A. The initial steps toward achieving the current CAT I operating minimums were taken on September 28, 1961. The first air carrier operations with minimums of a ceiling of 200 ft. and a visibility of 1/2 statute miles (RVR 2600) were authorized on May 11, 1962. The concepts developed to permit this reduction in operating minimums established the foundation for a “building block” approach. With this approach AWTA operations evolved in an orderly manner as airborne and ground based capabilities improved. The 1961 reduction in minimums to a ceiling of 200 ft. and 1/2 statute mile visibility (RVR 2600) was based on further improvements of airborne equipment, electronic ground-based navigation aids (NAVAID’s), ground-based visual aids and enhanced pilot training and qualifications. This 1961 reduction was authorized when the following conditions were met:

(1) Ground-based navigational aids included:

    • A complete, operational ILS

    • A maximum glideslope angle of 3 degrees

(2) Ground-based visual aids included:

    • High intensity runway lights

    • Full configuration approach lights with sequenced flashing lights

    • All-weather runway marking or runway centerline lights

(3) Airborne equipment included:

    • A flight director system or an automatic approach coupler (autopilot)

    • An instrument failure warning system or cockpit procedures for assuring the immediate detection of instrument failures or malfunctions

(4) Pilot-in-command experience, training, and qualification requirements included:

    • 100 hours of experience as pilot-in command in the particular type of turbojet airplane

    • Raw data approach to 200 ft.

    • Flight director and/or autopilot approach to 100 ft.

    • ILS approach (flight director and/or autopilot as appropriate) to 100 ft. followed by a landing

    • Engine-out ILS approach to a landing or missed approach

(5) Additional runway field length and crosswind component limitations included:

    • 15 percent or 1000 ft. of additional field length (whichever is greater) over normal regulatory requirements

    • A maximum crosswind component of 10 knots

B. A major change in the method of specifying the operating minimums for precision approaches evolved with the introduction of the decision height concept and the RVR concept. These changes were finalized by the publication of U.S. TERPS criteria in 1966. This conceptual change eliminated the ceiling requirement by introducing a decision height (DH) (see section 2, paragraph 489) and based landing minimums on runway visual range (RVR) reports, when available, instead of ground or flight visibility reports. This conceptual change was necessary because of the limitations in the methods used to observe or measure ceiling and visibility (see section 2, paragraph 495). Often ceiling and visibility observations were taken several miles from the approach end of a runway, and as a result were frequently not representative of the seeing-conditions encountered during the final stages of an approach and landing, especially in rapidly changing or marginal weather conditions. Operational use of RVR reports began in 1955 but they were not available at most major airports until the early 1960’s. Currently (1989), all operations using minimums below 1/2 statute mile visibility must be based on RVR reports.

C. In 1963, operating minimums were reduced further to DH 200/RVR 1800 for two- and three-engine airplanes (usually Category B or C) and DH 200/RVR 2000 for four-engine airplanes (usually Category D). These reductions were based on the “building block” approach established in 1961 and the added requirement for enhanced in-runway lighting systems such as high-intensity touchdown zone and runway centerline lighting. In 1964, the minimums for runways not equipped with a high-intensity touchdown zone and runway centerline lights were reduced to DH 200/RVR 2400. Improvement in visual aids were, and still are, a critical element in reducing landing minimums. These aids provide pilots with the necessary external visual references for manually controlling and maneuvering the aircraft during the final approach, flare, landing, and taxiing. The requirement for improvements in the overall airborne and ground based capabilities combined with a cautious incremental reduction in operating minimums assured that a high level of safety was maintained. Currently CAT I operations are still conducted in accordance with these concepts and criteria.

D. In 1988, CAT I operating minimums for Category D airplanes were reduced to DH 200/RVR 1800. This change established common CAT I minimums for all airplanes. The 1988 reduction was based on more than 20 years of successful experience with Category B and Category C turbojet aircraft operating to DH 200/RVR 1800, as well as research and analysis. This research has shown that the handling characteristics and seeing-conditions in existing turbojet Category D airplanes were equivalent to other turbojets.

Upon arrival at the minimum height or altitude for instrument flight and before passing a pre-established decision point, a pilot must establish adequate seeing-conditions to safely complete the approach and landing. Operating minimums are expressed as visibility (VIS), runway visibility values (RVV), or runway visual range (RVR). Criteria for establishing operating minimums must provide a reasonable assurance that a pilot can establish the required seeing-conditions before passing the decision point. This criteria provides this assurance if the weather conditions are reported to be at or above the landing minimum when the approach is initiated. To achieve this objective, the operating minimums specified for the procedure (VIS, RVV, RVR) must be compatible with the minimum height or altitude for instrument flight and the decision point specified for the procedure. Therefore, when the reported weather conditions are at the authorized minimums, a pilot should be able to establish external visual references upon arrival at the minimum height or altitude (DH or MDA), and before passing the decision point (DH, MAP, or visual descent point (VDP)). At this point a pilot must be able, by external visual reference, to maneuver to a landing without exceeding a descent rate of 1,000 fpm or exceeding aircraft limitations on touchdown. (See paragraph 497 for a discussion of factors affecting seeing-conditions.) For example, it would not be practical to specify a DH of 200 feet (HAT 200) with an operating minimum of RVR 700 since the first visual contact in a typical aircraft would not occur until approximately 130 feet above the elevation of the TDZ. The specified operating minimum must also permit adequate external visual references to be established early enough for a normal descent to landing (less than 1,000 fpm). For example, it would not be reasonable to specify an MDA equivalent to a HAT of 400 f e e t and an operating minimum of RVR 1600 for typical turbojet airplanes. In this situation, the pilot would not establish first visual contact until the airplane is within 4,000 feet of the landing threshold and would require a descent rate much higher than 1,000 fpm to land within the TDZ.