‘Cruise computer’ in POH

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    In the POH for my newly acquired 421C there is frequent mention of a ‘cruise computer’, to help calculate %power and fuel flow etc. I presume this is some sort of circular slide rule-like contraption, but one did not seem to come with my aeroplane. Does anyone know where I could acquire one of these gizmos?


    Yes, I noticed that too. If anyone has one perhaps we can use it as a model for making up a bunch of new ones, or the basis for a iPhone/android app?

    Also, does anyone recall the formula for LOP HP for 7.5:1 ratio pistons? It is basically a straight multiple of fuel flow (which is easier then calculating ROP % HP as that is affected by temp, MP, rpm, etc. )



    I recently bought a Kilo and had the same question about the power computer. It is indeed a circular slide rule made by Cessna back in the day. I couldn’t find one for a 310, but I did find one for a 206. According to my mechanic the power computer is based on the engine type in respect to MP, RPM, % power, fuel burn, etc. So you’ll have to make your own adjustments for speed (and multiply by 2 for the fuel burn) but it seems reasonably straight forward to recreate these.

    I’m drawing one out off of the 206 version at a size my old eyes can see (the original is only about 2-1/2 to 3″ in diameter), if anyone has any other information I’d love to hear it, otherwise you’ll be stuck with my version once it’s complete.


    The HP % formula for LOP operations is:

    FF X 13.7 / MAX RATED HP X 100

    Only works for LOP. For ROP use the POH.


      This is what your looking for. Back in the day our policy was to use this computer every 30 minutes of flight. Your constantly changing RPM to maintain the same % of power and fuel flow. Essentially, every 100 miles your flying in a different air mass. Simple to use and a lot better than doing the math equation from the charts.



      Please give an example of your LOP %HP calculation.


      quote :


      Please give an example of your LOP %HP calculation.

      Sure. Here is an example for my plane. Keep in mind, my MP is ALWAYS set at 31″. I never change it LOP…only the FF.
      If the temps are cold, I can run my FF to 17GPH and still keep the CHT’s below 380 degrees. In that example, here is how the formula works:

      17 X 13.7 = 232.9 HP
      232.9 / 335 (max rated HP) X 100 = 69.5%
      So at 17GPH, I am running my engines just under 70% of rated HP.

      In the summer (ISA + 10 degrees or more) I’m may have to reduce my FF to 15.5GPH in order to keep my CHT’s below 380 degrees. Here is the formula in that case:

      15.5 X 13.7 = 212.35HP
      212.35 / 335 X 100 = 63.4% of rated HP.
      CHT’s are cooler because I am running a lower power setting…even tho my MP is still 31″.

      To keep my engine cool using the RAM Book power settings ROP, I have to increase FF to 22GPH and at the 28 inches of MP the book shows I am producing about 70% of rated HP.

      Does that make sense?


      I believe 13.7 factor is for low compression engines.

      For 8.5/1 compression engines use 14.9.

      Example: 14.9 X 24 gallons/hour = 357.6 / 520 total HP = 68.8% HP ONLY LOP OPS.

      HP during LOP ops is only based on fuel flow, HP ROP is more complicated.




      Yes. Thanks!


        To me the above posts are a bunch od hogwash. Who cares if the engines are rated at 335 or 310 HP because that is only for takeoff power and has nothing to do with cruise I doubt the engines would make many hours if run at 41″ MP and 2700 RPM. They will run at 38.5″ MP and 2700 all day as that was what they were really tested for. RAM did a paper shuffle and got the FAA to buy off on 25 hours of max power. So you really have a 335 HP engine for a few minutes and then you revert back to the 310 HP regime. So figuring % of horsepower at 335 makes one feel good but doesn’t do any good that can’t be had by using any of the horsepowers available for the 340 and 414. If the CHTs run somewhere around 380 dF and you are happy with the fuel flow and miles per gallon (nautical miles per gallon)(BSFC), then use what you want. Go fast, long endurance, save fuel, build time, etc.

        All the Cessna and RAM numbers have no correlation to today’s reality as LOP was not on the books 40 years ago and RAM doesn’t believe in it. But if you want to run ROP and consume lots of 100LL, foul your plugs, dirty your oil and change cylinders at 800 hours go for it.
        Dick W


          I’m with you Dick, it’s all hogwash.

          All I see is $$$$$ when a customer comes to my Avionics shop and insists on having an expensive Engine Monitor installed on his airplane. It’s the same old story over and over. “This will allow me to save money on troubleshooting and I can run LOP and I’m going to save a ton.” For us in the industry it’s the gift that just keeps giving $$$$$.

          These engines are not Exotic European Racing engines. They are more like John Deere engines. We are well trained Pilots. Most of us are not Flight Engineers.

          There is a goog reason the mixture knobs are RED. You pull them back the airplane slows down. Pull them back too far and the engines quit.

          Fly Safe Fly Often


          quote :

          If the CHTs run somewhere around 380 dF and you are happy with the fuel flow and miles per gallon (nautical miles per gallon)(BSFC), then use what you want.

          I agree with that!
          However, to do that with any accuracy (either LOP or ROP), you need an engine analyzer to measure CHT. In my plane, the CHT difference between the hottest and coolest cylinder is often 60 or more degrees and since stock instrumentation doesn’t monitor individual CHT’s, an engine analyzer is a way to do that. If the owner doesn’t care about individual CHT’s, then I agree with Pat…an engine analyzer is probably a waste of money.

          OAT, speed, and altitude will affect CHT’s at any given power setting. Using an engine analyzer allows me to reduce my fuel burn by 10GPH while keeping all my CHT’s below 380df. Without the analyzer, I would have little idea what my CHT’s are.


          Pete +1


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