The air temperature was about 80 degrees, which is hot for an airport 7700 feet above sea level in the Rocky Mountains. I watched a pilot load his family into a 200-horsepower Beech Sierra, vacation done, ready to fly to… wherever.
WHY IT MATTERS
He fired up and taxied out, conducting the briefest of engine run-ups before taking position on the 8500-foot-long runway. I stood and watched as the Beech accelerated, oh so slowly. The pilot hadn’t paused to lean for a “maximum power” takeoff, and his slow progress along the runway showed it. He kept going, through 2,000 feet of runway, 4,000 feet, then, with more than 6,000 feet left behind, the little retractable gear airplane skipped its way off the pavement and very tentatively into the air. By this time, I’d trotted out to the edge of the runway, truly concerned about whether the airplane would claw its way into the sky — and clear the rapidly rising terrain off the end of the runway.
Apparently the pilot of the Sierra was concerned as well. Skimming the surface, and still in ground effect, he pulled up the landing gear as the Beech loafed past the far end of the runway. Now he was committed to fly, but if he couldn’t climb fast he’d hit the trees — before hitting the hill — less than half a mile ahead. That’s when he turned. Luckily for him (and his family), there was a small canyon off the end of the runway, about 90-degrees to his flight path. Turning right to line up with the trench, he disappeared from my view behind a hill… but was able to descend in the valley to pick up climb speed, then inch skyward as the canyon branched out into a broad valley.
They either got lucky, or this moronic method was their standard departure. Many pilots taking off at high density altitudes do not have the option of descending into a canyon after buzzing the earth in ground effect. This Sierra may not have been able to perform very well under these conditions, even if properly leaned. But if the pilot had taken the time to lean for takeoff, it certainly would have performed better — likely, much better.
MIXTURE MANAGEMENT: MAX-POWER TAKEOFF
Cruise leaning, which is only rarely taught to piston-engine pilots in any detail, is designed to deliver cruise speed (under “best power” conditions) or endurance (“best economy‘). For takeoff, though, you need to use a mixture setting that delivers best power and gives the added cooling effect of running at a richer mixture setting. You need to use a maximum power, takeoff mixture control.
Fixed-Pitch Props: Most of us learn in airplanes with fixed-pitch propellers, and little engine instrumentation. If you want (or in the case of a high density altitude takeoff, need) maximum takeoff power in such a craft, the procedure is to:
- Advance the throttle to FULL. (At high density altitudes, this will not yield full power.)
- Observe the static propeller speed (rpm while you sit still).
- Lean the mixture and watch the rpm increase as you develop more power.
- When the rpm reaches its maximum and begins to drop with additional leaning, enrichen the mixture just enough to re-achieve maximum static rpm.
- Release the brakes and go. You’re getting the most out of your engine for takeoff and initial climb.
Controllable-Pitch Props: Fly a plane like that Sierra, or anything else with a controllable-pitch (constant-speed) propeller, and you need a different technique (because the propeller will compensate for the power changes you make). You need to:
- Advance the throttle to FULL.
- Observe the Exhaust Gas Temperature (EGT) that results.
- Lean the mixture and watch the EGT increase.
- When the EGT reaches its maximum and begins to drop with additional leaning, enrichen the mixture to reacquire the maximum, or “peak” EGT.
- Note the indicated EGT. Then enrichen the mixture enough to lower the EGT 125 to 150 degrees Fahrenheit below the “peak” temperature.
- Release the brakes and go. You’re getting the most out of your engine for takeoff and initial climb, without burning it up.
Note 1: Many fuel-injected airplanes have markings on fuel-flow gauges that correspond to altitude. Simply lean the mixture until the indicated fuel flow matches the recommended rate for your present altitude.
Note 2: If the engine does not have an EGT gauge but does have a Cylinder Head Temperature (CHT) gauge, lean to find “peak” CHT, then enrichen to reduce that temperature by 100 to 125 degrees Fahrenheit.
Fully Instrumented Engines: If you’re lucky enough to have EGT information for each individual cylinder of the engine, use the technique described above while referencing the first cylinder to reach its “peak.”
- : This may not necessarily be the
- cylinder, but the “
first to peak
- ” is your takeoff leaning reference.
Turbocharged Engines: This one’s easy, unless otherwise addressed in the airplane’s Pilot’s Operating Handbook (POH), leave the mixture control at “full rich” for takeoff. Some manufacturers recommend a specific indicated Turbine Inlet Temperature (TIT) for a maximum-performance takeoff — if so, lean to obtain that indication.
MIXTURE MANAGEMENT: CLIMB
As an airplane climbs into thinner air, its normally aspirated engine loses manifold pressure — turbocharged engines lose air density as the turbocharger works harder and heats induction air as a side effect. In either case, then, you’ll have to lean to stay at your “best” mixture setting for climb. Lean to stay at peak rpm with fixed-pitch props, or to keep a constant EGT or TIT with more complex engines. As a rough rule of thumb, you’ll be leaning an additional five percent of indicated fuel flow for every 2000 feet of altitude increase — not a lot, but enough to make a big difference in continued, available power.
- : Some engines require different techniques, so don’t try any of this without first reading and complying with that airplane’s POH.
MIXTURE MANAGEMENT: APPROACH AND LANDING
Lastly, don’t blindly push the “red knob” all the way forward before landing. If the density altitude is high, you may need maximum power for a go-around or a missed approach — you certainly don’t want the engine to stumble and flood from a too-rich mixture when you push in the throttle.
- : In 2001, the FAA issued an
Airworthiness Directive to address an over-rich mixture condition
- Cessna 172R and S models, ‘which could result in rough engine operation or engine stoppage. The over-rich fuel mixture also contributed to the engine not restarting during flight when using published in-flight restart procedures.’ The problem occurred with the red knob full-forward as directed by the POH for certain flight conditions.
The “just right” amount to lean for approach and landing is something of an art. Unless you just took off from that locale, you don’t know for sure where the mixture needs to be for full throttle — the setting you’ll need for max-power if a go-around is necessary. You can, however, approximate what you need by moving the mixture control from full rich a bit towards lean, calculating and then looking for the approximate fuel flow or EGT/TIT you need to see in the case of a go-around. When you apply throttle and begin your climb, you’ll be ready to fine-tune your fuel flow to get the best results.
BOTTOM LINE: Many of us “flatland pilots” can go through our flying lives without ever having to lean for takeoff, climb or landing. But you need to get the technique done correctly the first time you need it “for real.” It’ll take some thought and practice, and you’ll need a moment at the end of the runway to configure the engine for takeoff at a high density altitude. Unlike that Sierra pilot, you might not have a canyon leading to lower terrain when you need it.