In the world of flight planning system calibration, these are some of the common root cause factors driving overburns.

1. Presented by Marc Brodbeck

2. Overburn = Actual burn > Planned burn
Various factors contribute to deviations between planned burn vs. actual burn
1. 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 0
5. Stronger headwind component or less tailwind component
6. Deviations from planned route, thus altering air-miles (ESAD) flown (less
tailwind/more headwind)
7. Fuel imbalance
8. Improperly trimmed airplane
9. ZFW/Gross Weight deviations from plan
10. Forward CG, causing additional drag
11. Excessive thrust lever adjustments
12. BTU (LHV) lower than nominal figures from Airbus/Boeing
13. ATC slow downs, early descents, and long arrival vectors
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3.  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

4. 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

5. 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

6. 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

7. 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

8. 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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9. 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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= ?+++

10. 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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11. 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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12. 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