The aviation industry consumes a relatively small amount of the world’s fossil fuels. It has a solid record of reducing its consumption and is driven to do so by the large economic impact of fuel costs on the industry. Fuel consumption has been reduced by constant change and improvement in engine and airframe technology, materials, operations planning, aircraft operation, and maintenance practices.
There are incremental gains to be realized in aircraft technology and Air Traffic control technology and procedures. But the predicted rate of industry growth through 2020 will exceed these gains, causing an increase in the industry’s overall consumption of fossil fuels. And there do not appear to be any new fuels on the planning horizon.
Several promising areas for breakthrough aircraft technologies have been identified, but all of them are very challenging. Similarly, major gains in Air Traffic Control efficiencies will not be easy to implement.
This article is a broad overview of fuel consumption in the aviation industry. It covers a range of topics, each of which could be expanded considerably. It is intended as an introductory reference for engineers, students, policymakers, and the general public.
Commercial aviation burns a relatively small 2%-3% of the world’s fossil fuels. Military and general aviation accounts for a small and declining proportion of that amount. Since fuel makes up about 12%—17% of an airline’s operating costs, the industry has clear economic incentives to reduce consumption. Strong competition in the airline business and its supplier industries has made such progress rapid and effective.
Aviation fuels are comprised mostly of kerosene, which is produced through the distillation of crude oil. Jet turbine fuels account for around 6% of refinery production worldwide. The biggest contribution to the reduction of fuel consumption has been the development of aircraft propulsion from piston-driven propellers to turbofans that use very exotic materials. Air frames have evolved from wood and canvas to aluminium, titanium, and carbon-fiber composites, with significant reduction in weight and an increase in strength.
Once aircraft are in service, operators maintain and operate them effectively in a variety of ways. These include drag reduction programs, flight planning systems, pilot techniques, advanced on-board flight control systems, maintenance, and trend analysis programs.
In the future, incremental gains in fuel efficiency will continue as weight reductions and engine efficiency gains continue, along with the utilization of more sophisticated control systems and manufacturing processes.
Air Traffic Control systems and procedures can be improved, leading to fuel efficiencies through reduced trip distances, and less holding or local maneuvering.
The industry, as a whole, illustrates how far fuel conservation can be taken.
TRENDS IN CONSUMPTION
In 2003, the Transportation sector accounted for 27% of the world’s energy consumption, using most of its share as common gasoline. Aircraft account for between 2 and 3% of the fossil fuel burned world-wide, and 6% of petroleum consumption.1-2-1
The vast majority of aircraft fuel is consumed by the world’s 18,000 or so commercial jet aircraft. Fuel consumption by military aircraft is estimated to have dropped from 36% in 1976 to 18% in 1992, and is projected to drop to 7% in 2015 and 3% in 2050.
Most industry associations and observers1-4’5-1 predict a continued growth rate in flight and passenger volumes, averaging at 5% per year for the industry.
Consumption per ATK
The industry has been able to lower its average fuel consumption dramatically in the last 40 years, even as the level of flight activity has soared (Fig. 1).[1,6]
The Available Ton-Kilometer (ATK) is a measure of production capacity for both scheduled and unscheduled passenger and freight aviation. An ATK is defined as the number of tons of available aircraft capacity multiplied by the number of kilometers these tons are flown. This measure isolates fuel efficiency discussions of technology and infrastructure from more complex discussions concerning fuel usage per passenger-kilometer, which is more of a market-driven measure. Airlines have several nontechnological paths to pursue in obtaining the most revenue for their fuel dollar. These include keeping aircraft as full as possible, matching aircraft type and schedule to routes and demand, and, thus, spreading fuel costs to cover more passengers and generate greater revenue.
ECONOMIC IMPACT OF ENERGY COSTS ON COMMERCIAL AVIATION
Impetus Towards Conservation
Like many other industries, the energy in aircraft fuel is crucial to airline operations. But fuel is a large percentage (12%-17%) of airline operating costs, usually second only to wages. The percentage varies with the type of carrier and its route structure. Fuel cost varies by as much as 30% between different airports due to transportation costs from refinery to airport, local supplier cost structures, volume discounts, and government tariffs or price support policies.
Given the highly competitive nature of the business, and its high-cost, low-margin characteristics, there is strong reason to pursue fuel efficiency. And the industry has been diligent and successful in its conservation efforts.
In addition to reducing consumption, airlines pursue several strategies to reduce fuel cost. These include fuel tankering, or carrying excess fuel from a low-cost airport to a high-cost one; local supplier negotiations; hedging; and so forth. These strategies extend beyond the scope of this article, but simply reducing consumption still constitutes the best long-term strategy for dealing with fuel costs.
PRODUCTION OF AVIATION FUEL
Crude oil delivered to a refinery is converted into upward of 2000 products, but the most profitable and high-volume products are gasoline, jet fuels, and diesel fuel. Naphtha jet fuels have been used in the military, but phased out in favor of kerosene-based fuels. The major jet fuel types are Jet A, Jet A-1, JP-5, and JP-8.
In terms of overall refinery production, jet fuel accounts for around 6% of output by volume. Only a fairly small number of refineries produce jet fuel.
Sans breakthroughs, kerosene-based fuels seem to be inescapable for the industry. aircraft, drag comes from several sources, but the largest are: induced drag, a by-product of lift, and parasitic drag, which is caused by the air friction and turbulence over the exterior surfaces of the aircraft, as the aircraft moves air out of its way, and by antennae, landing gear, and so on.
Fig. 1 Aviation fuel consumption per available ton-kilometer (ATK).
Induced drag has a strong relationship to weight: less weight means less lift is required. Induced drag also depends on the design of the wing and its airfoil (wing cross-section) and the angle of attack of the wing. Generally, this drag increases with the square of lift, and the square of aircraft weight.
Any reduction in aircraft weight will directly result in reduced fuel consumption. Reduction in an engine’s fuel consumption means that less fuel is required for a given aircraft and payload, hence there is a cumulative effect on its overall efficiency.
Aircraft manufacturers have continuously used a variety of means to reduce aircraft weight. The materials used for aircraft structures have changed from wood and canvas to plywood through various aluminium alloys and, since the 1970s, have included carbon-fiber composites used for simple panels and complex and critical components, such as engine fan blades.
In the Boeing 787 aircraft scheduled for first flight in mid-2007, carbon-fiber resin composites will make up approximately 50% of its structural weight, compared to 12% for the Boeing 777. The planned composite components extend far beyond the usual and into the wings and entire fuselage sections. Aluminium, titanium, and steel constitute the remainder of the structural weight. Airbus Industrie is more conservative, with about 25% composites in the airframe of the A380, which is scheduled to enter service in late 2007.
In the U.S., National Aeronautics and Space Administration (NASA’s) Advanced High Temperature Engine Materials Technology Program and the National Integrated High Performance Turbine Engine Technology (IHPTET) Program have investigated and promoted the use of polymer-matrix composites, metal-matrix/intermetallic-matrix, and ceramic-matrix composites for high-temperature parts of aircraft engines. These materials could allow the construction of higher-temperature engines with greater combustion efficiency, all at significantly lower weights. An example is the F136 military engine, which uses a titanium matrix composite in its compressor rotors.
Fig. 2 Specific fuel consumption (SFC) trend chart.
Airlines also work to manage and reduce weight throughout the aircraft and its operation. Excess weight can build up from moisture, dirt, and rubbish in the aircraft, unnecessary supplies, and excess passenger equipment. Boeing estimates that an aircraft will increase in weight by about 0.1%-0.2% per year, leveling off at about 1% in five to ten years. A 1% reduction in weight results in a 0.75 to one percent reduction in trip fuel, depending on the engine type.
Drag Reduction Programs
Drag increases required thrust, so aerodynamic cleanliness is an ongoing challenge.[13,12] Dirt and oil, skin roughness, dents, misaligned fairings, incorrect control rigging, deteriorating seals, mismatched surfaces, and joint gaps (e.g., doors and access panels) all contribute to drag and increased fuel consumption. The most sensitive areas of the aircraft are those where local flow velocities are high and boundary layers are thin: the nose area, the wing leading edges and upper surface, the elevator and rudder leading edges, engine nacelles, and support pylons. If not maintained, a modern transport aircraft can expect a two percent increase in drag within a few years as a result of these factors.
Aircraft Pre-Flight Planning
Before every flight, pilots and operations staff make decisions that affect the overall fuel consumption of each aircraft. Based on knowledge of the aircraft, schedule, payload, and weather, they prepare a load plan and a flight plan. The variables in these plans, and decisions made around them, have a major effect on fuel consumption for the flight.
Center of Gravity (C of G)
Operations dispatchers plan the fuel and cargo load to place the center of gravity within the correct range for safe operation. However, if possible, placing the C of G in the aft portion of this range will result in reduced fuel consumption. This is because when C of G is aft, there is less elevator control surface negative lift required to maintain the correct cruise attitude. This means less lift is required from the wings, resulting in less induced drag. Less negative lift from the tail plane also means less induced drag from this area.
Extra fuel, while comforting to passengers and crew, requires extra fuel burn due to the weight of this extra fuel. A better strategy is to accurately plan the flight to carry the correct amount of fuel and reserves. Elements of this strategy are to:
• Determine the accurate payload and use aircraft weight by tail number if possible.
• Plan the fuel load as required for safety and regulatory requirements, with optimum choice of an alternate airport, careful consideration of the rules that apply to the flight, depending on its origin and destination, and minimal “discretionary” fuel requests.
• If possible, use the re-dispatch technique to minimize contingency fuel requirements.
• Provide accurate, optimized flight planning using the latest origin, destination, and en-route weather information and planning techniques. This involves: choosing a great circle route to reduce distance traveled, if possible; flying pressure patterns and maximizing the use of prevailing wind to reduce enroute flying time; selecting cruise speeds that are, again, an optimum compromise between fuel consumption and schedule performance considerations; and using step climb techniques as required to move to newer altitudes as aircraft weight decreases during the flight.
Pilot Operations Techniques
Pilots can make incremental reductions to fuel consumption through a variety of techniques. They can delay starting the engine until the last minute, after Air Traffic Control (ATC) has issued departure clearances, so that such delays occur at the gate with the engines off. Pilots can minimize the use of the on-board Auxiliary Power Unit, a small turbine that supplies electrical power and compressed air at a higher fuel cost compared to ground power units. Where permitted, the aircraft can also taxi on one engine. Ground operations thrust and braking can then be minimized.
Moreover, pilots can utilize the appropriate flap settings, and retract them as soon as possible to reduce drag. They can follow minimum cost climb profiles whenever possible, but may be thwarted by noise restrictions and ATC congestion problems.
In flight, good control surface trim techniques can save as much as 0.5% in fuel burn by minimizing drag. The appropriate management of air conditioning packs can reduce fuel burn by 0.5%-1.0%. Pilots can use cargo heat and anti-icing judiciously.
There is an optimum point to begin descent into the destination airport. If the plane descends too early, fuel is wasted due to higher consumption while cruising at lower altitudes; if the plane descends too late, the descent speed is too high and energy is wasted. Pilots can delay lowering flaps and landing gear until the last minute: fuel consumption in this high-drag configuration is up to 150% of that in a “cleaner” configuration.
In all of these techniques, safety is the overriding concern. Pilots will always choose a conservative and safe option over a more economical one.
Flight Controls (Autopilot, FMC, W&B)
Modern transport aircraft have significant on-board flight control and management systems that can be used to reduce fuel consumption.
Some Airbus aircraft, for example, have a Fuel Control and Management Computer (FCMC) that can determine the C of G of the aircraft and continuously adjust it toward an optimum position for different flight regimes by pumping fuel to and from an aft-located “trim tank.”
Airbus also has a Flight Management Computer (FMC) that can plan step climbs. It also can show the pilots their current optimum altitudes and cruise speeds, in addition to the current actuals, taking into account upper wind forecasts for the flight’s planned route. The FMC calculates the optimum top of descent point. When in “managed mode,” the FMC uses a “cost index” to account for the carrier’s preferences between fuel costs, other direct operating costs, and time savings when calculating cruise altitude and speed.
Control of Engine Maintenance
Boeing recommends several procedures to maintain economical engine operation. These are on-wing washing, which reduces dirt buildup, and bleed air rigging, which compensates for leaks due to system wear. Bleed air is taken from the engine’s core and used for a variety of purposes where heated compressed air is needed, such as cabin pressurization and wing de-icing.
Regulatory agencies have mandated significant amounts of on-board data gathering for safety and accident investigation purposes. The industry has found ways to lever this data to provide information about engine health and performance. There are two methods used:
1. Post flight: flight data, gathered manually or electronically, can be loaded into various computer programs after the flight’s completion, often on a sampling basis.
2. In-flight: using online data link networks, airlines can downlink in-flight data from the aircraft, among many other types of routine operational reports. ARINC Incorporated of Annapolis, Maryland (GlobalLink) and SITA of Geneva, Switzerland (Aircom) provide Aircraft Communications Addressing and Reporting System (ACARS) services through a world-wide network of satellites,
Very High Frequency (VHF), and High Frequency (HF) ground stations used by airlines and business aircraft operators. Satellite services use four Inmarsat-3 satellites and constitute a global resource for appropriately-equipped aircraft, with the exception of polar regions.
The data are analyzed to determine overall fuel consumption and provide feedback on the success of flight planning and deterioration, if any, of the fuel efficiency of each engine.
This data usually provides the basis for engine trend monitoring, where parameters of interest are compared over time. The onboard computers can also capture short-term “limit exceedance” events, which are gathered on an exception basis.
Airlines, small and large, use in-house software, or software and services provided by many different companies, such as General Electric Aircraft Engines, to perform trend analysis on their engines. This software will predict and characterize trends based on the data provided, including analysis of the combustion efficiency and internal thermodynamics of the hot core sections of the engine. For example, a drop-off in fuel efficiency is probably a sign of wear problems. When certain thresholds of fuel flow, temperature, and so forth are met, the software provides alerts to maintenance staff. In rare cases, an engine may be scheduled for early removal and overhaul. For safety and economic reasons, this is in the operator’s best interests. Economic factors include both fuel efficiency and reduced maintenance costs derived as a result of early problem rectification.
THE FUTURE OF GLOBAL AIRCRAFT FUEL CONSUMPTION
Fuel efficiency gains are forecast to be about 2% per year for the foreseeable future. This includes gains from engines, airframes, and operational procedures.
Given a projected airline industry growth rate of about 5% per year, overall industry fuel consumption will continue to rise. If the industry continues to depend upon fossil fuels, it will become more and more expensive and may finally reach a downturn in growth as flights cease to be affordable for tourism and related discretionary travel.
While incremental gains in existing technology are still available, major future gains will depend on breakthrough thinking in airframe design or related technologies.
Aircraft designers will continue to reduce aircraft weight through new metals and composites and incremental reduction in the weight of on-board equipment. Active pitch stability features built into fly-by-wire, computer-assisted flight controls (autopilots) could provide a one to three percent reduction in overall fuel efficiency.1-8-1 The continued incorporation of wing-tip devices (“winglets”) will reduce induced drag, as will better manufacturing processes, which will smooth exterior surfaces.
Active systems used to increase laminar flow over the fuselage and wings are very attractive ways to decrease drag, but are fraught with technical challenges.
Fundamentally new designs, such as a blended-wing body, face different challenges, mostly in the realm of passenger acceptance. Similarly, shape-changing wings (morphing-capable) would allow an aircraft to use the most efficient wing size and shape for various flight stages. Coupled with support computers, this could also allow ailerons, rudders, and elevators to be eliminated.
There are potential breakthroughs in materials. Nano-technology promises to provide materials that are much different and potentially feature orders of magnitude increase in strength-to-weight ratios.
ROLE OF GOVERNMENT
While aircraft technology has been fertile ground for fuel conservation efforts, there are similar efforts underway in other areas. Air Traffic Control is a service that is either regulated by or provided by governments. As such, governments have a large role to play in reducing aircraft fuel consumption.
Clearly, overall trip fuel consumption depends on the length of the flight. If distance traveled and flying time due to holding or local maneuvering can be reduced, optimized, and streamlined, fuel consumption will be reduced. Industry estimates categorize this savings in the six to 12% range over a twenty-year period.
The industry has begun to use Global Positioning Satellites (GPS) to provide optimum point-to-point navigation capabilities. This is in contrast to classic airway navigation, which rarely offers direct or great circle routing, but rather a series of “legs” between fixed-position, ground-based radio navigation stations. When supplemented by a Wide Area Augmentation System (WAAS) and a Local-Area Augmentation System (LAAS), GPS-equipped aircraft can operate in instrument flight conditions for enroute navigation right down to so-called “nonprecision” approaches to the runway. This can reduce distance traveled, fundamentally reducing fuel consumption.
The industry has also continued to move to more advanced ATC systems and procedures. These are intended to streamline airport departure, enroute, and arrival procedures and timing to reduce waste and fuel consumption.
In 1995, the industry began trials of Future Air Navigation System 1 (FANS1) equipment and procedures. This equipment delivers routine ATC information to and from the cockpit via data link, and reduces the use of voice communications, which is a critical bottleneck for air traffic controllers. The industry is moving toward improving arrival and departure sequencing and enroute spacing and increasing flexibility for airline-preferred routing.
Eventually, the industry would like to see a single integrated global air traffic management system to safely optimize the use of scarce airspace (particularly near busy airports).
The aviation industry uses a small percentage of the world’s energy, but cannot survive in any form without it. The cost of energy in the form of fuel comprises a large percentage of industry operating costs.
In response, the industry has developed considerable expertise and sophistication in monitoring, controlling, and reducing its energy consumption on a per-unit basis. As such, this response serves as an example of how far one can go in pursuit of conservation.