Airlines in the US have been flying slower to cut fuel bills
Higher oil prices have made US airlines work to control costs. Between 2002 and 2012, the price of jet fuel quadrupled and fuel bills rose from 15% to more than 40% of the operating costs of US airlines, and their single largest operating expense. Airlines have made many efficiencies to cut fuel consumption, including now flying more slowly. Most of the fuel economies which have been implemented in the last decade will not be undone, even if oil prices were to fall (partly due to the possible future costs of CO2 emissions). There is an optimal cruising speed for each aircraft based on altitude. Flying faster increases the amount of fuel burnt. Historically, commercial aircraft have flown on average about 8% faster than their optimal cruising speed. Getting the aircraft to its destination quicker to pick up another load of passengers and minimise crew cost was worth the extra fuel expense. There is a trade-off between fuel consumption and time. But between 2004 and 2011, the average ground speed of seven major US airlines fell by 1.1%. More than anything else, however, airlines have focused on reducing excess weight.
The accompanying report: http://www.gao.gov/assets/670/666128.pdf
Airlines fly slower to cut fuel bills
By John Kemp
Between 2002 and 2012, the price of jet fuel quadrupled from 70 cents per gallon to over $3. Fuel bills rose from 15 percent to more than 40 percent of the total operating costs of U.S. airlines to become their single largest operating expense.
The results have been impressive. After peaking in 2005, jet fuel consumption in the United States has fallen by almost 15 percent, the equivalent of more than 200,000 barrels per day, according to the U.S. Energy Information Administration (EIA).
U.S. airlines’ fuel saving programme is just one example of how higher oil prices over the last decade have transformed transportation, and led to demand destruction which is likely to prove permanent. Most of the fuel economies which have been implemented in the last decade will not be undone, even if oil prices fall.
“There is a strong correlation between airline mission fuel efficiency and fuel price,” the National Center of Excellence for Aviation Operations Research wrote in a recent report (“The impact of oil prices on the air transportation industry” March 2014).
“There is ample evidence that airlines adopted new operational strategies to reduce total fuel burn for the same amount of traffic,” the centre concluded.
Some of the changes have been obvious. U.S. airlines have restrained growth in capacity and increased seat occupancy.
U.S. airlines measure capacity in available seat-miles while utilisation is measured in revenue passenger-miles.
Between 2007 and 2013, the number of available seat miles flown in the United States was cut by around 34 billion (3.25 percent) while revenue passenger-miles rose by 6 billion (0.8 percent).
The result is that seat occupancy, which the airlines call “load factor”, has risen from around 76 percent in 2004 to almost 83 percent in 2013, according to the U.S. Department of Transportation.
While airlines have mostly maintained capacity on major trunk routes, shorter and less profitable ones with lower load factors have seen the number of seats cut or been eliminated altogether.
Carriers have also shrunk the amount of space between seats to increase the number of passengers on each flight and saved more space and weight on the aircraft by installing thinner seats.
Other changes have been much less visible. One of the biggest fuel savings has come from flying aircraft more slowly.
From the perspective of fuel consumption, there is an optimal cruising speed for each aircraft based on altitude. Flying faster increases the amount of fuel burnt.
Historically, commercial aircraft have flown on average about 8 percent faster than their optimal cruising speed. Getting the aircraft to its destination quicker to pick up another load of passengers and minimise crew cost was worth the extra fuel expense.
The trade-off between fuel consumption and time is captured in the airline cost index and implemented in the carrier’s flight management system.
But between 2004 and 2011, the average ground speed of seven major U.S. airlines decreased by 1.1 percent, resulting in an even bigger reduction in fuel consumption, according to the centre for operations research.
Airlines have been pushing for other changes in crew behaviour and operations. Several airlines told the operations researchers they had instructed pilots to use only one engine while taxiing around the airport in order to save fuel.
Most airlines are also trying to maximise the use of ground power for aircraft instruments, heating, cooling and starting turbine engines when the aircraft is on stand rather than using the aircraft’s own auxiliary power units (which consume jet fuel).
One airline has stipulated ground power must be plugged in within 1 minute of the plane arriving at the gate.
More than anything else, however, airlines have focused on reducing excess weight.
In most cases, airlines found aircraft were carrying more water than was actually consumed on the journey. By modelling consumption by the number of passengers and the length of the flight airlines have been able to cut the amount of water loaded on board.
The number of magazines carried has been reduced, and those that are must “pay their way”. Airlines have removed onboard ovens from flights that didn’t need heated food. Safety equipment for a water landing has been removed from aircraft which do not fly over water.
One airline told the researchers that its weight reduction programme had cut the weight of a typical Boeing 777 by 700 pounds.
For some fleets, average weights have actually been cut by as much as 10-15 percent, according to the operations research centre.
USING BIG DATA
One of the most attractive targets for weight reduction is the amount of fuel carried on board. Aircraft must carry contingency fuel to deal with delays, storms or diversions but the reserves add significantly to aircraft weight.
Most airlines are now trying to trim the amount of contingency fuel by using modelling to estimate how much extra fuel must be carried to ensure safe operation of the aircraft based on weather conditions and the availability of alternative airports in case the flight must be diverted.
In fact, big data and computer modelling are revolutionising most aspects of aircraft operation, but changing behaviour is not always easy.
There is often a tension between trusting decisions about contingency fuel, water and flying speed up to the professional judgement of the pilots and allowing them to be determined by a computer model. In many cases pilot contracts limit the operational data which gets reported back to the airline and the ways in which it can be used.
“Two airlines noted the difficulty of enforcing the single engine taxi policy,” the operations researchers explained. “The reason for this is because pilot contracts with airlines often limit access to pilot specific performance data, which includes specific reverse thrust settings.”
Cutting fuel reserves has been a particular source of contention. “For pilots, fuel is like insurance, they take extra fuel to deal with uncertainties in flight. They more fuel the less they care if uncertainties like traffic or weather come up. For the pilot, carrying more fuel means less stress.”
But most airlines are now using computer models to encourage pilots to modify their decisions, and in some cases to compel changes in operating practices.
The result has been a huge improvement in fuel efficiency. Between 1991 and 2012, U.S. airlines cut their fuel consumption at an average annual rate of 2.27 percent per revenue passenger-mile.
Between 1991 and 2001, when jet fuel prices were stable, most of the improvement came from upgrades in the aircraft fleet. Older more fuel hungry aircraft were replaced by more modern and efficient ones. After 2004, however, most of the gains have come from network rationalisation and changes in operating behaviour.
Factors in aircraft fuel economy
Each model of aircraft has a maximum range speed for a given total load (fuel plus payload), which is the speed at which it is most fuel efficient Flying slower or faster than this optimimum speed increases fuel consumption per mile flown.
There is an optimum speed for efficiency because the component of drag resulting from airframe skin friction against the air increases at a square function of air speed, but the drag resulting from generating lift decreases with air speed. (These are technically called parasitic drag and induced drag, respectively.)
The desirability of a low maximum range speed to reduce environmental and climate impacts is at odds in aircraft design with the benefit to revenue streams of making that design speed higher, to increase the passenger miles flown per day.
Aircraft weight is also a factor in fuel economy, because more lift-generating drag (induced drag) results as weight increases. If airframe weight is reduced, engines that are smaller and lighter can be used, and for a given range the fuel capacity can be reduced. Thus some weight savings can be compounded for an increase in fuel efficiency. A rule-of-thumb being that a 1% weight reduction corresponds to around a 0.75% reduction in fuel consumption.
Flight altitude affects engine efficiency. Jet-engine efficiency increases at altitude up to the tropopause, the temperature minimum of the atmosphere; at lower temperatures, the engine efficiency is higher. Jet engine efficiency is also increased at high speeds, but above about Mach 0.85 the aerodynamic drag on the airframe overwhelms this effect.
This is because above that speed air begins to become incompressible, causing shockwaves form that greatly increase drag. For supersonic flight (Mach 1.0 and higher), fuel consumption is increased tremendously.
Although modern jet aircraft have twice the fuel efficient of the earliest jet airliners, they are only slightly more fuel efficient than the latest piston engine airliners of the late 1950s such as the Lockheed L-1649 Starliner and Douglas DC-7.
Nonetheless, jets have about twice the cruise speed. The early jet airliners were designed at a time when air crew labor costs were higher relative to fuel costs than today. Despite the high fuel consumption, because fuel was inexpensive in that era the higher speed resulted in favorable economics since crew costs and amortization of capital investment in the aircraft could be spread over more seat miles flown per day.
Today’s turboprop airliners have better fuel efficiency than current jet airliners, in part because of their lower cruising speed and propellers that are more efficient than those of the 1950s-era piston-powered airlines.
Among major airlines, those which have turboprop equipped regional carrier subsidiaries typically rank high in overall fleet fuel efficiency. For example, although Alaska Airlines scored at the top of a 2011-2012 fuel efficiency ranking, if its regional carrier—turbo-prop equipped Horizon Air—were dropped from the consideration, the airline’s ranking would be lower.
As over 80% of the fully laden take-off weight of a modern aircraft such as the Airbus A380 is craft and fuel, there remains considerable room for future improvements in fuel efficiency.
The weight of an aircraft can be reduced by using light-weight materials such as titanium, carbon fiber and other composite plastics. Expensive materials may be used, if the reduction of mass justifies the price of materials through improved fuel efficiency.
The improvements achieved in fuel efficiency by mass reduction, reduces the amount of fuel that needs to be carried. This further reduces the mass of the aircraft and therefore enables further gains in fuel efficiency. For example, the Airbus A380 design includes multiple light-weight materials.
Airbus has showcased wingtip devices (sharklets or winglets) that can achieve 3.5 percent reduction in fuel consumption. There are wingtip devices on the Airbus A380. Further developed Minix winglets have been said to offer 6 percent reduction in fuel consumption.
Winglets at the tip of an aircraft wing, can be retrofitted to any airplane, and smooths out the wing-tip vortex, reducing the aircraft’s wing drag.
NASA and Boeing are conducting tests on a 500 lb (230 kg) “blended wing” aircraft. This design allows for greater fuel efficiency since the whole craft produces lift, not just the wings.
The blended wing body (BWB) concept offers advantages in structural, aerodynamic and operating efficiencies over today’s more conventional fuselage-and-wing designs. These features translate into greater range, fuel economy, reliability and life cycle savings, as well as lower manufacturing costs.
NASA has created a cruise efficient STOL (CESTOL) concept.
Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research (IFAM) have researched a shark skin imitating paint that would reduce drag through a riblet effect. Aircraft are a major potential application for new technologies such as aluminium metal foam and nanotechnology such as the shark skin imitating paint.
Jet aircraft efficiency
Jet aircraft efficiencies are improving: Between 1960 and 2000 there was a 55% overall fuel efficiency gain (if one were to exclude the inefficient and limited fleet of the De Havilland Comet 4 and to consider the Boeing 707 as the base case).
Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use. Between 1971 and 1998 the fleet-average annual improvement per available seat-kilometre was estimated at 2.4%.
Concorde the supersonic transport managed about 17 passenger-miles to the Imperial gallon; similar to a business jet, but much worse than a subsonic turbofan aircraft. Airbus states a fuel rate consumption of their A380 at less than 3 L/100 km per passenger (78 passenger-miles per US gallon)
…… and there is more. See http://en.wikipedia.org/wiki/Fuel_economy_in_aircraft