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6 Answers

Negative density altitudes and the mixture control…

Asked by: 1093 views Aircraft Systems, General Aviation, Private Pilot, Student Pilot

Most of the talk on leaning and mixtures relates to high altitude / density altitudes due to summer and heat. In the winter, our DA can often be -3000'. 

When in cruise (and of course at takeoff/landing) should I remain full rich due to the nice "thick" winter air?  What other considerations with mixture may I be missing in winter?

(For example's sake in a C172 or any similar carburated fixed pitch aircraft)
 

6 Answers



  1. Heather H on Feb 15, 2013

    I just wanted to expand a bit.

    For a POH that suggests leaning only above a DA of 3000, that would mean most of my winter flying would be full rich which seems like I would have thought about this a while ago…

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  2. David Brown on Feb 16, 2013

    Heather,

    That is an EXCELLENT question. I have seen even in my warm location, days with high QNH and low temperatures where my engine is producing greater than rated power at low level. You will be enjoying this even more so.

    You mention a C172, so most likely a carburetted engine. These devices work on the principle of MASS AIRFLOW and thus self control themselves. More mass airflow, the more they suck fuel, less mass airflow the less they suck fuel.

    A TCM engine in say a Bonanza with an injection system controlled by RPM of the fuel pump is a different matter.

    As for leaning, at lower power settings you can lean just like any other aircraft in any other location. Helps of course if you have an engine monitor. But you can work out % power on the rich side of peak EGT by taking the % of MP relative to 29.9″ times the % of full rated RPM.

    In other words if you had 90% MP (around 27″) and about 90% rpm (around 2430) you would have near enough to 81% power if ROP. Here is the Advanced Pilot Seminars suggestion on the appropriate leaning for you to use with the above.

    • At 65% power, use richer than 100 ROP, or leaner than peak EGT.
    • At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
    • At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
    • At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.

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  3. Nathan Parker on Feb 16, 2013

    “For a POH that suggests leaning only above a DA of 3000″

    This is a misreading of the C172 POH. It says to lean in a climb when the DA is above 3000. In cruise, you should always lean in accordance with the POH.

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  4. David Brown on Feb 17, 2013

    “In cruise, you should always lean in accordance with the POH.”

    And there in lies the biggest problem. POH’s have been full of misinformation, contradictions and basically short of good information since the late 60′s.

    The Chieftan engine is a classic, following the book is not something I would let anyone do if it were my engine, plane or me in it. You should see POH recommendations on the Dyno…Frightening to say the least.

    Remember Section 2-Limitations is the only section the FAA actually focus on. Anything else is a manuals suggestion on one way to skin the cat. We could argue this for days, but when it comes to engine matters the POH is a very poor resource as are the publications that Lycoming produce at times.

    Yet at other times they produce very good ones, but these are usually about maintenance and limits, not handling or operation.

    Heather asked a question that the POH does not cover or cover well. So no point refrring her back there when the explanation is not sufficient to satisfy her needs.

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  5. Nathan Parker on Feb 17, 2013

    “Heather asked a question that the POH does not cover or cover well. So no point refrring her back there when the explanation is not sufficient to satisfy her needs.”

    Actually, the Skyhawk POH covers this material adequately and her misunderstanding is due to a misreading of the POH, or, more likely, making the mistake of listening to a flight instructor who never read the POH either.

    As for whether the POH or engine manufacturers are “poor resources”, they’re likely to be statistically better resources than random people on the internet, who probably are just passing along stuff they heard from somebody.

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  6. David Brown on Feb 17, 2013

    Nathan, if you read further above you will find that the sources are not some random on the internet. However in general terms yes I would agree. In this case the response to the question is not from some random.

    I dispute where the 172 POH covers this question adequately, in fact having reviewed, just so I was sure, I have reviewed a C172N and C172S manual and I can see exactly why Heather has asked the question.

    A famous Quote from a chap named Boswell, “It is not what you don’t know that hurts you, It’s what you know that isn’t so!”

    Nathan, the vast majority of flying instructors and the A&P’s that work on these things are the reason there is so much misinformation around. The fact POH’s are very poor resources as far as engines goes is rather sad. Weight and balance, emergency procedures, airspeeds etc are covered in great detail. The poor engine up front, nothing worth reading.

    How about you ask some other “randoms”, George Braly, John Deakin or Walter Atkinson, just a few randoms on the internet, who know more about this that most. Ask them about some POH’s, better still if you don’t believe me, book in for their Advanced Pilot Seminar in Ada OK in March. It will really open your eyes! And no I have no financial interest in sending you there.

    Heather, if you have any questions that you would like to ask me directly, email me at davidbrown ‘at’ advancedpilot.com

    Best Regards

    David Brown
    Advanced Pilot Seminars – Australia

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