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The heat energy per unit of time is given by the following formula:
Q = J x Hf x Wf
Where J is physical constant
Hf is the fuel specific heat (Fuel LHV) in BTU1/lb
Wf is the fuel flow in lb/h
As a consequence, the fuel flow required to produce a given amount of thrust is:
Q 1Q
Wf ==×
J × Hf FLHV J
The required thrust being fixed, the heat energy Q is also fixed. Thus, the higher the FLHV, the lower the required fuel flow.
1 BTU is the British Thermal Unit. It corresponds to the heat quantity required to increase the temperature of one pound of water from 39.2°F to 40.2°F. 1 BTU = 1.05506 kJ
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring

BACKGROUND
The following conclusions can be drawn from the above equations:
1.
Any deviation in the fuel LHV will result in a deviation in fuel flow

2.
As the heat energy remains constant whenever fuel LHV and fuel flow vary, the engine thermodynamic cycle is unchanged. The high-pressure rotor speed N2 and the Exhaust Gas Temperature remain unchanged.

3.
The only affected parameters are fuel flow (FF) and specific range (SR).

.FF .FLHV .SR .FLHV
=. =+
and
FF FLHV SR FLHV
The FLHV local and seasonal variations being a fact of industry, the accuracy can be increased by a FLHV measurement.
It is in any case essential to perform FLHV measurements, as variations in fuel quality exist throughout the world (crude oil quality) and in between flights. Airbus now has a fairly large database we have been receiving lots of samples from our audits worldwide as shown in figure B17.
Flight Operations & Line Assistance Getting to Grips with Aircraft Performance Monitoring

BACKGROUND
FLHV BTU/LB)

Figure B17 - FLHV values versus fuel density - Sample measurements
Figure B17 shows that the minimum fuel LHV encountered over a significant population of samples is 18400 BTU/LB. In routine performance analysis, this FLHV is rather difficult to obtain, because of the wide variety of fuel quality, depending on various world regions. Most of the airlines subsequently use the same value for all their analysis. Although this method is rather questionable if an accurate performance audit is intended, it is quite acceptable for routine analysis.
In this case, one should keep in mind the FLHV effect on the monitored fuel factor, especially when implementing the fuel factors in the airline flight planning systems, or in aircraft FMS systems. The monitored fuel should be corrected for the FLHV effect (see also chapter 0-2.4.3. Monitored fuel factor & 0-3.2.Keys for defining the fuel factor).
3.4.3.2. Data acquisition / transmission (Scatter/Bias) Before data is automatically collected by means of the various aircraft recording systems, some conditions are checked. In particular, the variation of a few parameters over a 100-second time period allows to identify cruise stabilized
　
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