In a landmark decision, the FAA has adopted a final rule allowing the use of HUD-based enhanced vision systems (EVS) for descent below published instrument approach minimums.

The eagerly anticipated rule change lets pilots continue straight-in Category I and nonprecision approaches below decision height or minimum descent altitude to 100 feet above touchdown zone elevation, where they would need to be able to see the runway or approach lights unaided to be legal to land.

In essence, the rule says that if the pilot can see the runway environment at published minimums by using the infrared EVS image, the approach can be continued to a height of 100 feet, after which “natural vision” must be used for landing. Pilots can rely on EVS during Cat I ILS approaches, as well as straight-in nonprecision approaches, by determining their “enhanced flight visibility,” a new term defined by the FAA as the average forward horizontal distance from the cockpit “at which prominent topographical objects may be clearly distinguished and identified” using the EVS. The FAA also said the rules apply to Cat II and III ILS approaches, but only by specific prior approval.

In its final rule, the FAA refers not to enhanced vision systems, but to enhanced flight visibility systems (EFVS), which it explains would include any electronic device capable of displaying a forward view on a HUD using an imaging sensor, such as forward-looking infrared (FLIR), millimeter-wave radiometry, millimeter-wave radar or low-light-level image intensifying.

Initially, the changes to FARs extend operational benefits solely to operators of Gulfstreams fitted with the infrared EVS sensor system developed by Kollsman of Merrimack, N.H. The $500,000 cryogenically cooled IR sensor package is a standard feature of the G550 and G450 and is offered as an option on the G300 and G500. Gulfstream gained its first certification for the IR camera system in October 2001 and has since installed about 50 of the devices on customer airplanes.

Used with the airplanes’ Honeywell HUD, the Kollsman EVS camera is mounted in a small fairing in the nose facing forward. Called the All Weather Window, it consists of a FLIR sensor operating in the one- to five-micron IR range, as well as an electronics-processing box and a glass pane through which the camera looks. The data from the camera system is processed and transferred to the raster HUD, where the IR image is displayed in shades of green.

Canada’s CMC Electronics hopes to gain certification later this year for a system it is testing aboard Bombardier’s Global Express, a design that uses an IR sensor developed by CMC and a Thales HUD. Federal Express, meanwhile, has announced it will equip its MD-10 and MD-11 and Airbus A300 and A310 fleets with the Kollsman EVS system by 2006. Max-Viz of Portland, Ore., is a third EVS manufacturer, but its design is presented only on head-down cockpit displays, and therefore is not covered under the new FAA guidelines.

The most important changes to the FARs (applying to Part 91, 135 and 121 operators) are contained in the language within FAR 91.175, which deals with IFR takeoffs and landings. According to the FAA, pilots flying precision approaches with an approved EVS can descend below the published decision height to 100 feet if the approach lights are in sight or the runway threshold and runway touchdown zone are visible with the aid of EVS. On straight-in nonprecision approaches, pilots can descend below MDA to 100 feet if the approach lights are available and are in sight or if the runway threshold and the runway touchdown zone are visible using EVS. On either a Cat I ILS or nonprecision approach the pilot can descend below 100 feet agl if the runway’s red terminating bars or the red side-row bars are visible and identifiable without reliance on the EVS image.

To obtain operational certification, the FAA said the EVS needs to provide the pilot with specific visual cues presented on a HUD centrally located in the primary field of view along the flight path. The HUD has to include flight-guidance symbology that is appropriate for the type of approach being flown, generally meaning it must show airspeed, vertical speed, attitude, heading, altitude and flight director commands, as well as lateral and vertical approach-path-deviation indications. A flight-path-angle reference cue and FPV (flight path vector) also must be displayed on the HUD that allows the pilot to maintain a safe vertical path from the DH or MDA to the desired touchdown point on the runway.

The speed with which the EVS rule emerged from the FAA seemed to surprise many. The agency issued its notice of proposed rulemaking on the subject only last spring. Considering that the EVS concept is still relatively new and FAA officials are generally known for taking a go-slow approach with novel or unproven technology, there was a general sense after the final rule’s publication last month that something remarkable had happened.

“It’s surprising that this occurred on the timetable it did,” said Jean Menard, director of sales for Max-Viz. “I don’t think anyone imagined that the FAA would ever move so quickly.”

Executives at Gulfstream, who had a hand in convincing the FAA to adopt the change to FAR 91.175, cheered the rule change’s passage, calling it a victory for business aviation.

“This ruling unequivocally validates the original concept and subsequent years of hard work,” said Preston Henne, senior vice president of programs, engineering and testing at Gulfstream, in a statement released soon after the final rule was made public. “The expediency with which the FAA made this ruling is a clear indication of the incalculable value this system will bring to both the business and commercial aviation industries.”

The FAA was careful to point out that the change to FAR 91.175 does not include synthetic-vision systems (SVS). Because SVS technology uses a database of the earth’s terrain to create a “virtual” view of the world on a flight display, it cannot show a real-time image of aircraft or vehicles on the runway. The FAA cited the ability of EVS to help prevent runway incursions as one of the technology’s chief benefits. The agency added that although the Chelton FlightLogic EFIS is an SVS that is certified for use on instrument approaches, it cannot be used by itself below DH or MDA.

During the rulemaking process, some in the industry asked the FAA to expand the EVS rule to include takeoffs. Since the system can be used to meet flight-visibility requirements on approach, it follows that some credit should be given for takeoff operations below established visibility, some argued. The FAA responded that such a suggestion was “beyond the scope of this rulemaking,” but by not disagreeing outright with those who commented the agency seemed to leave open the possibility that another update to the FARs covering EVS and takeoff requirements might be made at some point in the future.