Q: At or around sea level in a non-turbo airplane, like a 152 or a 172, should you lean your air fuel mixture for takeoff?
The reason I am asking this is after seeing a video of a flight in a 172, I noticed that the pilot leaned the mixture before takeoff. I wrote to him asking why he would do that and this was the answer I received: “Truth is, very rarely in a non-turbo aircraft will you take off with full mixture, even at sea level. Obviously it depends on the particular engine you’re flying with, but that’s been my experience in general aviation. It’s very common practice to lean the engine like that.”
I’m not a pilot, just an avid fan of general aviation. I realize there are always exceptions to the rule but, for the most part, I thought full rich at takeoff and landing was one of those written in stone rules that you did everything in your power not to break. I was really expecting something more involved than the answer “it’s very common practice.”
A: The topic of leaning Lycoming engines has always been a subject that brings to light several viewpoints, so I’ll try to answer your question without ruffling too many feathers of those who always seem to know all of the answers regarding this subject.
First and foremost: You should never lean the mixture at sea level for takeoff! You should always use full rich mixture for takeoff and climb. However, there are always some exceptions and density altitude is one that comes into the equation regarding leaning.
I’m sure almost everyone is familiar with the much talked about 5,000 foot concept that commonly was quoted as “never lean below 5,000 feet.” This statement has been around for a long time and isn’t necessarily true.
As an example, if you were taking off from an airport in Denver on a hot summer day, I’ll bet very few, if any, normally aspirated Lycoming engine would be able to produce full rated power under these conditions. To deal with this, you may adjust the mixture to achieve smooth engine operation. For fixed-pitch propellers, you may lean to maximum engine RPM at full throttle prior to takeoff. Common sense would dictate that you limit ground operation at full throttle to a minimum in this configuration.
If you can remember the following rule, I don’t think you can get into any trouble, keeping in mind that this applies only to normally aspirated engines: “You may lean at any altitude as long as you do not exceed 75% of rated engine power.”
This puts the responsibility directly on the pilot to calculate the power before making any decision to lean the mixture. I truly believe this is where the “5,000 foot concept” came from many years ago, considering that at that altitude normally aspirated engines are typically below the 75% power point.
For those folks who have an aircraft fitted with instrumentation, such as a CHT or EGT that help define how the engine is operating, it’s important — nay, vital — that these systems be calibrated to assure the information you are seeking from them is accurate. Ideally, these should be calibrated annually, but we all know 99% of owners will not spend the money to do this. For the engine types you are speaking of, this suggested annual calibration may be a bit of an overkill, but I still believe it has merit if you really want to know what’s going on in your engine.
I’d like to expand on this by including a couple of other instruments that should be included in the annual calibration. I would definitely put the engine tachometer at the top of the list before all others. The reason for this is that, quite simply, if the tach is giving you a lower RPM reading than the engine is actually turning, you may not be providing the required amount of fuel while leaning to support the power you are taking out of the engine.
In the specific engines (fixed pitch propeller applications) your question refers to, this may not be a big deal, because when leaning at 75% power or below there are two indications that will tell you that you’ve reached the optimum performance at that given power setting: Either the RPM reaches a peak or the airspeed indicator reaches a peak. Were you to slowly continue to lean the mixture you would soon notice some slight engine roughness, which should be corrected immediately by enrichening the mixture to obtain smooth engine operation. Always remember to move the mixture control slowly when making any changes and to always return the mixture slowly to full rich before increasing power.
One thing you must remember while operating any Lycoming engine is to always maintain the recommended limits for the cylinder head and oil temperature. On the engines you mentioned, the maximum CHT is 500°F and the maximum oil temperature is 245°F. Ideally, you should aim for a CHT of around 400° or below for continuous operation. Ideal oil temperature should be between 180° and 220°. Seeing these figures may help everyone better understand the importance of having calibrated instruments and eliminate any chance of exceeding the limitations set by the engine manufacturer.
Let’s be honest here: Some of the gauges used in aircraft built 30 years ago probably had questionable accuracy even when new. I’m certain we can all agree that they haven’t improved in accuracy over time, so why not have them calibrated?
Bill, I’d like to thank you again for your question and I might add that the response you received from the pilot in the video that “this is very common practice to lean an engine like that” was a shock to me as well. What really concerns me is that the procedure was shown in a video and seen by who knows how many pilots, including some student pilots who would take this procedure as the gospel. Needless to say, this type of procedure could lead to serious consequences.
I would like to encourage everyone to review the engine manufacturer’s publications referring to proper operation. Specifically in regards to leaning, refer to the latest revision of Lycoming Service Instruction 1094. As of this writing SI 1094D, dated March 25, 1994, is the current revision.
Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.