What is the effect of altitude on gas turbine engine performance?

The correct answer highlights a fundamental aspect of gas turbine engine performance related to altitude. As altitude increases, air density decreases, which significantly impacts engine function. At high altitudes, the lower air density means there is less oxygen available for combustion. This reduction can lead to decreased engine thrust because the engine has less air to mix with fuel, resulting in less efficient combustion.

However, during cruise conditions at high altitudes, gas turbine engines, particularly turbofans, can operate more efficiently despite the reduced air density. This is because they are optimized to take advantage of thinner air, which allows for higher cruising speeds and lower drag, ultimately improving fuel efficiency. The engines can maintain thrust levels that are acceptable for cruise, which is why many long-haul flights prefer to operate at high altitudes where air resistance is minimal.

This understanding helps clarify why option C is accurate, focusing on the relationship between air density and engine efficiency during different phases of flight.