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reading in the engine high speed database the related fuel flow by interpolating between two bleed ratings (OFF, LO/ECON, NORM, HI).
In practice, bleed flows between both packs can be different. This item is more significant on A320 aircraft types, where the old standard of Flow Control components (including flow control valves) had a less restrictive industrial tolerance than the newer standard. Airbus published a specific Service Information Letter (SIL) to inform airlines of this issue. This SIL is given in Chapter H -Appendix 4 – Airbus Service Information Letter 21-091.
The APM program will evidence that some troubleshooting may be required on this specific ATA 21 item. The Trouble Shooting Manual (TSM) contains a couple of entries in the form of crew observations in section 21-51. The procedures will lead relevant trouble shooting procedures.
2.7.3. Aircraft model specifics
Two aspects need to be underlined to appreciate the results of the APM program.
2.7.3.1. The thrust / drag uncertainty The aircraft drag assessment assumes the invariability with time of thrust at N1/EPR relationship. However a scatter exists from one engine to another. This
relationship can therefore essentially vary with the production of built material and slightly over time.
CL at flight condition for a measurement point corresponds to CD through the drag polar (quadrant (A) ; thrust is to compensate drag and is related to N1 (quadrant
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
CRUISE PERFORMANCE ANALYSIS
(B) with the possibility of engine-to-engine model alteration (N1 (EPR) /thrust relationship).
2.7.3.2. Engine-to-engine model N1 (EPR) / thrust relationship
alterations whereby DFFA – aerodynamic part – (quadrant may shift).
An observed .N1 or .EPR does not necessarily indicate an aerodynamic deterioration of the airframe. An altered N1 /thrust or EPR / thrust relationship with respect to the reference engine is, in many cases, responsible for such a deviation. This is also valid for new engines as well. Engine test-cell-gathered N1 or EPR versus thrust ratios cannot be transmitted to cruise high Mach / high altitude conditions with an acceptable confidence level.
N1 meas : N1 measured on the aircraft DSR = (FFA - FF th ) / FF th (%) (overall aircraft)
FFA : fuel flow measured on the aircraft DFFA = (FFC-FF th) / FF th (%) (thrust/drag balance)
FFC = FF (N1 meas) via the engine model DFFB = (FFA - FFC) / FFC (%) (engine contribution)
FF th : provided by the overall aircraft model
Figure D4 – Illustration of the thrust at N1/EPR uncertainty
2.7.4. Processing rule
The recorded data are processed considering the following rule. If one of the mandatory parameters is missing, an average value will be taken in replacement to run the APM program, but the final result will be biased accordingly. This is particularly true for the fuel lower heating value (FLHV) or the center of gravity location.
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