8.2.2 Extra distance and Taxi time
From a theoretical standpoint there is an optimal route minimizing the emissions from the point where the aircrafts takes off to where it lands. Unless the wind gives another opportunity to route the flight, this distance equals the GCD between destinations. Inefficiencies in the Air Traffic Management (ATM) system and flight operations lead to a considerable amount of unnecessary fuel burn and emissions since all flights are longer than the GCD.
The pilot has to follow his flight plan and pass certain predefined navigational points which usually not coincide with the great circle. In the older version ag NTM-calc there was a constant distance added to the GCD which was proportional to the distance itself. Looking at EUROCONTROLS statistics, it is possible to quantify this in the European context. The average extension I Europe is 5,6%, this figure does not include the LTO but includes the deviations made to reach the exit and entry points of the Terminal Manovering Areas approximately 100 km from the airports.
Figure 1. Mean En-route extension for air traffic in EUROCONTROL. Note that this not include flights within the TMA (from EUROCONTROL PRR 2009).
When departing and arriving at airports aircrafts are sequenced into the “flow pattern” around the airport. The size and the capacity of the airport determine how much extra lateral flying those patterns adds to the total distance. This varies a lot with time of the day and also with current weather situation. In the statistics from EUROCONTROL there are figures on the arriving traffic. The extra lateral flying is expressed in the additional time used for approach, where a “normal” approach according to the published routes is used as norm. The European average is 2,1 minutes additional time per approach and flight. Asuming an average speed of 300 kt this corresponds roughly to 20 km. London (Heathrow) sticks out as the worst European airport with a additional time close to 10 minutes per approach and flight.
On ground the amount of taxi is another unknown factor in each case. NTM-calc uses a standard value for taxi which as a mean is a good approximation. But for large/small airports the calculations yields to little/to much emissions. The statistics for arriving and departing traffic on the 20 biggest airports in Europe is presented in the figure below. It includes the above described additional time airborne approaching the airport and the taxi-phase for departing traffic.
All commercial flights file a flight plan describing the flight. Fuel onboard in pounds or kilograms as well as in hours and minutes of flight time is one of the compulsory blocks in a flight plan. Reserve Fuel is included in this block.
How much reserve fuel the aircraft carries varies within different organisations and airline companies. Forecasted weather at the destination might be close to the limit for landing so then there has to be enough reserve fuel on-board to reach a designated alternate airport (also defined in the flight plan) and perform a landing there. Extra fuel to perform holding are usually carried when going to larger airports. Long flights over open water are another case which usually stipulates extra fuel to be carried.
The reserve fuel adds to the total take-off weight and hence it influences the emissions.
A complete analysis of reserve fuel has not been able to do with the funding limits in this project. Thus the decision became to “forget” the fuel reserves, or in other words assuming the reserves to be consumed when the range has been reached.
Jane’s is used as a data source. The data for “Range Adjustment”, can be found there. Users of the NTM database should be aware that by using the X (constant) and Y (derivative) for the emissions, the following is implicit:
- For a very short distance, the takeoff mass includes very little cruise fuel (and proportionally small fuel reserve, implicitly). The results are almost according to Piano, for the parts below and above 3000 feet, respectively.
- For a very long distance, the takeoff mass nearly maximum cruise fuel (and proportionally large amount of fuel reserve, implicitly). The results for the climb are according to Piano (without the fuel adjustment), and for the cruise are according to Piano and adjusted by the fuel factor.
To show the error level by “forgetting” to remove the reserve fuel in our estimation of fuel consumptions for the NTM cases, Table 9 is presentenced. Here the tanked and reserve fuel amounts for a number of representative aircraft have taken directly from maximum range runs with Piano, with maximum passengers in typical layouts. The chosen aircraft are in different engine and size categories. Reserve policies may be different for the examples.
|Tanked fuel [kg]
|Reserve fuel [kg]
Table 4.4. Tanked and reserve fuel for different aircraft (max. range runs with Piano)
There is a very clear correlation between the aircraft size (roughly proportional to the tanked fuel amount) and the reserve percentage. With our treatment of the NTM cases, it is still possible to compare results for different, similarly sized aircraft with each another.