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I'm practicing for my EASA PPL(A) exams and one of the questions I came across looks like this:

How much taxi fuel must be consumed before take-off to reduce the aircraft mass to the maximum take-off mass?

Maximum ramp mass (MRM): 1150 kg
Actual ramp mass: 1148 kg
Maximum take-off mass (MTOM): 1145 kg (1,00 P.)

Possible answers:

  • 2L
  • 3L
  • 5L
  • 4L

Now, my thinking was Fuel needed to be consumed = Actual ramp mass - MTOM => 3L

But it seems the correct answer is 4L. I don't understand why.

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  • $\begingroup$ This looks like an ATPL question. Is this really on the EASA private pilot exam? $\endgroup$
    – John K
    Jan 18 at 4:18

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Your thinking is correct, the fuel needed to be consumed is indeed the actual ramp mass - MTOM. That results in 3 kg of fuel.

What you forgot is to convert the weight into a volume (note that the answers use litres). According to Wikipedia Avgas has a density of 0.72 kg/L, so the answer is:

$$ \frac{1148 \, \text{kg} - 1145 \, \text{kg}}{0.72 \, \text{kg/L}} \approx 4.17 \, \text{L} \approx 4 \, \text{L} $$

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    $\begingroup$ It is a good question! Converting between volume and weight is extremely important to get right in aviation. One example that comes to mind is the "Gimli Glider" accident (see wikipedia or youtube). $\endgroup$
    – ghellquist
    Jan 17 at 16:01
  • $\begingroup$ (Very much non-pilot here) If the exact answer is slightly above 4, and the only answer alternatives given are 2, 3, 4, and 5, shouldn’t the correct answer be 5, since burning 4 liters still leaves you 0.17*0.72 ≈ 0.12 kg overweight? Admittedly, 0.12 kg is probably within the measurement error, but I would expect that one would want any rounding to go in a "safe" direction. $\endgroup$
    – Ture Pålsson
    Jan 18 at 9:05

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