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Fuel - Specific Ground Range - Page 4.36

  • brentonrule
  • Topic Author

brentonrule created the topic: Fuel - Specific Ground Range - Page 4.36

Hi All, Wondering what I am missing here...

Page 4.34 and 4.35 give a very specific and etailed description of how to calculate both Best range and Specific Ranges. Best Range is simply GS/Fuel Flow ie: GNMPG. Specific Ground Range (SGR) is simply Fuel Flow/Ground Speed in Gallons or Kg.... So.

Exercise 4.17 practice chart last line asks for the best SGR and at what power. This requires a check of each power setting at various Density Altitudes and Power Settings (a laborious task) that delivers 0.17gal/NM at 75 and then 0.16gal/NM at all other settings.

My answwer was 65% power as this gives the best GS and with all else equal would get you there in the shortest time.

Your answer uses the Best Range or Miles/Gallon as the basis of the answer.

Please let me know what I am missing here. Have i misread something in the question?

Thanks, Brenton :S

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  • Richard

Richard replied the topic: Re: Fuel - Specific Ground Range - Page 4.36

Hi Brenton,

There's some confusion arisen in the discussion of Specific Air Range and Specific Ground Range (both of which will be made a little clearer in the next edition of the book).

Basically, the formulas for SAR and SGR that you will be using in the exam are TAS/FF and GS/FF respectively. These give you your air nautical miles per gallon and ground nautical miles per gallon respectively.

The alternative form given in the textbook on page 4.35 (i.e. FF/TAS and FF/GS) can be used if you want to work out the amount of fuel needed to move one mile. If you were selecting for best range, you would then choose the setting that gave you the lowest value: the least fuel burnt per mile.

In fact airlines do this when they calculate how much it costs to move a passenger one mile in say a A380. They calculate the fuel needed per mile, work out how much that fuel would cost and then divide that by the number of passengers.

As you saw with your workings, you got 0.17 for 75% and 0.16 for the other settings. That's correct but unfortunately, you won't see the differences in the power settings unless you round to say 4 decimal places. Then you will see that 55% power does indeed give the lowest amount of fuel burn per mile.

Since there are only very slight differences in the answers, I would recommend you stick to using the SGR formula: GS/FF. This gives you the nm per gallon and the differences in the values you calculate for different power settings will be more obvious. In that case of course you would choose the highest value: the value that is giving you the largest distance for each gallon of fuel.

Bottom line:
When trying to find the correct power setting, use SGR = GS/FF.
In the exam you won't be expected to do an 8-way interpolation. In fact they don't even give you the TAS table - they just give you the figures to use :)
I understand you had to resort to looking at the groundspeed because you felt the SGR values you had calculated were all identical (due to rounding the decimals). In reality though, the GS is already factored into your calculations. The power setting with the highest GS will most likely not be the setting for best range: it may well be burning fuel at a horrendous rate to get that speed.



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  • wadner1

wadner1 replied the topic: Re: Fuel - Specific Ground Range - Page 4.36

I am having issues with this as well

Refer to page 18 of the work booklet.

Given -

TAS during cruise................... 153 knots
Fuel flow during cruise..............20.4 gph
Usable fuel on board at take-off.....150 US gals
Wind component at cruise level.......25 knot tail wind
Fixed reserve........................15 gallons

The maximum range that could be planned in an Echo aeroplane, ignoring climb and descent, for the charter flight described above is closest to -

1 1000 nm
2 1024 nm
3 1050 nm
4 1075 nm

Submitted Answer was Answer 4 (Incorrect).

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  • Richard

Richard replied the topic: Re: Fuel - Specific Ground Range - Page 4.36

Hi Sammy,

You first need to find your available flight fuel using the fuel at take off of 150 gallons. You are given the fuel at take off so you don't need to consider the taxi allowance.

Flight fuel available = (Total Fuel - Fixed Reserve) ÷ 1.15 = 150 - 15 ÷ 1.15 = 117 gals.

You have 117 gals available and you will burn them at 20.4 gal/hr. Therefore, your fuel will last 117 ÷ 20.4 = 5.74 hours.

What's your ground speed during those 5.74 hours?

TAS is 153 kt but you have a 25kt tailwind so your groundspeed is 178kt.

5.74 hrs @ 178kt = 5.74 x 178 = 1022 nm

The closest answer is (2) 1024 nm.

By the way, since you are getting these questions from your Exam Prep attempts, you can always go back and review your attempts even after you have submitted the exam. There you will see the answers worked fully for you. If you are not seeing these worked answers, let me know your ARN and I'll check it out for you.

EDIT: I just realised you are registered with the previous Cyberexam system which doesn't have the worked answers.



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