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4 Answers

Performance vs. efficiency (at high altitudes)

Asked by: 9420 views ,
General Aviation, Student Pilot

In general, I know that performance decreases with altitude.  Performance is also decreased if we have a high density altitude.

However, isn't efficiency increased at higher altitudes? 

It seems as if these two statements contradict themselves.

An aircraft is more efficient at high altitudes b/c it consumes less fuel.  In addition, bad weather and turbulence may generally be avoided by flying above the storms. 

However, the density of the air is also decreased at higher altitudes...and I would think this, as well as another of other factors, would cause a decrease in performance.


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4 Answers

  1. Andrew Leonard on Nov 30, 2010

    Performance is kind of a loose term applied to several aspects of flying. For examle, you have climb performance, engine performance, and so forth. Most of them are affected in similar ways. As you go up in altitude, your climb performance is going definitively down the higher you climb.
    Effeciency has almost entirely to do with fuel consumption by the aircraft’s engine. The way I understand it, your efficiency goes up to a point, after which your fuel effenciency is going to decrease as your engine is unable to produce power in thinner air.
    Hopefully this clarifies things a bit. I’m not a huge expert on engines, though.

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  2. Student Pilot on Nov 30, 2010

    Andrew, yes.  And that’s exactly what I thought!  I was just wanted other people’s input as well.  I wasn’t ENTIRELY sure…

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  3. Kent Shook on Nov 30, 2010

    I’m going to focus on climb performance and cruise performance, as well as how engine performance affects them.
    Climb rate is a function of excess horsepower – Horsepower available above and beyond what it takes to hold an aircraft in level flight at climb speed (Vy). As you climb, if you’re flying in an airplane with a normally aspirated engine, the amount of power available is going to go down as the density of the air decreases. At around 7,500 feet, a normally aspirated engine will only be able to develop 75% power, and at around 11,500 feet it’ll only be able to develop 65% power. Since you probably need at least 40-50% power to maintain level flight depending on conditions, climb performance will be severely reduced at higher altitudes.
    Since those 65% and 75% numbers are normal cruise power settings pilots often use, those could be considered the most efficient cruise altitudes. If you’re going to cruise at 75%, you can do so down low, say 3,500 feet, or up higher at 7,500 feet and expect roughly the same indicated airspeed and the same fuel flow. However, at 7,500 feet your true airspeed will be higher, and thus more efficient. Same thing with going at 65% up at 11,500.
    However, that must be balanced with the length of the trip. Since fuel flows are significantly higher at climb power, the time spent in the climb at that higher power level must be considered. For example, if you’re only going 100nm, it probably doesn’t make sense to climb to 11,500 feet even though your cruise would be most efficient there, simply because you’d spend such a large portion of the flight climbing to that altitude that you wouldn’t be cruising there long enough to make up for the added fuel burn in the climb.
    Finally, if you’re looking for efficiency, you must consider winds aloft as well. If you’re westbound, generally you’ll have headwinds which will be stronger the higher you climb. Your true airspeed may improve, but the winds may negate the gain entirely. However, if you’re going to have a tailwind up high, it may make sense to climb higher than you otherwise would to take advantage of them.
    Performance and efficiency are obviously affected by several factors, and those factors may change with every flight. But, that’s one thing I love about flight planning is putting all those factors together and determining the most favorable cruise altitude – And, of course, performance and efficiency are only part of the equation, you also need to consider safety and passenger comfort as well. Hope this helps!

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  4. Brian on Nov 30, 2010

    At high altitude efficiency (miles covered per gallon of fuel) is increased because TAS go up. Fuel burn (rate burned per hour) for the given power setting remains relatively unchanged.* 
    *Note: Power setting refers to a percentage of power, such as 65 percent.
    Cessna 172R 5-17 & 5-18 out of the POH:
    2,000 feet PA – 65 percent power – 7.5 GPH – 106 TAS == efficiency 14.1 miles per gallon 
    12,000 feet PA – 65 percent power – 7.5 GPH – 113 TAS == efficiency 15.1 miles per gallon
    See it?
    PS: PA refers to pressure altitude in case your not used to that nomenclature. 

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