of navigation system technology. Currently the FAA sees a need for a full DME infrastructure and a minimal VOR network to remain for the foreseeable future. However, as the NAS evolves and navigation technology improves, a satellite-based system may become the core of the navigation system.

Benefits and Costs
The FAA believes the proposed rule would not impose an obligation to change current navigation systems, meaning no costs to aircraft operators. In fact, cost savings might result, said the agency, because the proposed rule would enable the use of advanced area-navigation routes that are typically more direct and, therefore, shorter than the current federal airways and jet routes. Estimates of cost savings from flying advanced Rnav test routes that the FAA has established are in excess of $30 million annually, the agency claimed.

In addition, eliminating the middle-marker as a required ILS component would result in net cost savings to owners of middle-marker facilities who choose to decommission their facilities. In looking at the docket of this proposed rule, few substantive comments have been submitted. That’s because all 20 submitters to date requested an extension of up to 90 days beyond the January 31 comment period. Those asking for more time included AOPA, Alaska Airlines, Arinc, Air Transport Association, Boeing, Continental Airlines, Delta Air Lines, Northwest Airlines, Regional Airline Association and United Parcel Service. At press time the FAA was preparing to reopen the comment period to at least March 31.

For further information contact Lawrence Buehler, Flight Technologies and Procedures Division, Flight Standards Service, AFS-400, FAA, 800 Independence Ave. SW, Washington, D.C. 20591; telephone: (202) 385-4586.

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