ANN's Daily Aero-Tips (05.15.06): Fuel Tutorial #5: Percentage Of Power | Aero-News Network
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Mon, May 15, 2006

ANN's Daily Aero-Tips (05.15.06): Fuel Tutorial #5: Percentage Of Power

Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.") It's part of what makes aviation so exciting for all of us... just when you think you've seen it all, along comes a scenario you've never imagined.

Aero-News has called upon the expertise of Thomas P. Turner, master CFI and all-around-good-guy, to bring our readers -- and us -- daily tips to improve our skills as aviators, and as representatives of the flying community. Some of them, you may have heard before... but for each of us, there will also be something we might never have considered before, or something that didn't "stick" the way it should have the first time we memorized it for the practical test.

It is our unabashed goal that "Aero-Tips" will help our readers become better, safer pilots -- as well as introducing our ground-bound readers to the concepts and principles that keep those strange aluminum-and-composite contraptions in the air... and allow them to soar magnificently through it.

Look for our daily Aero-Tips segments, coming each day to you through the Aero-News Network. Suggestions for future Aero-Tips are always welcome, as are additions or discussion of each day's tips. Remember... when it comes to being better pilots, we're all in this together.

Aero-Tips 05.15.06

We're still talking about it -- avoiding fuel exhaustion through better fuel planning. Today let's look at percentage of power.

From Advisory Circular 61-23C, the Pilot's Handbook of Aeronautical Knowledge:

Fuel Consumption

The rate of fuel consumption depends on many factors... particularly the percentage of horsepower used for flight at cruising speed.

Percentage of power

Percentage of power is the measure of power development as compared to that engine's maximum rated horsepower. For instance, a 150 horsepower engine being run at about 112% is operating at about 75% power. Any number of combinations of manifold pressure, propeller speed and mixture setting may result in a given percentage of power.

If you're flying an airplane with a fixed-pitch propeller the concept of percentage of power is not terribly usable—not because it doesn't apply, but because you have little control over the variables. You typically won't even see reference to percentage of power in these airplanes' performance charts.

If you're flying a high-performance piston airplane its handbook usually gives only a few combinations of manifold pressure and rpm for specific percentages of power. All are based on the sometimes-elusive "book" leaning technique. Many Pilots Operating Handbooks (POHs) have gotten away from the whole idea of percentage of power, offering instead simple "Recommended Power Settings" performance charts for a limited range of unidentified power settings.

Unless you precisely calculate percentage of power using charts or products that take mixture and a wide range of MP/RPM settings into account, you can't truly determine percentage of power. Remember that the horsepower curve is directly related to fuel flow—percentage of power defines fuel requirements.

Aero-tip of the day: Percentage of power is one of the prime determinants of fuel burn. But unless you follow "book" technique to the letter, your actual percentage of power—and the resulting fuel flow -- will differ from what's in the POH.

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