Fuel Efficiency Basics

Presented by Marc Brodbeck

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Content

Other fuel savings opportunities

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Thrust, Drag, Lift, Weight

Efficiencies by phase of flight

1 Fuel Cost Impact

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Fuel Cost Impact, ~30% of total operating expense

3% savings is normally achievable using various levers

Fuel

SalariesLanding Fees

Maintenance

Regional Capacity

Depreciation

Aircraft Rent

Distribution

Interest

Other

Profit

Fuel Cost Impact

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4 forces of flight, each one that can be optimized may lead to a lower total fuel burn (cost)

1. Thrust: Generated from the engines; produces force to move the aircraft through the air

2. Drag: As a lifting mass moves through a body of air, it will generate some form of drag, drag has a counter effect on thrust, thus should always be minimized

3. Lift: The airframe body and predominately its wings generate lift to keep the mass aloft

4. Weight: Downward effect from gravity that pulls the mass towards the ground, when lift > weight, the airplane is climbing

Drag

Weight

Thrust

Lift

High bypass engines, lower SFC, e.g. GTF

Winglets or PIP

Wing Area + Winglets + raked wing

Carbon Fiber or light weight

materials

Flight Theory

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Thrust

Engines produce thrust by converting fuel into thrust/energy– Therefore, the optimum engine from a fuel

efficiency perspective is one that produces the most thrust (energy) for the least amount of fuel burned

Thrust is used to:– push the aircraft through the air to offset drag – move the wing through the air to generate lift

Thrust specific fuel consumption (TSFC) or sometimes simply (SFC) specific fuel consumption is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption per unit of thrust

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Thrust

Turbojets were first generation transport engines (JT3)

Turbofans 2nd generation engines (CF6) “High Bypass” turbofans could be

considered 3rd generation (GE90)– Huge N1 forward fan produces a

percent of total thrust Engine SFC has improved over +40% in

the last 30yrs– Higher bypass engines

• Leap-X• GTF (Geared Turbo Fan)

– More time on wing• Lower Maintenance cost

– Lighter weight components, higher core temperatures (more energy)

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Drag

In simple terms, drag is the resistance acting on the airplane as a result of the airplane's movement through the air

Airframe, wings, engines & pylons, etc., e.g. anything in the windstream will generate some amount drag

Creating lift (wings) also generates drag The goal of wingtip devices is to reduce

induced drag, as induced drag is due to the global effects of generating lift

A device (or devices, e.g. 777 PIP or Winglets) which helps to reduce drag will reduce the need for thrust (lower fuel consumption) to maintain the same speed/altitude

Raked Wing

Scimitar Winglet

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Lift

Wings generate the majority of lift. Lift is used to offset weight (gravity) and keeps the airplane aloft– Airfoil design has improved over the

years with advanced 3D modeling and wind tunnel testing

Larger wings create more lift, but also create more drag– Wings must be large enough to

generate adequate lift during takeoff and climb• Flaps and various wing devices

are used to generate more lift during these critical phases

• Winglets generate some lift Wing size & airfoils are generally

optimized for a particular mission, e.g. long range cruise

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Weight

The aircraft weight is the sum of the DOW (Dry Operating Weight) + Payload (passengers, bags, and cargo) + Fuel Burn + Fuel Reserves

The heavier (higher) the aircraft weight, the more lift and thrust are required to achieve stable flight, thus higher fuel burn

Programs to reduce excess weight are a prominent component of fuel efficiency:– Using lighter weight materials

(carbon fiber brakes) – Lighter weight seats – Removing excess potable water– Using lighter onboard equipment,

pantry items, or reduced catering– Lighter weight cargo containers– Excess fuel

Carbon Brakes

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Fuel Efficiency by phase of flight

Highest fuel flow (burn) rates occur during take-off and climb

However, largest amount of fuel burn occurs during the cruise segment

The majority of fuel optimization focus is therefore on the cruise segment

Phase Time (min) Fuel (kgs) Distance

(nm)Fuel Consumption Rate

(1000 kgs/hr)Taxi-out 15 404   1.6

Takeoff 1.5 447 1  17.9

Climb 27 5,919 184 13.2

Cruise 1001 103,595 8,149 6.2

Descent 21 408 122 1.2

Approach 3 209   4.2

Taxi-In 7 188   1.6

Total 1076 111,170 8,456 6.2

A350XWB-1000 8,456 nm trip fuel consumption

Source: Piano-X

Avg. 6200 kgs/hr consumption

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Ground Ops

Prior to departure and after arrival, the APU (a small Auxiliary Power Unit, in the tail) is run at the gate to provide electrics and power the heat/air-conditioning system

APU’s can burn 115 US gallons (435 liters)/hr on a 747-400 Many Airlines/Airports invest in ground power and air to support the aircraft at the

gate APU usage during ground ops is jointly managed by flight crews, ground crews, and

technical (maintenance) crews

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Taxi-out and Departure

Fuel savings are possible during taxi-out by taxiing from gate towards departure runway on 1 engine (engine warm-up times of 3-5 minutes are required)

Delaying APU start until 5-10 minutes before pushback also can save fuel Takeoff runway in the direction of flight is a fuel savings technique Takeoff Flap selection and acceleration altitude can also save fuel:

– Lower takeoff flap setting reduces drag on initial climb out– Retracting takeoff flaps and accelerating quickly to climb speed at low altitudes

(800ft AGL) increases efficiency

Clean up flaps on schedule Remain at clean maneuver speed until

within 90°of intended track Trade speed for altitude

1st waypoint115°from departure direction

Departure Restriction

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Cruise Altitude and Speed selection

Cruise Altitude and speed selection can be optimized to decrease fuel consumption– Balance of on-time performance vs. fuel efficiency

Generally higher cruise altitudes more efficient and generally have smoother rides and are clear of weather– Optimum altitude is generally one that yields the best fuel mileage

Cruise speeds are typically planned/flown which minimize total cost via flying cost index; generally slightly (3%) faster than Max Range Cruise (MRC) which is the best fuel mileage cruise speed, but slower than Long Range Cruise (LRC)

Normal Cruise Speed envelope for the CRJ700 is M.74 to M.78

Chart based on nominal Weight of 66,000lbs at 35,000ft

Key Speed Measures (figures in still air): MRC = M.74/427kts (NM/lb of 0.1405) LRC = M.76/439kts (NM/lb of 0.1392)

– Speed-up Mach = M.80/461kts (NM/lb of 0.1348), this is 22kts faster than LRC

Operating @ high speed M.80 vs. LRC/M.76 is a burn penalty of ~3.2%– On a 1200nm cruise segment (~3hrs)

this is 281lbs more burn (8 minutes faster)

Operating in the speed band of M.75-M.77 has a small impact on overall % fuel mileage Operating in the speed

band of >M.79 has a larger impact on overall % fuel mileage

Flying Efficiently

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Descent and Approach

Since cruise fuel consumption is at optimal fuel burn, flights are generally flown at cruise altitudes, as long as possible, until an idle (thrust) descent can be initiated– A descent initiated too early results in early level offs, which require more thrust

at lower altitudes (more fuel) Airlines, OEM’s, and ICAO work w/ ATC navigation services (ANSP) providers to

improve the descent profiles into major airports (goal: reducing noise and burn during the descent/approach phase)

Optimum TOD

Continuous descent approach (CDA)

Stepped descent (early)

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Landing rollout and taxi-in

Fuel savings can be achieved by reducing drag (and therefore requiring less thrust) during the approach phase, by using less than full flaps

Flights can utilize idle reverse thrust after touchdown, to minimize fuel-flow during rollout

Once engine cool-down times have been observed (generally 3-5 minutes) the aircraft can taxi-in to the gate on 1 engine

APUs are generally not turned on after arrival if sufficient ground electric/air exists

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Other fuel savings opportunities

Small incremental items add up to large

benefits(~3% of fuel expense)

Engine core washing ~2x per year (improves

SFC)Reducing fuel

reserves (weight) towards regulatory

minimums

APU no start SOP for narrowbody

fleets

Replacing older technology fleets

with new tech (engines/wings)

A 4D optimized flight planning system

(savings ~1%)