Recently, a student pilot posted a question on the Ask a CFI site a question about performance verses efficiency at higher altitudes. The poster began by noting that he or she knows that performance decreases with altitude and that performance is also decreased if there is high density altitude. He or she then wrote:
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.
In response, Andrew Leonard posted that performance is kind of a loose term that can be applied to several aspects of flying such as climb performance, engine performance etc. On the other hand, he noted that efficiency has almost entirely to do with fuel consumption by the aircraft’s engine. Hence, fuel efficiency will go up to a certain point and after that an aircraft’s efficiency will decrease as an aircraft’s engine will be unable to produce power in thinner air.
Meanwhile, Kent Shook’s response focused more on climb performance and cruise performance and then how engine performance affects them. He also pointed out that one must consider the length of the trip, the amount of time spent climbing at a higher power level and the winds that you encounter.
At the end of his response, Kent noted that:
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.
All points well worth noting.