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I'm learning to fly in a C152 and I've found the fuel calculation to be a little ambiguous. Based on information from my instructor, Internet and the Cessna 152 Information Manual (1980), I've home cooked the following formula and wanted to run it by the community for some verification.

I hope this is an appropriate use of this forum and please excuse my newb-ness. Here's my formula:

Fuel Required =
    5.6L unusable +
    5L run-up and taxi +
    24L/hour flight (rich) +
    18L (45 minute) reserve

Based on this formula, a one hour lesson conducted in the training area and below 3,000 feet AMSL requires a minimum of 52.6 litres in the tanks, calculated as follows:

5.6L unusable + 5L run-up & taxi + 24L flight + 18L reserve = 52.6L total

Or 26.3L in each tank. Since we're measuring fuel by the surprising method of dipping a stick in the tank, I'm realistically looking for around 25L per tank, and maybe topping it up to 30 per tank to be on the safe side.

Is this correct and reasonable?

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  • $\begingroup$ Hi Marky Mark, which jurisdiction do you fly in to apply a 45min reserve and use the metric system? (Or is it just the Cessna being a Reims-Cessna?) $\endgroup$ – SentryRaven Mar 18 '15 at 7:06
  • $\begingroup$ @SentryRaven It's a US made Cessna. The manual is in US customary units, but I converted the fuel volumes from gallons to litres since I'm not in the US. The reserve time was given to me by my instructor. $\endgroup$ – Mark Micallef Mar 18 '15 at 13:03
  • $\begingroup$ I should also add that fuel here is sold by the litre. Hence the need to convert. $\endgroup$ – Mark Micallef Mar 18 '15 at 13:06
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    $\begingroup$ @Pondlife Thanks! I should have used two more brain cells and looked there myself. :P $\endgroup$ – SentryRaven Mar 18 '15 at 13:22
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    $\begingroup$ I'm in Australia and the airport is YMMB. It's a mid-size GA airport in Melbourne, and home of the aero club. $\endgroup$ – Mark Micallef Mar 18 '15 at 13:24
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The short answer is: Yes, your calculation is reasonable.

Fuel consumption values are not an exact science, they are average values and rounded in many cases or based on pilot experience. Using 24L/hr in your example for traffic patterns is the right assumption, as you will most likely remain in rich configuration. Even if you lean a bit within the pattern, you would not gain enough fuel efficiency to offset these 24L/hr.

Your 5L run-up and taxi obviously depends on the airport you are based at or use the formula at. If this value was your calculation for a smaller airfield, you might need to adapt your formula when flying to larger regional airports or airports with high-traffic, where your taxi time could be significantly longer.

Be reasonable however with topping up your values. Going from 52.6L to 60L is the equivalent of 5.3KG, which you will need to consider in your weight and balance calculation.

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Normally, the practice in my experience is to fill the tanks in all cases to minimize the situations in which fuel starvation would be possible. For this reason, in the United States most trainers are rented wet. Exact fuel calculations usually are only relevant when flying cross country, and even then using generous reserves is a wise practice. During a lesson there are usually lots of unplanned activities, like the ole 30 minute run-up, waiting on the taxiway to take off behind 3 other aircraft that have some kind of problem, and in-flight distractions, so cutting it close on fuel is a bad idea.

A second issue is that if you fill the tanks only for what you need, any additional rentals will each have to fill the tanks, too, which can cause delays and throw off the rental schedule. For example, let's say the C152 is rented 4 times that day. If each pilot only filled the tanks for what he/she needed, then the fuel truck would have to come 4 times. Sometimes it takes a while for the fuel truck to arrive. If you need to taxi to a pump, that is another delay.

For all of these reasons, the normal practice for a student is to fill the tanks. If you are the owner/operator of the aircraft, you may choose to parsimoniously load the fuel you absolutely require, and save some money, but it does not sound from your post as though this is the case.

IMPORTANT CAVEAT: Overweight

If the aircraft would be overweight if you were to fill the tanks, then the above advice obviously cannot be followed. In that case, assuming it is a rental, the logical thing to do is two computations: your estimated need (which are already doing), and the maximum possible. If the need is greater than the maximum possible, you will need to change your flight plan. If the maximum is significantly greater than the need (enough for two or more lessons), then you should fill to the maximum, so that the next student does not have fill the tank, too. If the maximum is only moderately over the need, and the next student will have to fill it anyway, then you may as well fill it just to your calculated need.

As far as your calculations themselves are concerned, they seem right, aside from the use of the evil French units of measure.

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    $\begingroup$ OP is asking for correct methodology, not your particular experience in renting planes. $\endgroup$ – Erich Mar 30 '15 at 3:00

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