Final Approach
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The aircraft operated by most air carriers are usually equipped to satisfy the navigation requirements of a variety of approach procedures. Precision approaches include Global Positioning System (GPS) autoland, GPS LNAV/VNAV and Category (CAT) I, II and III ILS approaches. Many runways at larger airports utilize the Instrument Landing System (ILS) to provide guidance to pilots during instrument conditions along a well-defined path made up of lateral and vertical elements called the localizer and glide slope respectively. A non-precision approach is a procedure where lateral track information is provided by a local navigation aid (navaid) or satellite, but vertical guidance is received through barometric referencing or other means not directly associated with the specific runway. As expected, precision approaches provide for operations in much lower ceiling and visibility conditions. When very low visibility conditions exist, CAT III approaches are mandated which require autoland or HUD (heads up display) guidance. Arrival delays should be anticipated during these operations due to limited spacing and runway options. When VFR (Visual Flight Rules) conditions exist, pilots are encouraged to use all available navaids as a back-up even during visual approaches. In addition, many airfields employ instrument procedures during VFR conditions in order to manage aircraft sequencing or noise restrictions. At some point during the vectoring or feeder segment, the flight will be “cleared for the approach”. An approach clearance by ATC authorizes the crew to execute the procedures for landing. As mentioned earlier, it is the cockpit crew’s responsibility to determine approach legality. The current weather conditions must be compared to the procedures and equipment available, both ground-based and airborne. Downgrades in onboard automation or displays may dictate higher landing minimums and/or unavailability of certain procedures. Likewise, the inoperative status of any ground components may result in additional landing restrictions. An adverse condition of any required approach facilities is usually reported by ATC or the ATIS, but also may be detected by onboard alerting systems. Most authorities designate a specific location in the procedure where the current weather must be at or above weather minimums in order for the aircraft to continue on the approach. If the flight passes the designated position with reported weather at or above minimums, it may continue to the missed approach point or decision height, as applicable. If weather conditions are below minimums at the designated position, the procedure must be aborted and other alternatives considered i.e. diversion, holding, etc. Runway wind conditions must be addressed by the crew during the final approach and landing. Depending on the wind direction, intensity, and presence of gusts, adjustments may have to be made resulting in a higher planned approach speed. Maximum crosswind limitations vary among equipment type and weather conditions. In the event the requirements for completing the approach and landing are not satisfied, a “go-around” is executed and a standardized “missed approach procedure” and/or ATC instructions must be followed. Options available following a missed approach include entering holding to wait out whatever unacceptable condition resulted in the aborted landing, diverting to an alternate airport, or most commonly, accepting ATC vectors to initiate another approach. Many aborted landings are initiated by ATC or the cockpit crew due to traffic on the runway. In most cases a prior arrival failed to clear the runway in a timely manner, but a delayed take-off by an aircraft sitting in position at the threshold can also result in an aborted landing. If the runway is in-sight and clear when at the decision point, the cockpit crew continues the descent until initiating the landing “flare” maneuver where the descent rate is reduced just before touchdown.