What happens to an unsupercharged aircraft reciprocating engine when operating at full throttle from sea level to 10,000 feet?

Operating an unsupercharged aircraft reciprocating engine at full throttle from sea level to 10,000 feet results in a loss of power due to reduced air density. At higher altitudes, the atmosphere becomes thinner, meaning there are fewer air molecules available for combustion in the engine's cylinders.

As the engine relies on air to mix with fuel for combustion, the decrease in air density significantly affects the engine's performance. Specifically, the engine must work harder to intake enough air to maintain optimal combustion conditions, leading to a reduction in the overall power output.

At sea level, the engine functions optimally with a rich air-fuel mixture, but as altitude increases, the thinner air results in a leaner mixture, reducing engine power. This is a critical consideration for pilots flying at various altitudes, as they must be aware of how engine performance changes with density altitude.