Hi for question no.15 in review questions set 2. Just wondering why the main fuel tank full of 100 gallons is 261 kg and not 272 kg and full auxillary tank of 80 gallons is 215 kg and not 217.6kg.

Your concern says, very clearly, that the confusion relates to what fuel SG values you are using.

Keep in mind that the old "standard" SGs of 0.71 and 0.72 (although still widely used) related, specifically, to 100/130 and lower octane fuels.

Both figures are largely irrelevant out in the real world of today but still figure in the exams and general use. If you have a small aircraft with smallish tank quantities, it doesn't matter all that much. For larger piston aircraft, the variation in weight can be significant.

Easiest way to find out what the SG for your fuel load is .. ask the refueller ... they pay for their fuel based on SG so they will know and be able to tell you. The actual SGs will vary according to source and temperature on the day.

Remember that, if the question stipulates the weight at take-off then the fuel in the mains would be less the 3 gallons taxi fuel. If you start up with 100 gallons in the mains, you will have 97 gallons at take off. It makes no practical difference to the final answer, after all the taxi fuel is an allowance only and on any one occasion you may not have burnt exactly 3 gallons - but that's CASA. The conversion factors CASA use in the exams are the ones in ERSA CON. The figures you have given for the exercise in the book do look a bit suspicious. I'll check them out. Thanks for your feedback. btw, how old is your book?

Hi Bob I got the performance 2003 textbook

OK. As you can imagine, a lot has changed in the last 15 years. That Question now gives the correct numbers.