GPH variance, full rich vs properly leaned in a C152

7 Answers

1. Best Answer
   
   

   
   

   John D. Collins on Dec 22, 2012

   It all depends on how conservative you want to be. The manual suggests that using cruise power settings at or below 65% using full rich mixture verses using their recommended leaning for climb and cruise will result in a fuel burn 13% higher. If you want to be very conservative, assume 3 gallons for taxi and climb and 3 gallons for reserve, this leaves 16.5 gallons for cruise. At 6 gallons per hour (a high value), that allows for 2 hours and 45 minutes during the cruise phase. One mistake you do want to avoid is to base your fuel burn solely on the distance as the winds aloft may change or be higher than anticipated. What won’t change is how long the engine will run until it quits due to fuel exhaustion. Just because your plan suggests that you have plenty of fuel to complete a cross country, doesn’t mean you will make the engine run any longer in terms of time. Always keep aware of the time you have been airborne and divert early in the flight, long before you get into your reserves.

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   Bill Trussell on Dec 22, 2012

   Heather,

   It is important to remember that the performance charts for any aircraft are produced as part of the flight test program and represent an accurate picture of the performance under certain conditions, including those given in the notes on the chart. Your question is a good one in one sense that you are trying to be conservative in your flying. Bravo! On the other hand, why would you think that you would ever forget to lean if you are using your check list?

   Climbing down off my soap box now and off to your question…. there is no accurate way to extrapolate the peformance figures to get to the one you are seeking. A better measure would be to set a fuel consuption figure somewhere between that required for climb ( consumption vs. time to climb) which is the very worst case, and the book figure for cruise. This would place your number somewhere between 6.1 GPH on average for cruise and 9 GPH for climb. This range is too broad to be practical but it illustrates a point.

   A better figure to use is an average fuel consuption based on a number of typical fights including takeoff, climb, cruise and approach and landing. This would be measured by departing with full fuel, and filling up after you land, taking gallons used over time on the hobbs meter. Doing this test at full rich a few times would get you the number you are looking for. I am not advocating that you become a test pilot but this is the process required to get to your conservative number.

   For a flight test I would highly recommend that you use your POH figures for each phase of flight, calculate your total consumption and then note your conservative number as an effort to be even safer than the book figure, then make sure you use the checklist…..you will be glad you did!

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   Heather H on Dec 22, 2012

   John & Bill, both your answers are so very helpful, I appreciate you taking the time to help. I think since since training is over, I’m being extra-extra cautious. That’s great, but I’m probably going overboard. Most of my future planned flights I’d like to do are in the 2 hour range, 170-200nm.

   I’ll leave this open to further discussion/comments.

   Have a great Christmas and holiday season.

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   Mark Kolber on Dec 23, 2012

   The difference is also going to depend on your altitude. Higher altitude, less air density, smaller fuel flow. The, of course, at or above certain altitudes, you’ll get a reminder to lean, like a rough running, sputtering or even quitting engine.

   For planning purposes I generally assume the worst cruise fuel burn and leave an hour reserve. My working assumption is that the reduction of power during descent will make up for the increased fuel burn for takeoff and climb. It forgoes the extended calculations and since my bladder holds less hours than a fully fueled 172, it seems to have worked out fine. And, on refueling it’s even turned out to be accurate. It’s a planning shortcut for light aircraft my CFI taught me early on and has worked well from the Tomahawk i trained in through 6-seat Bonanzas.

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   Bob Watson on Dec 23, 2012

   If you’re no longer training, then you can do your own studies to see (of course, you could also do this while you’re training, but CFIs like to mix things up during a flight which is good for learning but bad for experimental data collection).

   What I mean is that you could plan out a sample trip, do all the flight planning like you would if you were trying to impress your instructor (fuel burn by flight segment/leg, time en-route taking into account the weather, etc.) and fly the flight plan as close to what you planned as possible. Then, when you refuel, find out how much fuel you used vs. what you expected to use from your plan. Identify any deviations from your plan that would influence fuel consumption (e.g. longer than usual taxiing, deviations from the planned route/altitude, etc.) and with that data, you’ll be able to see how accurate the book numbers are to how you actually fly. After that, you’ll know how much to adjust them for future flights. Of course you’ll still want to add all the appropriate safety margins. For example:

   Estimated fuel required: 10 gallons
   VFR reserve: 3 gallons
   Fudge (safety) factor: 2 gallons

   So, with these numbers, you wouldn’t leave with less than 15 gallons, but the gas truck should put in 10, after you return.

   If you fly the same plane all the time, you should get to be pretty good at estimating how much fuel it’ll take to fill it back up after each flight. IMO, it’s a good habit to make a point of comparing your estimated to actual fuel burn after every flight, even (or especially) if you’re just “ball-parking” it.

   Another tip, the Cessna manuals don’t make this particularly easy (when compared to the Piper Cherokee manuals, anyway), but if you can find the 65% (or 75% or whatever you like to cruise at) power numbers for the different altitudes you tend to fly at, you’ll notice that the fuel consumption is pretty consistent at the different altitudes. So, by using your favorite power setting (throttle or throttle & prop in complex airplanes) for the altitude you can assume the same fuel burn. But, remember to read the fine print. Most performance charts specify some mixture setting (e.g. 25 degrees rich-of-peak, or something similar)

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   Bob Watson on Dec 23, 2012

   Just to clarify the apparent disagreement between what I said and what Mark said about fuel consumption and altitude.

   If you adjust your throttle and mixture to produce the same power at different altitudes, the fuel burn will be about the same at every altitude. You will, of course, have to apply more throttle as you climb to produce the same power that you had at a lower altitude, but the net fuel consumption will remain pretty close to constant.

   If you don’t change the throttle, but still adjust the mixture as you climb, you’ll use less fuel as you gain altitude, but your [non-turbocharged] engine will be producing less and less power as you gain altitude.

   Basically, fuel = power (when the correct mixture is maintained).

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   Mark Kolber on Dec 26, 2012

   I agree with Bob and his clarification (which clarified my thoughts). Heather, if you go through that cruise chart in the 152 manual, you’ll see that for every altitude (and temperature) at which the 152 is =capable= of producing 65% power (about 10,000′ at standard temperature) the fuel burn will be about the same – about 5.3 GPH according to the chart.

   What proper leaning will give you is the ability to produce that fuel flow and power at that altitude. A too rich (or too lean) mixture will end up producing less power than optimum (and end up wasting fuel).

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