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3 Answers

Leaning for taxi/takeoff at high density altitude airports

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Aircraft Systems

Prior to flying this aircraft, i have only flown a fuel injected cessna 172 at low elevation, low temp airports. Now i am flying a 172s at 3000ft and 100 degrees outside.   I feel there is a problem with the mixture, but with my limited experience Im not sure. 

For RPMs below 1400 the mixture needs to be almost fully aft, but at this mixture the engine will die if power is added much above 1600. I am leaning after startup per the cessna procedure of first going to 1200, finding best power, and then reducing to 1000.  If i go to 1800+ rpm, the mixture is put to about 1/2" aft from full.  At this mixture, the engine will not operate below 1400 without choking and getting near quitting.  The engine will not idle (below 1000rpm) at any mixture setting.

 

is this normal?

 

the aircraft just came out of a 100 hour and nothing was changed on the mixture, as i thought it might have been. This leads me to think its normal?

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3 Answers



  1. Mark Kolber on Jun 02, 2014

    I flew in Colorado for 20 years and never heard of the “Cessna procedure” you are describing.

    Are you trying to use the taxi setting as a takeoff setting? That would definitely be too lean. Fuel and power needs for takeoff are far greater than for taxi.

    SOP for takeoff just about everywhere I flew in a 172 was to set for best power (highest rom) at normal runup power (1709 rpm, enrichen 3 turns of the mixture vernier as an estimate of the additional fuel needs for takeoff power and engine cooling, and confirm proper rpm is being achieved on the takeoff roll (as one would do anyway.)

    For taxi, we typically guesstimated. But if you wanted to be more precise I guess you could use the procedure you describe. But realize that the goal on taxi is simply to avoid plug fouling and it would be quite rare to go above 1600 rpm for taxi, where power needs are pretty low.

    .

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  2. snowpilot on Jun 02, 2014

    I should not have said cessna procedure, it is found here, in a lycoming service bulletin.

    http://www.lycoming.com/Portals/0/techpublications/serviceinstructions/SI%201497A%20(04-12-2011)/Engine%20Procedures%20for%20Flight%20Training%20Operations.pdf

    I guess my biggest concern is that the engine does not idle at the mixture setting that is needed for maximum performance at high rpm. If i go to 1800 rpm or more, set the mixture for best power and leave it there, the engine will die at less than approx 1400rpm. I have little experience in higher altitude ground operations. Ive always had an engine that can idle at full mixture (which is what ive always used for takeoff in the cold, low elevation northwest)

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  3. Mark Kolber on Jun 03, 2014

    Yes, I’m familiar with the bulletin which describes both leaning for ground ops and for best takeoff power.

    If what you are describing is correct, that, in a 172, “If i go to 1800 rpm or more, set the mixture for best power and leave it there, the engine will die at less than approx 1400 rpm” there is definitely something wrong. That should definitely not be happening.

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