Re: [Q] Required Fuel or trip fuel

From:         D.P.Rhodes@lboro.ac.uk (Darren Rhodes)
Organization: Loughborough University
Date:         25 May 96 14:40:22 
References:   1
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In article <airliners.1996.776@ohare.Chicago.COM>, Upul Abayasingha <b.u.abayasingha@cranfield.ac.uk> wrote:
>I would like to know how the airlines calculate required fuel for a given
>jouney including reseve fuel, for long-haul flights . I know little bit
>about the theory using engine SFC.  etc...  How accurate is this method
>and what kind of safty factor airlines use? Is there any books regarding
>this matter, and any softwear available ?

On most aircraft, the Flight management system will take into account the
thrust setting and fuel flow rate to constantly determine range during flight.

However to determine the fuel required for the stage they will probably use
the following analysis.

Basically fuel required is calculated using the Breguet range equation:

ESAR=V/C*L/D*ln(W1/W2)

ESAR - equivalent still air range (nm)
V    - Cruise speed (kts)
C    - Engine SFC (lb/hr/lb)
L/D  - Aircraft Lift/Drag ratio (-)
W1   - Take-off Weight (kg)
W2   - Land weight (kg) (i.e. Take-off weight - Fuel burnt)

The equation can be rearanged in terms of fuel weight/Take-off weight
required:

Wf/Wto=1-e^(-ESAR*C/(V*L/D))

Typically Wf/Wto will be around 0.2-0.3 depending on range flown.

Standard reserves for a long range aircraft are 5 percent trip fuel, 250nm
diversion and a 30min hold at 1,500ft.

For short/medium range aircraft, the reserves are 5 percent trip fuel, 200nm
diversion and a 30min hold at 1,500ft.

These can be taken into account in the Breguet range equation by increasing
the ESAR value by around 500nm for short/medium range aircraft and about 750nm
for long range aircraft. More detailed values values can be determined by
applying the Breguet range equation to the diversion and hold segements as
well as the cruise/climb/descent.

Check out "Synthesis of Subsonic Aircraft Design" by Egbert Torenbeek, which
is in the Cranfied Library. It's not very easy to read, but it's in there.
Also check out "Aircraft Design: A Conceptual Approach" by Daniel P. Raymer.
It's a easier to read but leans more towards military aircraft.

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