Fuel burn vs fuel density
Thread Starter
Joined: Dec 2004
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From: UK
Fuel burn vs fuel density
Whazup!
Just relocated to the UK and busy converting my ICAO licence. Writing last four exams in January. What a busy six months it was! This is my first ever posting, so I hope I'm at the right forum. Question on flight planning:
The fuel burn is 200kg/hr with a relative fuel density of 0.8. If the relative density is 0.75, what is the new fuel burn?
Does it stay constant at 200kg/hr or does it change to 188kg/hr?
I've had various answers from various sources. It is one of the sample questions from the JAA web-sight. Any help would be appreciated!
Cheers! I.
Just relocated to the UK and busy converting my ICAO licence. Writing last four exams in January. What a busy six months it was! This is my first ever posting, so I hope I'm at the right forum. Question on flight planning:
The fuel burn is 200kg/hr with a relative fuel density of 0.8. If the relative density is 0.75, what is the new fuel burn?
Does it stay constant at 200kg/hr or does it change to 188kg/hr?
I've had various answers from various sources. It is one of the sample questions from the JAA web-sight. Any help would be appreciated!
Cheers! I.
Joined: Sep 2002
Posts: 2,188
Likes: 6
From: La Belle Province
No doubt there's a "canned" answer for this "canned" question, but....
From an energy basis, it should be the same - you need the energy extracted from 200kg of fuel every hour, whether that energy is contained in 200/0.8 litres, or 200/0.75 litres, doesn't matter. (It will if your tanks fill before you can get all the fuel mass you need into them, of course).
That's for flight planning.
If the question were "instantanoeus", it would depend on your engine's fuel system - if it's a FADEC type controlling for a power output, then I'd say it'd stay at 200kg/hr, for the same reasons of energy/power considerations. If your throttle is controlling fuel flow (which would be likely volumetric) then it'd drop by the 0.75/0.80 ratio (as would your engine power output) and you'd end up having to advance your throttle to get back to 200kg/hr at a higher volume flow rate.
From an energy basis, it should be the same - you need the energy extracted from 200kg of fuel every hour, whether that energy is contained in 200/0.8 litres, or 200/0.75 litres, doesn't matter. (It will if your tanks fill before you can get all the fuel mass you need into them, of course).
That's for flight planning.
If the question were "instantanoeus", it would depend on your engine's fuel system - if it's a FADEC type controlling for a power output, then I'd say it'd stay at 200kg/hr, for the same reasons of energy/power considerations. If your throttle is controlling fuel flow (which would be likely volumetric) then it'd drop by the 0.75/0.80 ratio (as would your engine power output) and you'd end up having to advance your throttle to get back to 200kg/hr at a higher volume flow rate.
Fleet Manager
Joined: Apr 2001
Aviation Qualifications: ATPL
Posts: 7,447
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From: various places .....
The other practical consideration is that fuel sg usually only is super critical for ultra-longhaul operations, typically record attempts, where the maximum full mass is required for the tank volume - subject to maximum weight limits.
This is when you will see combinations of particular fuel origins, temperature etc. to maximise sg on the day.
This is when you will see combinations of particular fuel origins, temperature etc. to maximise sg on the day.
Joined: Jun 2001
Aviation Qualifications: ATPL
Posts: 1,187
Likes: 59
From: OZ
John T,
SG is regularly a major consideration for my mob. The nightly LAX-MEL may well be able to lift the weight of extra fuel, but the tanks have no spare volume. SIN-LHR, too, can be fuel volume limited for us. It's really a pain in the bum to have the weight available but not the volume for extra fuel.
SG is regularly a major consideration for my mob. The nightly LAX-MEL may well be able to lift the weight of extra fuel, but the tanks have no spare volume. SIN-LHR, too, can be fuel volume limited for us. It's really a pain in the bum to have the weight available but not the volume for extra fuel.
