Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Subsequently, a number of fuel-conscious airlines developed the concept of a long-range cruise (LRC) speed schedule, usually based on Mach number. LRC was
introduced as a compromise between maximum speed and the speed that provides the highest mileage in terms of km per kg of fuel burned in cruise (maximum range
cruise, or MRC speed), taking some account of costs associated with flight time. LRC is defined as the fastest speed at which cruise fuel mileage is 99% of fuel
mileage at MRC. At the time LRC was introduced, it was not possible to fly at lower speeds, closer to MRC, because of the stability needs of the autothrottle and/or the
autopilot. At speeds close to MRC, the autothrottle would continuously "hunt" which could give rise to an increase in fuel burn.
In the mid-1970s, fuel conservation was further enhanced by development of the performance management system (PMS) for aircraft such as the Boeing 737 and 747
and the McDonnell Douglas DC-10. Later aircraft (such as most Airbus aircraft; the Boeing 757, 767, 747-400, and 777; and the McDonnell Douglas MD-11) included
the flight management system (FMS) as a built-in feature. PMS or FMS computer systems can be used to minimize overall trip cost, which is a balance between fuel-
and time-related costs. The most efficient cruise speed (ECON) can be calculated on a real-time basis by using the cost index facility on the system. With full-time
autothrottle, late-model aircraft can fly between MRC and LRC to optimize fuel savings.
Figure 8-1 shows the relationship between the difference in block time and the difference in fuel consumption for various cruise speed schedules-such as constant
Mach number, LRC, MRC, or ECON-for the Boeing 747-400 (block time is the time between engine start at the airport of origin and engine stop at the airport of
destination; block fuel is the fuel burned in this time). Data in Figure 8-1 are consistent with those in Fransen and Peper (1993). The data presented in Figure 8-1
suggest that reduction of fuel use by further speed optimization is likely to be small.
Bradshaw (1994) investigated the variation in block fuel and block time for Airbus aircraft. The report shows higher figures (up to 10%) for the potential reduction in fuel
burn from reduction of cruise speed. However, the report also concludes that reduction of Mach number will involve penalties such as significant increase in block time
and the subsequent effect on direct operating costs.
8.3.1.3. Aircraft Weight Reduction
A negative effect on the mobility efficiency of aircraft can be caused by additional non-essential weight. Additional weight is introduced if an aircraft takes more fuel
onboard than that required by the fuel flight plan. Tankering is the term for loading of fuel used for subsequent flight segments. The main reasons for tankering of fuel
are commercial-for example, in cases where the cost of fuel consumed in carrying additional fuel is more than offset by the difference in the price of fuel at the
departure point and a destination where fuel could be loaded. Factors that can affect fuel costs and decisions on tankering include the following:
. Genuine high fuel costs because of expensive distribution infrastructure and local taxes
. Fuel availability at some remote airports
. Government-imposed fuel pricing (at some Eastern European airports, the price of aviation fuel is more than 50% more than at Western European airports)
. Monopoly distribution of fuel, which can involve cross-subsidies from large to small airports and expensive manpower practices
. Concern over fuel quality (e.g., water content) at particular locations
. Slot availability (where limited aircraft turnaround time allows insufficient time for refueling, an aircraft may have to tanker to minimize the risk of losing slots; problems
in this area are exacerbated at congested airports, where there may be limitations in runway and/or terminal capacity).
Estimates from British Airways suggest that additional fuel burn as a result of tankering is on the order of 0.5% of total aircraft fuel consumption.
Apart from tankering, all commercial flights must carry a certain amount of additional fuel, often mandated by national legislation, for safety reasons. The minimum
amount of fuel required for the planned route is calculated by taking into consideration the weather forecast, the route, the weight of the aircraft, and other factors
(including an allowance for diversion to secondary destinations). In most cases, this calculation is carried out with a computerized database. The pilot is presented with
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