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9 Answers

lean mixture vs rich mixture

Asked by: 7550 views , , ,
Aircraft Systems, Private Pilot, Student Pilot

The feet given in the Pilot's Operating Hand book (POH) are in density altitude or pressure altitude? I think its density altitude as we know that destiny altitude is used for performance. However, for example, altitudes shown in the aeronautical charts are given in MSL. 

My POH says to start leaning once reach the 3000 feet, but it does not say in which type of feet (MSL), (Density altitude), (Pressure Altitude)

How can you determine what altitude the POH is talking about? 

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9 Answers



  1. KDS on Aug 04, 2019

    First, let’s look at the formula for density altitude:

    density altitude = pressure altitude + [120 x (OAT – ISA Temp)]

    Which brings me to an old military joke about the private who asked the sergeant how to convert the European road signs for kilometers into miles. The sergeant said to multiply by five, then divide by eight, and subtract one to get the miles. The private said he could see why he would multiply by five and divide by eight, but why would he subtract one. The sergeant smiled and said that was because that’s how far he would travel while he was doing all of that math.

    I’ll be anxious to hear the views of other folks here, but I’ll venture to say most people just look at the altimeter and say “Well, I guess I should start leaning here”. What I’m getting at is leaning is an excellent practice, but it isn’t an exact science.

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  2. Mark Kolber on Aug 05, 2019

    We lean based on the density altitude.

    Leaning and enrichening is the way we adjust the fuel/air mixture to obtain optimal engine performance. Keeping it simple, density altitude is essentially performance altitude. High density altitude means less air molecules per volume so we need to change the mixture to account for it.

    I demonstrated this to a flatland pilot just last week. Our airport is about 250 msl, Density altitude reported on the AWOS was about 1200. During the runup I demonstrated leaning for takeoff. Even at that fairly low density altitude, there was a noticeable increase in power as we pulled the mixture control back to reach max engine performance. He was surprised how far back the mixture control came.

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  3. KDS on Aug 05, 2019

    Adding to what Mark wrote, there is a classic accident that illustrates his point. Jessica Dubroff was killed in 1996 while departing the Cheyenne, Wyoming airport. The mixture was set to full rich. The elevation for the airport is 6,160′. Her flight instructor who was the PIC made several mistakes and that mistake was cited as one of the contributing factors in the accident.

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  4. Mark Kolber on Aug 05, 2019

    KDS, BTW, that formula is the rule of thumb, not the real density altitude formula. It’s fairly close for most purposes.

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  5. KDS on Aug 05, 2019

    Thanks Mark and thanks for mentioning that very important point about leaning. I suspect you have seen quite a few pilots from low altitude environments find themselves in trouble in your part of the world.

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  6. Mark Kolber on Aug 06, 2019

    No longer my part if the world.

    One of the interesting ones i saw happily involved no accident. Piper Archer cleared for takeoff. Reads back, pushes throttle forward, engine quits immediately. I keyed the mic, “Tower, you might ask if the Archer leaned for takeoff.” Archer came back with, “oh! Forgot about that.” Good thing it happened in the ground,

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  7. Warren Webb Jr on Aug 17, 2019

    It sounds like your POH may be for a C172. In the Section 4 takeoff and enroute climb checklists it says “above 3000 feet”, but in the Section 4 Amplified Procedures and Section 5 Takeoff Distance chart, it says “above 3000 feet elevation” so it is an MSL reference in the POH. However I had interesting experiences with leaning in more than one model. When the new C172R’s came out, the fuel servers apparently weren’t calibrated exactly right. These models had a lower rpm operating range and actually made us feel a little uncomfortable at first, so I checked to see if leaning made any difference. I was flying from a nearly sea level airport and when leaning before takeoff would get a surprising increase in the rpm. Over time this discrepancy stopped occurring – not sure if it was because of hours on the airplane or whether the fuel servers were adjusted or replaced. The proper rpm would occur at full rich. A turbocharged C182 was another model in which this occurred even though it was supposed to be left at full rich at all altitudes. When I flew once at Leadville CO (nearly 10,000 elev) of course the mixture was out a substantial distance. The bottom line objective is to lean for maximum rpm (or power) for takeoff and climb so that can be checked for any takeoff and climb by just pulling on the mixture control. If no increase return to full rich. From what I’ve seen the rpm increase and elevation at which it occurs can vary on both takeoff and enroute climb from model to model and day to day. If departing a low elevation airport and landing at a high elevation location, nearing destination go to a full power Vx or Vy climb and lean for max rpm so you will be prepared for a go-around.

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  8. Warren Webb Jr on Aug 17, 2019

    Sorry that should be ‘fuel servo’.

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  9. KDS on Aug 17, 2019

    Thanks Mark. A very interesting example.

    Yes, I keep thinking of you as a Colorado resident.

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