New rules for all-weather operations are being introduced in Europe. These include provisions for approach operations with “operational credits”, but what is an operational credit?
Approach operations are classified according to the weather conditions in which they can be used; for example, a category I (CAT I) approach allows a pilot to fly using instrument guidance to a height of 200 ft above the runway. If the pilot does not have the runway in sight at 200 ft (“decision height”), she has to abandon the attempt to land and climb away (“go-around”). There are also minimum visibility requirements. The CAT I approach typically requires a “runway visual range” of 550 m. Other approaches that don’t provide such precise guidance have higher decision heights than CAT I and require greater visibilities. The decision height and minimum visibility depend on the type of avionics and ground equipment available, any obstacles or high ground in the approach path, and the runway lights and markings.
Category II approaches (“CAT II”) may have a decision height as low as 100 ft. Category III approaches (“CAT III”) can allow descent to landing without seeing the runway. CAT II and III require autoland systems or head-up displays (HUD), sophisticated radio installations on the ground, arrays of landing and approach lights, a long runway, an extended area clear of obstacles, and multiple redundancies, including backup power supplies. Upgrading an airport to support CAT II/III and maintaining that capability is expensive. Some airports are not suitable for CAT II/III because of the topography.
An operational credit allows the use of lower weather minima within the same classification of operation. Operational credits use the enhanced performance of ground or airborne equipment to extend either the instrument or the visual segment of an approach.
Approach operations have an instrument segment during which the pilot flies solely by reference to instruments. Guidance for the instrument segment is provided by a radio signal processed by the avionics on the aircraft. This could be an ADF, VOR or ILS receiver or a system deriving position information from multiple sources, including GPS. The instrument segment continues down to decision height, the point where the pilot must have sight of the runway. After decision height is the visual segment. In the visual segment, the pilot flies visually, looking out of the window to complete the approach and landing.
An operational credit that applies to the instrument segment of the approach uses some capability of the flight guidance system to stretch the instrument segment. “Special Authorisation Category I” (“SA CAT I”) is being introduced in Europe. SA CAT I will require special avionics, including a radio altimeter and an ILS signal that gives reliable indications closer to the runway than a standard Cat I ILS. This will allows a pilot to fly further along the approach on instruments so that the decision height can be as low as 150 ft.
An operational credit that applies to the visual segment of an approach uses some new capability to allow the pilot to fly visually in conditions where he/she previously couldn’t. Enhanced Flight Vision Systems (EFVS) use infra-red sensors combined with a HUD to allow the pilot to see through weather conditions, including fog and snow. At decision height, the pilot can use the EFVS and, if the runway is in sight using EFVS, the approach can be continued. EFVS doesn’t affect the instrument segment of the approach, so the decision height doesn’t change, but an approach can be completed in lower visibility.
When the new regulations for operational credits are fully implemented, some aircraft operators will use lower weather minima, especially to some secondary airports that don’t have all the infrastructure required for CAT II/III low visibility operations.
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