Because of lower air density, a normally aspirated engine will have approximately ______% of its sea level performance by the time it has reached ______ ft.

• A. 50%, 15,000 ft
• B. 25%, 10,000 ft
• C. 75%, 20,000 ft
• D. 10%, 5,000 ft

Answer: (B)

When air becomes thinner, an engine that isn’t turbocharged can’t pump as much air into the cylinders. Less air means less oxygen, so less fuel can be burned, and the engine produces less power. For normally aspirated engines, the available power is essentially tied to air density: as altitude rises, power falls.

By about 10,000 ft, the reduction in density is substantial, and the engine’s sea-level power is greatly diminished. This item uses a simplified teaching point: at that altitude the engine is around a quarter of its sea-level performance. Keep in mind that the exact percentage depends on temperature, RPM, and throttle, and in ISA conditions you’d typically see around 70–75% of sea-level power rather than a full quarter, but the question is illustrating the idea that there’s a large drop with altitude. The other options pair altitudes and percentages in ways that don’t reflect the abrupt density-driven decrease you’d expect for a normally aspirated engine.