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6 Answers

Estimating endurance

Asked by: 1807 views General Aviation, Private Pilot, Student Pilot

I'm a private pilot working on my CPL. I'm a latin guy so not a full English proficient, thank you for understanding.

Before my checkride, the inspector asked me about C-172 fuel consumption rate, in my school everybody says that 172 consume 8 gallons of fuel per hour. We have/had (some involved in accidents) 172 N, Q and P, I think . All have 6 cylinders engines. I answered him based on that rate, but he told me that is a common error to estimate endurance based solely on the aircraft published cruise fuel burn rate, but didn't told me anything else. I know gross weight, altitude and wind are determining factors on fuel burn rate but have never heard about how to calculate aircraft actual fuel consumption considering those factors. Was I wrong?? Please help me with that.

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6 Answers

  1. Kris Kortokrax on Feb 09, 2015

    What you should have been taught by your instructor is to look at the cruise performance chart. For the CE 172N, it is found on page 5-16 of the POH. Based on your altitude, temperature and power setting, you can determine the fuel burn.

    One size doesn’t fit all.

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  2. Dan S. on Feb 09, 2015

    I wanted to add here that wind and weight have nothing with fuel burn rate, directly. They will effect how LONG the flight takes, but the only things that truly effect fuel burn rate is altitude, temperature, and power setting.

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  3. Wedge on Feb 17, 2015

    What your inspector may have been getting at is accounting for ground and climb fuel burn. Don’t forget that you must subtract a certain amount of fuel for taxi and takeoff. Also, our climb burn is certainly much higher than in cruise. Cessna makes this calculation easy with a ‘time, fuel, and distance to climb’ chart. Make sure to remove that fuel in your calculations before estimating endurance with your cruise burn. You will find all of this information in section 5 of your POH. Pay close attention to notes for temperature, wheel fairings, etc. on all charts.

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  4. Wedge on Feb 18, 2015

    I forgot to mention that the climb burn should be removed but re-added since you are flying for that time as well. The equation I have use is:
    Fuel on board in time = ((total gallons – taxi and climb gal used) / cruise gph) + climb time in minutes

    I hope that’s somewhat easy to read… It’s just removing the ground and climb fuel used and replacing it with a time. We don’t know the gph in climb but we are given the total amount of fuel used and the time it takes to use it.

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  5. Mark Kolber on Feb 18, 2015

    Wedge, I think your answer that the examiner was looking to take climb and descent into consideration is spot on.

    I’d add that the examiner probably referred to it as a “common error” but didn’t go further is that it’s relatively irrelevant to real-world flying.

    First, to be really accurate, you would have to also figure the fuel burn for the descent and approach, which is not given in the tables, at least in the piston POHs I’ve seen. So even following the tables, the calculations aren’t really accurate.

    Second, the fuel burn for descent and landing is typically at a lower (or lowering) power setting, which will, for the most part, balance off whatever increased fuel use you had at the higher power settings for the climb.

    Third, as we all know, those tables are based on a new, clean airplane with no dents and dings, flown by a pilot who is using excellent coordination technique and ideally leaning the engine. I’m not too sure how many of us fit into that category 😉

    The cruise fuel burn is probably going to get us into the exact same ballpark as using the fuel/time/distance to climb tables, except perhaps for the time element. And for that a simple rule of thumb will work also (almost 24 years ago, when I did my first dual cross country, my CFI suggested adding one minute for each 1,000′ of altitude for the climb and it worked out very well for the many years I used it).

    Yes, the methodology needs to be learned, understood and tested. But like VFR checkpoints 10 NM apart on a cross country, is more of a training exercise than a reflection of the world.

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  6. Kris Kortokrax on Feb 18, 2015

    Checkpoints??? In this age of iPads and Android tablets, does anyone remember what a checkpoint is?

    The time, fuel, distance chart is based on Maximum Rate of Climb, standard conditions and no wind. That’s not going to do me much good, because I like to be able to see over the nose in case there is another aircraft in the sky and seldom find a no wind day to fly. One size doesn’t fit all and like you said, Mark, we’ll be in the ballpark using cruise numbers. It is important to use the correct cruise number, though.

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