Anatomy of Overburns

Presented by Marc Brodbeck

Overburn = Actual burn > Planned burn

Various factors contribute to deviations between planned burn vs. actual burn1. Temperatures greater than planned (+3% per +10 deg. ISA)2. Cruise altitude lower than planned (+1% per 1000ft below optimum)3. Cruise altitude more than 2,000 feet above optimum altitude (+2%)4. Speed faster than planned or appreciably slower than max range cruise, when

MRC was planned/CI 05. Stronger headwind component or less tailwind component6. Deviations from planned route, thus altering air-miles (ESAD) flown (less

tailwind/more headwind)7. Fuel imbalance8. Improperly trimmed airplane9. ZFW/Gross Weight deviations from plan10. Forward CG, causing additional drag11. Excessive thrust lever adjustments12. BTU (LHV) lower than nominal figures from Airbus/Boeing13. ATC slow downs, early descents, and long arrival vectors

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Temperatures exceeding forecast can have a slight impact on actual fuel burn. This is mainly due to an increase fuel flow required for deviations >ISA, e.g. ISA+20 would require approx. 1.054 (5.4%), more fuel flow, compared to ISA.

Higher ISA temps also increase TAS (~1kt/degree), so the actual range impacts are small

Verification of Data: FP Wind Matrix /

Temp data APM/AHM Position Reports

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Temperature Deviation

Lower cruise altitude than planned For each weight and speed, there is an optimum altitude; that which provides the greatest

amount of fuel mileage per lb of fuel. Due to outside influences (ATC, weather, etc) the altitude flown is 2000ft lower than

planned, the impact will be approx.. 2% less efficient.

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Verification of Data: FP altitudes APM data Position Reports

Cruise altitude more than 2k below or 4k above optimum altitude For each weight and speed, there is an optimum altitude which provides the greatest

amount of fuel mileage per lb of fuel (Specific Range). If required to fly off optimum, the range efficiency may be decrease. For higher

altitudes (above optimum) the thrust required to maintain that speed/altitude for a given weight will be slightly higher, thus also reducing range efficiency.

Verification of Data: FP altitude profile APM Position Reports

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Max Altitude

Optimum Altitude

Departure Destination

FL400

FL360

FL320

FL300

FL280

FL260

Off Speed Flying; Speed faster than planned or appreciably slower than planned For each weight and altitude, there is an optimum range speed; that which provides

the greatest amount of fuel mileage per lb of fuel. If required to fly slower than this Max range cruise (MRC) speed [CI 0], the fuel

range efficiency decreases.

Verification of Data: FP Speed profile APM Position Reports

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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

Stronger headwind component or less tailwind component For each segment of the plan, a ground speed is computed, based on the forecast

wind vector along that leg. If actual wind encountered alters the wind vector from plan, the actual ground speed

and thus the zone time/fuel burn will vary. Wind vector errors >20kts will likely cause noticeable burn deviations. ESAD; Equivalent still air distance, is the ground distance corrected for the affects of

wind velocity.

Verification of Data: OFP Wind Matrix / Temp data APM Position Reports

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Formula for ESAD (nautical miles):ESAD = (TAS * Ground mileage) ÷ (TAS + WV)Where;TAS = True Airspeed (cruise speed in knots)WV = Wind velocity

Deviations from planned route, thus altering air-miles flown (less tailwind/more headwind) For each flight plan, there is a minimum burn route. If a direct route is accepted, the ESAD for the new direct routing may in fact be

greater than the minimum burn route, thus increasing time/fuel burn. Any direct off planned route, which either increases/decreases the wind vector speed

for the leg, may impact actual time/fuel burn.

Verification of Data: OFP

route/altitude/speed APM Position Reports ATC position reports

(ASDI)

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ZFW/Gross Wt. deviations from plan

For each flight plan, there is a planned assumption on the Zero Fuel weight (ZFW) and Gross weight (GW) of the aircraft.

If the actual weight is greater than the flight plan figure, the fuel burn will increase. The approximate increase is at a rate of 3% per hour of flight time for the Weight.

For a flight of 3hrs, this increase burn would be approximately 9%. For a flight of 14hrs, +42%.

1000lbs additional weight vs. planned will result in 420lbs additional burn If the aircraft requires more thrust (higher drag) to achieve speed/altitudes compared

to nominal figures, the actual ZFW may be higher than anticipated. This is possible due avg. passenger and bag weight assumptions (and actual CG).

Verification of Data: FP Planned ZFW WB system actual ZFW APM Drag/FF reports Thrust Required vs. Nominal

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= ?+++

Forward CG location

For each a/c there is a large CG %MAC envelope. Flight planning figures are generally based on a nominal GC %MAC figure, e.g. 25% The actual location of the CG during cruise can impact cruise drag. Forward/Aft of

nominal value, will increase/decrease drag, up to +/-1%

Verification of Data: FP Planned

ZFW/CG %MAC WB system actual

ZFW/CG %MAC APM Drag/FF

reports

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LHV (BTU) lower than nominal

Jet fuel has variable energy content (LHV = Lower Heating Value) Flight planning figures are generally based on a nominal figure, e.g. Boeing =18,580

BTU/lb, Airbus = 18,590 BTU/lb BTU for Jet fuel around the world can vary: 18,484 – 18,645 BTU/lb The actual BTU of the fuel being burned can impact Specific Range There is a relationship between LHV and Specific Gravity (SG/Density), e.g. higher

density fuel = lower LHV = less energy A higher density fuel may result in a 0.5% decrease in Specific Range

Verification of Data: FP Planned BTU APM Specific Range reports Fuel vendor plane-side BTU

(if avail.)

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ATC slow downs, early descents, and long arrival vectors Due to numerous ATC letters of agreement and procedural design issues, aircraft

rarely fly on the optimum trajectory (lateral + vertical). Non-clean maneuvering consumes approximately 150% compared to clean

maneuvering, and 250% compared to idle-descent. If the aircraft is required to descent 50nm before the optimum top of descent point,

descent fuel will be approximately 200% compared to an idle-descent / decelerated approach.

Longer than planned IFR arrival procedures (extended downwind vectors)

Verification of Data: FP

route/altitude/speed Position Reports ATC position reports

(ASDI) DFDR (if available)

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Optimum TOD

Continuous descent approach (CDA)

Early ATC descents