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Aviation Fuel Consumption Strategies

Getting the most from your dollar at the pump.

The debate over ‘Peak Oil’ (PE) is still in play today: have we passed it already- or is it due in 2010- 2013- or possibly much later this century. PE essentially refers to ‘The time of maximum production’- or when the world’s oil consumption reaches its highest point at around 97 million barrels per day (MBPD).

The cost of fuels derived from crude oil are going to rise continually unless a viable and cheaper replacement fuel product can be produced on a large enough scale to compete against- and eventually replace today’s fossil fuels. In the meantime- the economic impact of ever-increasing fuel costs are being felt across the world by every user sector- not least aviation.

Around the globe- private leisure flying is down- and both airline ticket prices and commercial charter rates are increasing to compensate for the current ‘into-wing’ (fuel) prices that continue to climb. Currently- the average into-wing General Aviation Jet A and 100LL fuel prices per US Gallon ranges from US$10.00 ($3.00/liter) in the United Kingdom- and $5.00 ($1.35/liter) in the United States. Ten years from now- though- it’s very possible we’ll look on these numbers as being unbelievably cheap! The airplane has seen such incredible global success since it came into being at the beginning of the 20th century- because no other machine is able to provide a better- safer- speedier and more cost effective transportation system for the majority of the traveling public.

The whole purpose of flying transportation is the speed - unfortunately though- attainment and the sustainability of a high-speed flight does require large amounts of fuel to power it.

There are over 200-000 flights daily that occur within US airspace alone. Most flights are made by turbine powered aircraft. According to a study undertaken by NASA- the daily Jet Fuel consumption for North America in 1998 was almost 75 million US gallons per day (that’s almost 285-000-000 liters per day). Expect that number to have been eclipsed today.

The science of engines
All hydrocarbon burning engines require a fuel-air mixture ratio that allows a sustained- energy release. This ratio is correctly termed the Stoichiometric ratio. A piston engine requires a fairly narrow Stoichiometric ratio band centered around 14.7 of air units to 1.0 fuel unit. Any mixtures above- or below this ratio are considered ‘Rich’ or ‘Lean.’ It is possible to run close to 12.0 to 1.0- but normally detonation will occur- causing considerable internal damage.

A turbine engine can require anywhere between 45.0 to 1.0 up to 130.0 to 1.0. This ratio along with the tolerance of the mixture envelope is determined by the engine designer- as up to 80% of the air that flows through a turbine engine is not meant for combustion- but instead for cooling and flame stabilization.

Aircraft engines fly in environments that have various reduced temperature and air density ranges commonly found at altitude - and these can potentially affect engine performance in different ways. Therefore one needs to allow for such factors- especially when applied to how much fuel must be burnt for a given desired flight profile to be achieved.

Aircraft piston engines will burn less fuel at certain altitudes- but the power that they produce is greatly reduced if they are not normalized by the means of an engine driven geared supercharger- or an exhaust gas drive turbo-supercharger. Unfortunately the act of normalizing an engine also increases the amount of fuel that is required to power the engine. We normally see a beneficial reduction in fuel consumption without too much of a detriment to overall speed and power- when a piston aircraft is flown in the band between 5-000 and 12-000 feet.

Interestingly- tests conducted in the 1930s indicated that the effect of altitude on compression-ignition engines (diesels) was minimal over a wide range of air-fuel ratios.

Aircraft that are powered by turbo- propeller engines thrive in altitude band 10-000-20-000ft- depending on the ambient temperatures at altitude- and lastly- aircraft that are powered by turbine engines that create thrust by expelling a jet of rapidly moving and expanding hot exhaust gases out of their jet-pipes usually reach optimum power and efficiency at much higher altitudes. Often the behavior of airflow over and around a wing and through an engine- as determined by design is the limiting factor on the altitude that any given turbine engine can attain and operate at.

Preventing compressor stall and subsequent flame-out are almost as limiting at the Maximum Exhaust Gas Temperature (EGT) or the Inter-Turbine Temperature (ITT) as determined by the engine manufacturer. Generally speaking any flights below 30-000 feet causes a marked increase in fuel consumption without any benefit in increased speed or available power- for the same setting on a turbine powered aircraft flown at a higher altitude. But why is this?

1) With altitude increase- the ambient temperature decreases. A Turbine engine is effectively a heat pump that depends on thermal efficiency to achieve optimum power output. There are greater temperature differentials at altitude- i.e. the difference between the temperatures of the jet efflux and the ambient air is greater when the ambient air temperature is lower.

2) The compressors of a turbine engine are more efficient at higher revolutions per minute (RPM). The pressure of the air drops and becomes less dense with increase in altitude- these changes in the physical properties of the air being let into the engine allows the compressor to attain higher RPMs.

3) The same changing properties of ambient air at altitude also aerodynamically benefits efficiency of the airframe- as overall friction and drag is reduced- thus requiring less thrust than would be required at lower altitudes to retain a given Mach number or True Air Speed (TAS).

Getting to altitude efficiency
The above altitude effect (and its ability to reduce your overall fuel consumption) can only be harnessed by getting your aircraft up to altitude as expeditiously as possible. Many significant range increases achieved by aircraft manufacturers on newer aircraft models are only possible because the manufacturer has devised a way of getting the new model up to altitude faster than its predecessor.

The installation of winglets - for example - is often an easy way of increasing the aspect ratio of a wing- thus increasing the amount of lift that it produces. Every aircraft manufacturer is required to accurately produce and supply a Flight or Operations Manual to the buyers of their aircraft. These manuals can come in a variety of formats- including separate books that break out Performance- Loading- and more. Obviously this is where you will find specific instructions on aircraft configuration- power setting- climb angle and speed to achieve the best rate of climb for your aircraft at a given weight- air density and temperature.

As a matter of interest- however- recently I was told that one of the best ways of getting a King Air- or similar turboprop up to altitude as fast as possible- is to leave a notch of flap out with climb power adjusted to set your climb angle and speed to fly at 1.3 times your stall speed at your given weight. Unfortunately your progress (time) to your destination will be hampered by this climb strategy- and therefore you may wish to employ the ‘constant parabola’- ‘step’- or ‘cruise’ climb procedure- which relies on the combination of forward speed and rate of climb.

In general- a constant parabola climb can be achieved by setting power above your cruise setting- but below your best rate of climb setting- and with every increase of 2-000 feet- you should increase your TAS by 10 Knots. This method works because you are using less power than you would in a climb- and yet you are reducing your overall journey time by traveling at a higher than normal climb speed.

Regardless of what method you employ to get to altitude efficiently; before you get there you must know at what flight level to level off on- to either maximize the advantage of- or minimize the effect of whatever the upper level wind is. Good flight planning- along with an eagle eye on your ground speed read-out on your wrist watch- R-Nav- GPS- FMS- or whatever you’ve got as an aid- are needed.

You are still not ready to relax yet though. Next you must set the desired cruise power setting- which may be for best range (the slowest)- or best speed (the fastest). Finally you must trim the aircraft up properly so you may achieve the best ‘planning’ or air penetration deck angle and directional point. Any power/speed configuration that normally nets you a reduction in fuel consumption will be dashed if you are flying one-wing down- your nose is too high or too low- or you are slipping or skidding. The best transportation pilots are the smoothest. They achieve their silkiness by understanding that a large part of their job is to manage the autopilot- while it flies the aircraft.

Tips on maintaining efficiency at altitude
It is important to remember that when at altitude- your specific range decreases with a headwind. This can be minimized however with an increase in cruise speed that counteracts the headwind (see ‘constant parabola’ climb principle above). When upper level headwinds do increase- you probably will need to adjust your flight level too. Obviously an altitude increase requires higher fuel consumption than is required in cruise. Therefore sometimes by decreasing altitude you will decrease your overall potential energy- but it will decrease fuel consumption if you get less of a headwind at the lower level.

Captain Richard Theriault- Worldwide Jet Charter- explains- “Long-range cruise will help in reducing your fuel bill- but owners tend to want speed when they fly. They always seem to be asking 'when do we get there?' For oceanic flights it can be next to impossible to use this technique due to speed assignments by ATC. While flying as high as possible does cut down on fuel burn in jets- efficient flight planning and procedures also help.

“In a jet- one of the most efficient ways to control fuel burn is to control how long the engine runs. Look at your winds and plan accordingly. One case in point: I flew a Lear 25 from Grand Junction- Colorado- to Reading- Pennsylvania non-stop one day 3+42 airborne time. I was able to do so because I had almost 200 Knots of tailwind at FL370. At our normal cruise level of FL410- I would not have been able to do this flight non-stop!”

Another corporate pilot who wished to remain anonymous emphasized the importance of careful pre-flight-planning. “We operate twin turboprops. I influenced my flight department to start using www.fltplan.com. Through the use of this free website I have come to a happy medium between hot weather and cold weather operations on fuel burn and cruise speed. This enables a very accurate ‘expected’ fuel burn and time for any ‘fltplanned’ trip.

“Armed with the knowledge of specifics- each user is enabled to have a starting place for what the trip will need over all. Unforeseen routes - say into or out of the North East that are routed low and wide- with stronger than normal headwinds - holds (very few)- alternates and reserve are all part of the equation for how much fuel to ‘tanker.’ “Fltplan.com does a good job at setting basic default altitudes - but also enables viewing winds aloft- ISA- true airspeed- fuel burn and all altitudes up to the maximum certified altitude on the ‘Weather’ page. There it is possible to select the best combination of trip time- fuel burn- and true airspeed. We try to stay at the high altitudes and almost every time the controllers require a descent that puts us into the higher fuel burn altitudes way too early... sometimes we get discretionary descent clearances- but most times we don't.”

Our turboprop pilot touched on another important issue relating to high priced aviation fuel: ‘Tankering’. Jim Deuvall of the CAVU Companies explains more: “We offer EFB-Tankering- a PC- or PDA-based- program that calculates fuel savings in dollars. The software is aircraft type-specific- so it calculates the differential fuel flow at the higher weight of carrying extra fuel.

“The user can select different cruise flight levels- speed (.8MI- Long Range etc.)- relative temperatures- takeoff weight (w/o tankering) and flight duration. Finally- the program looks at fuel prices at the departure and arrival airports and factors any landing fee rebates for purchasing ‘X’ number of gallons. A spreadsheet then appears comparing various amounts of fuel remaining at the destination- the number of gallons to tanker- the number of gallons to purchase at the destination- and the dollar savings over buying all at the destination.

“As an example for a Falcon 7X- one line on the spreadsheet might read:
600 517 100 $398.22
“That information can be read as follows: You want 600 gallons at the destination? Then you will need to tanker 517 gallons (the 17 gallons if used to transport the extra fuel weight three hours in this case at FL250- normal ISA- TOW 60-000lbs). Purchase 100 gallons at the destination (to take advantage of a landing fee rebate)- and save $398.22 over buying 600 gallons at the destination.” (Find out more at www.CAVUcompanies.com).

Larry D'Oench of Flight Audit LLC reveals that another way of reducing your overall fuel consumption is the technique of single-engine taxiing. “…The airlines have used it for years with no adverse consequences. If any departure delay is anticipated or if it will be a long taxi- aircraft weight permitting... start one engine (two in the case of the tri-jets)- and taxi out with one shut down. At the appropriate point- bearing in mind warm up and check list requirements- start the remaining engine and depart. “Upon landing and after the required warm-down time- shut one engine down as you taxi in to the FBO. Collateral benefits are not having to taxi with one engine in reverse to keep your speed down and reduced noise and pollution- and using less fuel and money.”

Dale Hawkins from Duncan Aviation- Battlecreek- concluded this look at Fuel Strategies- observing “...a big proponent is keeping your aircraft well maintained- running to its optimum level- while spending the extra cash to keep all control surfaces as clean and tefloned as possible.

“I see aircraft that have been abused- sitting outside with the Paint baking to a dull finish. Some with paint flaking off and large patches just plain missing. If you think of the aircraft flying in its very best configuration- you think of very clean lines and smooth surfaces to reduce drag wherever possible.

“All Engines should be electronically controlled to maximize fuel savings and power. If there are Service bulletins or means by which you can to add engine computers to your engines- then they should be installed... Buy the fuel smartly and control consumption as best you can.”

Jeremy Cox is the Vice President at JetBrokers- Inc; a professional aircraft sales company. He currently holds valid A&P IA FCC Licenses and a Commercial and Instrument Ratings. Mr. Cox is actively involved in the Greater St. Louis Business Aviation Association- and has also held various roles with the NBAA- PFA- EAA and AOPA. Jeremy has amassed twenty-five-plus-years of diverse technical and operational experience- and has been a Director of Maintenance for several different companies.

More information from www.jetbrokers.aero

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