Leaning the Mixture for Takeoff and Landing
An e-mail expressing concerning about leaning the mixture prompted the following information. Some pilots are afraid of asking questions because they don't want to appear foolish in front of their peers. Remember the only "dumb" question is the one not asked.
"I was hoping you could give me some guidance for leaning for takeoff and landing at an airport close to our cabin in the mountains of North Carolina. The airport elevation is 2,020 feet (no laughing, oh, go ahead and laugh). The strip is 4,400 feet by 75 feet. I expect density altitude there to reach 4,000 feet to 5,000 feet during the summer. I own a 1974 Cherokee Six-300. My POH does not say anything about this subject. Thanks, Tom”
Well, Tom, several things come to mind when operating at your strip.
According to the FAA, density altitude is the “pressure altitude corrected for non-standard temperature variation.” This is the textbook definition. I prefer to think of density altitude as “the altitude at which the airplane thinks it is flying.” This is because the airplane will perform at the existing density altitude the same as it would at the same altitude in the standard atmosphere.
There are actually four variables that determine density altitude … altitude, pressure, temperature, and humidity. We don’t have an effective means of dealing with humidity, so we limit density altitude calculations to pressure altitude and non-standard temperature.
Occasionally a pilot will confuse the term “high density altitude,” thinking this means the airplane will perform well. The term means the air experiences a low density that is associated with a high altitude in the standard atmosphere.
Density altitude can be computed with a calculator, flight computer or a rule of thumb. The rule is that for each 10-degree Fahrenheit above (or below) standard temperature, add 600 feet (or subtract, if the temperature is below standard) to the physical altitude. This is a fairly accurate rule of thumb that will rarely result in an error of more than 200-300 feet from a calculator solution.
Standard temperature at any particular altitude can be determined by multiplying the operating elevation in thousands of feet by 3.5 (standard lapse rate) and subtracting from 59 (standard sea level temperature). For example, at the 2,020-foot strip, the elevation in thousands would be 2.020 times 3.5, or 7.1 from 59 = 51.9 degrees. The standard temperature would be 52 degrees. If the temperature is 85 degrees, subtract 52 for a difference of 33 degrees. This would equate to 3.3 (each 10-degrees above standard) times 600 or an additional 1,980 feet added to the field elevation (2,020 plus 1,980) for a density altitude of 4,000 feet.
Leaning the mixture to obtain maximum engine performance is predicated upon the density altitude, not the physical altitude.
Caution: Do not lean turbocharged or supercharged engines for takeoff.
But before addressing how to lean the mixture, remember for safety’s sake, to incorporate the rule of thumb to insure the airplane will become airborne within the runway distance available.
RULE OF THUMB — Determine if the Runway Length is Adequate for Takeoff
10 times the square root of the percentage of liftoff distance required is equal to the percentage of lift off speed that should be attained in that distance.
This might sound like a complicated rule of thumb, but it’s not. We use the halfway point (50 percent) of the runway as the percentage of lift off distance required. This is because an airplane will stop much easier than it will accelerate. So if the airplane reaches the half-way point of the runway and doesn't have sufficient takeoff speed, the airplane can easily be stopped on the remaining runway. This rule of thumb is not valid for a downslope runway or one with wet grass.
Then, 10 times 7.07 (the square root of 50 percent) equals 70.7 percent of the liftoff speed to be obtained by the halfway point of the runway.
There is the possibility of experiencing some problem with rate of climb after takeoff, so use caution if obstructions are present.
“Is the possible problem with rate of climb after takeoff a result of the mixture setting?”
The rate of climb after takeoff is a function of the density altitude, not the mixture setting. You can get an idea of the expected rate of climb performance with the following rule.
RULE OF THUMB — Density Altitude Takeoff Distance
Constant-Speed (Variable Pitch) Propeller — Density Altitude Takeoff Distance — To the standard sea level takeoff distance, add 10 percent for each 1,000-feet density altitude up to 8,000 feet. Add 15 percent of the standard sea level takeoff distance for each additional 1,000-feet density altitude above 8,000 feet.
Leaning for Takeoff
I hate to sit at the end of a runway and run up the engine to adjust the mixture. In addition to the possibility of picking up rocks and debris that may damage the propeller, adequate cooling is not available to the engine and its accessories. If the runway is too short to comfortably allow adjusting the mixture during the takeoff roll, use full power to adjust the mixture. Adjusting the mixture at the magneto check rpm isn’t advisable. Even without a power enrichener valve (maybe the engine is fuel injected, so you don’t have it), the fuel flow changes at higher power settings making it inadvisable to adjust the mixture with partial power.
Since your runway is long enough, I prefer to start the takeoff roll and begin leaning the mixture immediately. At the first sign of engine roughness, increase the mixture (push it in) until the engine runs smoothly. Warning: Sometimes when the mixture is pulled out the engine is going to stop. Don’t panic and shove the mixture control back in to the starting point or you’ve defeated the purpose of the adjustment. Make a small, smooth increase by pushing the mixture control back toward rich until the engine runs smoothly. This will give maximum power for the takeoff. Fine tuning of the mixture can be accomplished after climbing 400- to 1,000-feet AGL.
"I assume the sequence is: full power, gauges in the green, adjust mixture, and airspeed alive. Is this the correct sequence? Also, does adjusting the mixture during the takeoff roll affect reaching 70% of your takeoff speed by the halfway point?"
That is the correct sequence for the takeoff. The rule of thumb remains the same, obtain 70 percent of the liftoff speed prior to or at the halfway point of the runway. Adjusting the mixture just allows you to obtain that speed sooner.
And don’t forget to adjust the mixture for landing in the event that you need to make a go around. I'm not suggesting that a go around is caused by pilot technique, rather because an animal or airplane may be on the runway.
Blue skies, tail winds and safe flying!