Descent
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The descent profile is determined by both ATC limitations and optimal aircraft performance. An aircraft operating at typical cruise altitudes (31,000 to 41,000 feet) will nominally initiate the descent at 100 to 130 nautical miles from the destination airport. The distance varies primarily due to ATC restrictions/procedures but also may be influenced by equipment type and environmental conditions such as winds aloft and turbulence. The initial descent takes place with about 30 to 40 minutes remaining in the flight, at which time the crew begins their approach and landing preparations. An “In Range” message is often transmitted to the destination station either through ACARS or by VHF radio. This message includes the latest touchdown estimate, special passenger requests (wheelchairs/connections), and if not already transmitted, any maintenance discrepancies. The station transmits or uplinks the arrival gate assignment, ground power unit status and any other relevant status message such as a “tow-in only” requirement for the assigned gate. During the descent, ATC may issue crossing restrictions which can be part of a published standard arrival procedure or as a response to a traffic sequencing requirement. If the clearance is not issued as an immediate descent, it is the responsibility of the cockpit crew to determine a top-of-descent (TOD) point which satisfies the crossing restriction. Ride conditions notwithstanding, it is desirable from an efficiency standpoint to delay the descent as long as possible, then descend at or near engine idle at an optimum speed and meet the restriction within a few miles before the fix to give a margin to ensure the restriction is met. Factors which must be taken into consideration during descent planning include wind direction and intensity for the relevant altitudes, possibility of speed restrictions if not already assigned, high barometric local pressure at the transition level and turbulence. The FMS is the primary resource available to the crew for descent planning as restrictions can be programmed directly and a profile calculated. There are other ad hoc methods for determining the distance required to lose a given amount of altitude. The “3 to 1” rule is still used by most pilots to back up the FMS solution in which 3 miles are required for every 1000 ft of altitude loss, e.g. 30,000 feet would require 90 miles. Adjustments are then made to accommodate headwinds/tailwinds and anticipated speed restrictions. Destination weather and the expected approach/runway procedures are major considerations in planning the arrival. The primary source of this information is the ATIS previously described, although holding delays, weather conditions and runway operations may be passed along via ATC and/or dispatch. ATIS provides the current weather, instrument approach procedures in use, and active runways, as well as details concerning runway and taxiway closures, windshear reports, precise visibility values for individual runways, braking capability, bird activity, temporary obstructions (e.g. construction), land and hold short operations utilisation and any other relevant safety-related information. Once the crew has received the destination weather and approach information, they begin setting up the navigation equipment for the expected arrival procedure. Of primary concern are the current weather conditions vs. the available approach procedures. Low ceilings and visibility mandate specialized procedures which in turn require specific navigation equipment necessary for executing the approaches. If the current weather is below the minimums available for the procedure in use, or the necessary equipment is unavailable or inoperative, the crew must consider other options which include holding (if weather improvement is anticipated) or diverting to an alternate airport. Either course of action requires coordination with ATC and the airline’s dispatch office. After the crew has programmed the navigation systems and FMS for the anticipated procedure, the PF briefs the approach, using the published approach procedure as a reference. Some low visibility weather situations may mandate that the Captain always perform the PF duties to meet standardization requirements during critical low visibility operations. In addition to designating the PF, the approach briefing includes information about the required navaids, key segment and crossing altitudes, approach minimums vs. current weather, obstacles and terrain awareness, and the missed approach procedure. The cabin crew activities during the descent include preparing the cabin and galleys for landing, forwarding connecting gate information to the passengers, completing customs-related documents, forwarding any cabin-related discrepancies to the cockpit and verifying that seatbelt compliance requirements are satisfied. The Captain’s descent PA announcement usually includes updates of arrival estimates and weather conditions. Any anticipated adverse weather or delays are usually briefed to both cabin crew and passengers. As the aircraft descends below the transition level, the PNF works on completing the descent checklist which includes monitoring the pressurization, correcting any accumulated fuel imbalance, and calculating and/or reviewing landing data (approach speeds, runway limits). While passing through 10,000 feet, the Captain alerts the cabin crew (by chime or PA) that the sterile cockpit period is in effect and that the final cabin preparations for landing should be completed.