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3 Answers

Performance chart interpolation

Asked by: 4007 views , , ,
FAA Regulations, General Aviation, Instrument Rating

Hi. So I'm working on my IFR rating, and was given the task to write up a flight plan from KTCY to KRNO. The plane I fly is a C172 and it's service ceiling is 14k, so technically it can fly over the mountains to Reno. However, the performance charts for time distance and fuel to climb only goes up to 12K. And since this is supposed to be an IFR flight, flying East requires an odd-thousand cruise altitude. The lowest MEA victorway route I could find without routing around the mountains was 11.5K, so I would need to fly 13K cruise altitude. So my question is, can I interpolate the performance chart up to 13K? Or would that be "test pilot" flying and considered unsafe?

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3 Answers



  1. BJ Miller on Nov 02, 2015

    I don’t know the strict legality of it, but I can think of several reasons right off the top of my head why I would not accept a student doing it. The first question you need to ask yourself is “What would my insurance man say if something went wrong?” I certainly wouldn’t be the guy standing there with an interpolation of a chart to support my decision. Neither Lycoming (assuming a 180 hp model) nor Cessna will have your back if you choose to interpolate performance charts.

    Also, without “getting into your cockpit”, I would ask that you reinvestigate your decision to fly directly over the mountains in a C-172. If it is simply to save time on the Hobbes, then you are making the choice for all the wrong reasons. Mountain flying requires that you consider your “outs”. The average C-172 is not well suited for mountain flying. In ABQ, we use the 210 hp version because the standard 180 hp just couldn’t get it done. Most of the local pilots out here who taught me to avoid the mountains to maximum extent practical. From a power standpoint, you will probably not leave yourself much of a margin at the altitudes that you are proposing to fly. Once again, you are robbing yourself of an “out” should the winds shift over the mountains. If you can find a slightly longer route that allows you to preserve power and perhaps even has some emergency “outs” (i.e. better terrain, airports, highways), then you should be asking yourself “why not this route?”

    If you haven’t done so already, I recommend doing a little reading on mountain flying. I learned to fly in southern California and I can’t remember even bringing it up out there. I can tell you that most of what I just passed along to you, I learned from folks who are much more experienced in this type of flying than myself. The Air Safety Institute has a decent course on this that you can take for free in about 20 minutes. It might give you some things to consider. Without knowing your level of experience, I hope that I am not talking down to you, but my guess is that there is a lesson that your instructor wants you to learn beyond the nuts and bolts of “paper” flight planning. A good IFR pilot is first and foremost a good (safe) decision maker. As a former SoCal flyer who now lives in the mountains, I can’t stress enough how a key piece of your mountain planning should be arming yourself with the information to make mountain flying planning decisions.

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  2. Russ Roslewski on Nov 02, 2015

    Regarding proper altitude for direction of flight, I suggest you re-read 91.179, with special attention paid to the difference between (a) and (b).

    The tradeoffs you are facing (routing around, over, through the mountains, and having to select an appropriate altitude given your aircraft) are undoubtedly the whole point of your CFI having you plan this flight.

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  3. Kris Kortokrax on Nov 05, 2015

    As Strother Martin said in “Cool Hand Luke”, “What we’ve got here is failure to communicate.”

    Lycoming and Continental would indeed “have your back” if you choose to interpolate. Interpolation is determining a value which falls between to stated values. For example, your charts show values for 10,000 feet and 12,000 feet, but you need a value for 11,000 feet.

    What you are proposing is called extrapolation. In other words, you are attempting to come up with a value outside the range of stated values.

    Extrapolation is mentioned frequently in AC 23-8C Flight Test Guide for Certification of Part 23 Airplanes. However, in this case, the extrapolation is being done by engineers. It is done in a conservative manner. If you are asking this question, it is unlikely you possess the knowledge to perform the extrapolation.

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