Fuel burn vs fuel density

[Ibanez]

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.

[Mad (Flt) Scientist]

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.

[john_tullamarine]

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.

[Ibanez]

Thanx for the replies! Some questions in these exams are far from practical! Anyways, I'll go for answer b - 200kg/hr.

Hope to be in the air again soon.

HAPPY FLYINGS!: :ok:

Regards, I.

[mustafagander]

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.