Sensitivity Analysis

A total of eight simulations were conducted. A Control run with no wind effect was followed by an Observed wind run, representing a full day of air traffic with the impact of the wind profile generated using the radiosonde data as described in 5.2.1. Another six runs were then performed to analyse the sensitivity of RAMS Plus to wind speed and direction changes, using the scenarios described above.

The fuel burn calculations apply only to sections of flight inside the simulation area (i.e.

the UK airspace). As discussed previously, the air traffic sample provided from NATS, contained detailed routing information only inside UK airspace. With the scenario defined in RAMS Plus in this way, only emissions within UK airspace could be calculated. Moreover, as mentioned in Chapter 4, due to the unavailability of detailed data on airport configurations and ground movements, RAMS Plus cannot be used to calculate emissions of the landing and take off cycle activities. Therefore, the only modelled wind effects are those related to enroute flight above 3000 feet within the UK airspace. More precisely, only the two-dimensional impact of the wind vector at altitude is modelled (i.e. the impact of head/tail winds and side winds). The effects that are not modelled are, for example, changes to runway choice due to wind direction, or changes to the flight trajectory to make best use of the wind (original flight plans are used).

In the RAMS Plus software the flight profiles are a great circle approximation. A true great circle path is possible only if the flight changes direction every instant. A great circle path is approximated with a direction change at every given time or distance. The smaller the time or distance, the more direction changes are made and thus the closer the approximated path is to a true great circle. However, a modelling trade-off exists between the number of direction changes and system performance when calculating the flight profile. The direction changes are dictated by two factors, the number of navaids and sectors in the flight plan and default simulation parameters (ISA, 2002).

Obviously, the number of navaids and sectors is a natural result of the fight plan and airspace definitions. On the other hand, the default simulation parameters exist to ensure great arc profiles for scenarios with flights that have long distances between navaids or sectors (i.e. 100 to 1000 nautical miles) (ISA, 2002). When computing flight profiles in RAMS Plus, flight points are inserted at a given time or distance interval to

approximate the great circle heading. This chosen value is important in large-area scenarios or traffic samples that may have large distances between navaids or sectors.

A reasonable value, suggested by ISA (2002) is around 5 minutes or 25 nautical miles.

Smaller values do not give much more accuracy, but slow down system performance.

Therefore, in order to ensure that RAMS Plus made sufficient calls to the ATMOS utility for long flight profile segments and simulated them correctly, the great circle approximation was set to 25 nautical miles. This ensured the consistent application of wind along the flight profile, taking into account bearing changes.

The first step in this sensitivity analysis was to compare changes in total fuel burn with wind field changes in different scenarios. Table 5-2 shows the total fuel burn values and percent changes with respect to the Control or “no wind” scenario. Results show that wind profile changes had a small impact on the total fuel burn of the traffic sample and that the largest change is for the Speed 100 scenario (i.e. doubling of wind speed compared to Met Office data). One possible reason for a limited net impact on the fuel burn changes may be due to tail winds cancelling out some additional fuel burn from head winds on return journeys.

Table 5-2. Total fuel burn by scenario

Scenario Fuel Burn (t) Change from Control (%) Change from Observed (%)

Control 1394.27 may not have a large effect on air traffic (unlike for example the jet stream). In addition, simulations were run with a single wind profile matching 12 GMT measurements when the lowest wind speeds are recorded. However, Figure 5-9 shows that the highest observed wind speeds are observed during low level traffic early in the morning and late during the day, suggesting that only a limited number of aircraft would have been actually exposed to these extremes. Therefore, application of the wind profile with temporal resolution and inclusion of a larger traffic sample would not largely influence the overall change in fuel burn.

Since the examination of overall fuel burn effects appears to be masked, analysis of individual flights was conducted. The maximum decrease recorded for one flight is

Chapter 5 Wind impact at altitude

approximately 12 percent (in Speed 100 scenario) while the largest increase is approximately 10 percent (Table 5-3). This indicates that although the average impact is small, some flights experienced a substantial change in fuel burn due to the wind field. Changes in wind direction showed a limited average impact on individual flight fuel burn. Flights that experienced any fuel burn alteration, due to wind direction changes, increased or decreased their fuel burn by an average of 1.0 and 0.75 percent respectively. For flights that experienced an increase in fuel burn, the magnitude of change for all three scenarios with wind direction modifications (i.e. Dir 30, Dir 90, and Dir 180) is almost the same, with somewhat smaller values found in the Dir 180 scenario of 0.66 percent. In addition, the wind speed modification scenarios result in increases or decreases of individual flight fuel burn almost linearly with respect to the magnitude of the wind speed increase.

Table 5-3. Fuel burn changes of individual flights (Pilot Study traffic sample) Change from Control Focussing on one flight, Figure 5-9 shows aircraft speed for the Control run (no wind) and Observed wind scenarios. There is an increase in the flight speed due to the introduction of the wind profile. In addition, Table 5-4 shows a comparison of fuel burn consumption for the same aircraft for its two legs of flight in these two scenarios. For example, the Observed scenario shows approximately a 2 percent decrease in fuel burn for one leg of the flight at flight level 310. The example shows that the only effect on fuel is from changes in the speed, so a reduced or increased time at a particular altitude/fuel burn rate will alter the fuel burn of an aircraft. In RAMS Plus fuel burn rates (kg per minute) are called from BADA input data, therefore the time in flight will affect total fuel burn. Figure 5-10 shows that the flight trajectory remains the same in both scenarios. Figure 5-11 shows the route for this flight, annotated with wind speed and direction for the two flight levels, therefore these figures show that the same wind speed and direction are applied to the legs of an each flight for the same altitude.

Therefore, the cause of the change in fuel burn is the increased (or decreased) journey time.

Figure 5-9. Example of flight's speed change (Control vs Observed scenarios)

Table 5-4. Example of fuel burn difference (Control vs Speed 100 scenarios)

Wind No wind Wind adjusted

Flight Level (ft) Flight bearing (o ) Wind speed (knots) Wind direction (o ) TAS (knots) Duration (min) Fuel (kg) Ground speed (knots) Duration (min) Fuel (kg) Change (%) 310 71.52 7 327 419 0:03:24 131.37 420.64 0:03:19 128.57 -2.13 330 71.52 14 260 419 0:01:48 96.21 408.64 0:01:48 95.82 -0.41

WS = 7kts WD = 327⁰

WS = 14kts WD = 260⁰

Chapter 5 Wind impact at altitude

Figure 5-10. Example of flight's trajectory (Control vs Observed scenarios)

Figure 5-11. Example of flight's altitude (Control vs Observed scenarios)