2.1. DFFA interpretation
Case 1 - DN11 and DN12 >0 and thus DFFA1 and DFFA2 >0
DFFA> 0, i.e. higher apparent drag or lower thrust at N1than model
Case 2 - DN11 and DN12 <0 and thus DFFA1 and DFFA2<0
DFFA<0 i.e. lower apparent drag (or higher thrust at N1) than model

2.2. DFFB interpretation
Case 1 - DFFB1 and/or DFFB2>0
higher fuel consumption than model
Case 2 - DFFB1 and/or DFFB2< 0
lower fuel consumption than model
Case 3 - DFFB1 >0 andDFFB2<0
.DFFB1 .> .DFFB2 .. DFFB > 0: higher consumption from engine part .DFFB1 . < . DFFB2 .. DFFB < 0: lower consumption from engine part
Case 4 - DFFB1 < 0 and DFFB2> 0
.DFFB1 . > . DFFB2 .. DFFB < 0: lower consumption from engine part .DFFB1. < .DFFB2 .. DFFB >0: higher consumption from engine part
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring

RESULTS APPRAISAL

2.3. DSR interpretation
Combining paragraphs 2.1 and 2.2 gives the following possibilities with regard to specific range deviation.
Case 1 -DFFA > 0 and DFFB> 0 .
 DSR < 0
Compounded effect resulting in specific range deviation (worse than book value)
Case 2 -DFFA< 0 and DFFB > 0
1) if .DFFA . > .DFFB .. DSR > 0 Higher engine fuel consumption than model is being compensated by an apparently better than nominal aerodynamic condition resulting in better specific range than book value
2) if .DFFA. < .DFFB.. DSR < 0 Partial compensation of resulting in worse specific range than book value
Case 3 -DFFA> 0 and DFFB< 0:
1) If .DFFA .< .DFFB .. DSR < 0 Partial compensation of resulting in worse specific range than book value
2) if .DFFA .< .DFFB .. DSR > 0 An apparently worse than nominal aerodynamic condition is being compensated by lower engine consumption than model, resulting in better specific range than book value.
Case 4 -DFFA < 0 and DFFB < 0 .
 DSR > 0
Compounded effect resulting in specific range deviation (better than book value)
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring
RESULTS APPRAISAL

3. EXAMPLE
This paragraph is based on a cruise performance analysis that was performed for an A310-304 fitted with CF6-80C2A2 in year 1990.
Figure E2 shows manual readings that were taken at that time. The three stable points identified from the manual recordings of Figure E2 were processed by APM. Only two of these were retained by the statistical procedure and are framed in Figure E3.
The result of DFFA and DFFB (with DFFA < 0 and DFFB > 0 and DFFA < DFFB) is a marginal deviation in DSR (-0.56%). Higher engine fuel consumption than model is very often observed and at times is partially compensated by an apparently better than nominal aerodynamic condition as exemplified in the APM outputs shown in Figure E3.
　
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