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Flight Phase 1: Pre-flight, Taxi and Take Off
This phase involves the flight commencement, from when the aircraft is ready to 'start engines' through to take-off. The aircraft requires pushback from the gate and is towed into a position where it can safely start its engines, to taxi under its own power to the take-off point for the runway in use.
For most of today’s flight the crew will communicate with Air Traffic Control via data link. Prior to the flight, the flight plan track, enroute winds and temperatures will be uplinked to the aircraft from the Airline Operations Centre. Prior to engine start ATC will provide a start clearance and pre departure clearance via data link.
Click here to learn more about airline preparation prior to flight
During the flight any deviations from the planned track, clearance to climb to the next flight level and clearance for the arrival will be conducted by data link. The only voice communication with Air Traffic Control will be the clearance for takeoff, during approach and the clearance to land.
What used to happen?
At times, aircraft pushback was delayed due to other aircraft being on the airport apron, or taxiing was interrupted due to other aircraft movements - each resulting in unnecessary emissions once the aircraft's engines were started. There were also delays experienced prior to take off as the aircraft waited for other aircraft to arrive or depart ahead of it.
Prior to engine start it is normal practise to start the aircraft Auxiliary Power Unit (APU) in the tail of the aircraft. This unit will provide aircraft power and air conditioning until the aircraft engines have been started. Minimising the use of the aircraft’s APU is a significant opportunity for airlines to reduce emissions.
What are we doing today?
Today’s flight will use air conditioning and electrical power supplied from the Auckland Airport aerobridge for the aircraft turnaround. The APU will be started approximately 15 minutes prior to departure to provide compressed air for the start of the aircraft main engines. At a typical electrical and pneumatic load, the Boeing 777 APU will consume 60 US gallons per hour. On a B747 the consumption averages 110 US gallons per hour.
Click here to learn more about emission savings from reduced use of the APU
‘Just in time’ fuelling is a concept used by a small number of airlines that includes Air New Zealand. It involves the tanker loading 2000kg less fuel than the initial estimate and then remaining connected to the aircraft until the captain is able to “fine tune” the fuel load requirements at 20 minutes prior to departure. The final amount of fuel is then carefully loaded to minimise an “over-fuel” with the final calculations to confirm the correct fuel load occurring 5 minutes prior to departure.
Click here to learn more about ‘just in time’ fuelling
Air traffic controllers now manage aircraft engine starts to minimise aircraft holding prior to take off. Controllers and pilots also endeavour to manage the aircraft taxi speed to arrive at the take off clearance point without having to slow down and speed up, in order to minimise the fuel burn involved in powering up to accelerate the aircraft.
Aspire 1 will receive a priority clearance from air traffic control in Auckland Control Tower, for taxiing and departure. Air traffic control will provide the pilot with the optimum engine start up time following pushback, to achieve an unimpeded taxi through to the departure point for the runway in use.
Aircraft engines are designed and optimised to minimise fuel burn at power settings consistent with an aircraft in cruise. On the ground at close to sea level and very low power settings the engines are at their worst in terms of efficiency. From a fuel conservation perspective it is more efficient to taxi a twin engine aircraft on one engine.
Operational considerations that need to be addressed in crew operating procedures are the requirement for the engines to have operated for 5 minutes prior to takeoff (warm up to prevent thermal shock), the implications of a far higher jet blast behind the aircraft when a single engine taxi commences (e.g. ground equipment, apron staff behind aircraft), and the high level of break away thrust required at heavy aircraft weights. For today’s flight, both engines will be used for taxi because of the aircraft weight.
The prime consideration for the takeoff phase of a flight is safety. All takeoffs involve complex calculations to determine the power settings, decisions and rotate speeds that ensure that at all times in the event of a critical failure (e.g. loss of an engine) the aircraft will be able to stop on the remaining runway or once a decision speed known as V1 is achieved, continue the takeoff with a failed engine.
Most takeoffs today involve reducing power setting up to 25% less than full power. This has made a significant contribution to improved reliability of engines and the average engine time on wing increasing from 20,000 hours to numbers in excess of 30,000 hours. The application of reduced power for takeoff does have a minor adverse impact on the total fuel consumed during the takeoff and initial climb.
Why are we doing it?
Important environmental improvements on the airport surface can be realised by minimising aircraft ground run time. When demand exceeds available capacity, significant run time can be spent by aircraft sitting in queues at the runway hold-short line while waiting take off clearance.
By moving any anticipated queuing back to the gate, aircraft can be delivered to the hold-short line at the proper time and sequenced for take off with minimum ground run time.
Optimal runway usage planning will increasingly play a more significant role in reducing aircraft emissions as promising new technologies which utilise collaborative surface operations in maximising airport throughput become available. For the moment, more efficient surface operations for even just a few flights during busy hours can provide significant delay reductions, as well as emissions reduction benefits. Estimated fuel saved on this flight from reduced use of the APU is 60 US gallons, and through ‘just in time’ fuelling, the average Boeing 777 fuel load has dropped by 170 US gallons. Even more significantly, the fuel burn saved by not having to carry this extra fuel is 68 US gallons.
What does this mean in terms of the bigger picture?
The challenge in providing an aircraft with an unimpeded taxi and take off is in the effect that this can have on other aircraft in the vicinity, both on the ground and in the air. The role of air traffic control is to balance these conflicting objectives to ensure that, all things being equal, all aircraft on and around the airport are provided with optimum vectoring so that they all receive some benefit, while none are unfairly penalised. Air traffic control must also balance the airport and airline requirements for utilising the runway to its fullest capacity while maintaining 'on time performance' for each flight.
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?
Useful Links for this Flight Phase
Airline preparation prior to flight
Emission savings from reduced use of the APU
See how emissions savings in this flight phase contribute to savings across the entire flight
