At what altitude range does a turbofan engine achieve maximum efficiency?

Turbofan engines achieve maximum efficiency at higher altitudes where the air is less dense, allowing the engine to produce thrust more efficiently. This results in improved fuel consumption and performance. The altitude range of 33,000 to 37,000 feet is optimal for commercial jet operations, as it balances the benefits of reduced drag with engine performance, allowing for longer flight durations and better fuel economy.

Flying at this altitude enables aircraft to take advantage of thinner air, which reduces the amount of power needed for thrust and allows the engines to operate in a more efficient phase of their thrust and fuel consumption curve. In contrast, the other altitude ranges mentioned do not provide the same level of efficiency due to denser atmospheric conditions, higher drag, and increased fuel consumption relative to the thrust produced.