Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
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CRUISE PERFORMANCE ANALYSIS
2.4. APM output data
Before detailing the APM output data, the following lines will remind the principle of the Airbus APM program.
Based on the flight mechanics equations, and thanks to some of the parameters
recorded in-flight, it is possible to determine the amount of lift or lift coefficient (CL).
The aerodynamic characteristics of the aircraft are known from the IFP model. The
drag to lift relation and the calculated lift allows to get the corresponding amount of
drag (CD).
In the flight mechanics equation, the drag is the required thrust to maintain the
flight. The thrust at N1 (in the example) is deduced from the IFP engine model,
giving us the N1 as per the book level or theoretical N1 (N1TH).
Second, at a given N1, the IFP model allows us to determine what the fuel flow is.
The fuel flow corresponding to the measured N1 (N1A) is called the Calculated
Fuel Flow (FFC). The fuel flow corresponding to the theoretical N1 (N1TH) is
called the theoretical fuel flow (FFTH).
The APM output file provides for each engine:
. DN1 = N1 – N1TH or DEPR = EPR -EPRTH
. DFFA = (FFC - FFTH) / FFTH x 100 (%)
. DFFB = (FFA - FFC) / FFC x 100 (%)
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
CRUISE PERFORMANCE ANALYSIS
The APM output file also provides average figures for the two (or four) engines:
. DFFAM = (FFCM - FFTH) / FFTH x 100 (%) . DFFBM = (FFAM - FFCM) / FFCM x 100 (%)
. DSR = (FFTH - FFAM) / FFAM x 100 (%).
DSR represents global aircraft performance degradation (in %), in terms of Specific Range degradation.
DFFB is the deviation of fuel flow due to engine deterioration.
DFFA is the deviation fuel flow due to "apparent" airframe deterioration.
Some aspects need to be underlined to better appreciate results of the APM program:
DFFB is only linked to N1/EPR and FF recordings, and is independent of the EPR thrust relationship and of the associated engine model. This means that a high level of confidence can be given to the DFFB value.
DFFB is also linked to the fuel lower heating value (FLHV). The Airbus nominal value is 18590 Btu/lb. The FLHV is used to calculate theoretical parameters such as the fuel flow (FFTH), the N1/EPR (N1TH/EPRTH).
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