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Flight Phase 4: Descent and Approach
What used to happen?
Efficient management of the descent and landing phase of a flight is often the biggest challenge facing an air navigation service provider. Congested metropolitan areas, such as the San Francisco Bay, are typically served by multiple airports, handling hundreds of operations each day. Consequently, the need of the service provider to manage arrival and departure capacity often overshadows the goal of providing environmentally friendly and efficient descent profiles for arriving aircraft.
Typically, a flight arriving from Australia or New Zealand into the San Francisco Airport enters the Oakland Center high altitude radar airspace approximately 225 miles offshore of the airport. Upon confirmation of radar and radio contact, the flight is given an arrival clearance direct to the fix PIRAT then to the Woodside VOR (OSI), direct to the airport. Additionally, the controller will issue a descent and speed restriction for the aircraft to “cross PIRAT at eight thousand (ft) and two-five zero knots”. This arrival clearance was constructed to provide procedural points where arriving aircraft level off at a greatly reduced speed in order to ensure that descending aircraft cross underneath aircraft departing on offshore routes destined for Southern California, and to match speeds with other arriving aircraft for sequencing purposes. The aircraft would then be turned over to the Northern California Terminal Radar Control for the final arrival and approach phase of flight.
This requirement for low altitude level flight improves the ability of the center and approach control operations to sequence the aircraft for arrival, and simplifies the separation of the flight from conflicting departures. However, the aircraft produces significantly more emissions in low altitude level flight, than in a continuous, low power descent to the ground.
What are we doing today?
Today’s descent and approach into San Francisco will be optimized for efficiency using what is known as a Tailored Arrival or a TA for short. The TA into San Francisco is a sophisticated application of a type of emissions optimised arrival known as a Continuous Descent Arrival (CDA). CDA allows an airplane to fly a continuous descent path to land at an airport, rather than the traditional “step downs” or intermediate level flight operations. The airplane initiates descent from a high altitude in a near “idle” engine (low power) condition until reaching a stabilization point prior to touch down on the runway. CDA results not only in fuel savings and decreased emissions; it also significantly reduces noise beyond the airport.
The Tailored Arrival takes the principles of the CDA a step further by creating the most beneficial flight path available as a result of the integration of all known aircraft performance, air traffic, airspace, meteorological, obstacle clearance and environmental constraints expected to be encountered during an arrival.
Click here to learn more about Tailored Arrivals
The Tailored Arrival into San Francisco is part of an operational trial being conducted in partnership between the FAA, Boeing, NASA and the lead airline partner, United Airlines. Initial trials were conducted by these partners in 2006 and 2007, to prove the concept and begin to gather information and Air New Zealand joined the project shortly after regular Tailored Arrivals flights began in December 2007. Several other airline partners are currently using Tailored Arrivals including Air New Zealand, Japan Airlines, All Nippon Airlines.
The final approach to land and actual landing are performed with flaps and undercarriage extended. In this high drag configuration the fuel burnt per minute is very high. It is therefore desirable to delay the extension of flap and undercarriage to a point as late as possible in the approach while not compromising the airline's "stabilised approach" safety requirements. In addition, dependant of the runway length, wind and wetness of the runway, techniques such as using a reduced flap setting for landing and reverse idle during the landing roll provide further opportunities to reduce emissions.
The "delayed" extension of flap, undercarriage and the use of reverse idle are, dependent on runway length and weather conditions, standard operating procedures within Air NZ. For tonight's landing at San Francisco, weather conditions permitting, the extension of the undercarriage will occur at 1500ft with the final landing Flap 25 extended at 1000ft. (NOTE : Full landing flap on the B777 is Flap 30).
Why are we doing it?
Tailored Arrivals provide the potential to significantly decrease the fuel required for aircraft to conduct an arrival into an airport, while still maintaining or even increasing the airport’s capacity. These reductions in fuel burn equate directly to reduced emissions, making Tailored Arrivals both environmentally friendly and financially beneficial. In a time of increased environmental impact analysis, and of critical airline need to reduce their fuel costs, Tailored Arrivals provides a welcome operational efficiency for all concerned.
What does this mean in terms of the bigger picture?
On a Boeing 777-200ER arriving into San Francisco from New Zealand, the Tailored Arrival saves on average 200 USG gallons= 1912 kgs reduced CO2 emissions. The fuel saved from the use of the delayed flap procedure on a typical landing is a further 80 US gallons = 760 kgs reduced CO2 emissions.
The airlines participating in the San Francisco Tailored Arrivals are seeing fuel savings of between 500 and 2800 pounds per flight. Total fuel savings and emissions reductions for the airlines involved over the first 6 months of the San Francisco Tailored Arrivals activity are depicted in the following table, with “actual savings” referring to those accrued and “potential savings being the savings that could have been achieved if all flights in the period had conducted Tailored Arrivals.
Click to see how these emissions savings contribute to savings across the entire flight
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What does this mean in terms of the bigger picture?
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See how emissions savings in this flight phase contribute to savings across the entire flight
