In an internal combustion engine, the fuel-air mixture is correct at full rich only at:

In an internal combustion engine, the fuel-air mixture is ideally set to "full rich" at sea level because at this altitude, the air density is highest. The "full rich" setting means that the mixture contains more fuel relative to air, which is necessary for optimal combustion under standard atmospheric conditions.

At sea level, the atmospheric pressure allows for a higher concentration of oxygen in the intake air, which supports the combustion of that richer fuel mix effectively. As altitude increases, the air becomes less dense, resulting in lower oxygen levels. Consequently, if the mixture remains at the "full rich" setting at higher altitudes, it could lead to inefficient combustion, potentially causing engine performance issues because too much fuel would not burn completely due to insufficient oxygen.

Therefore, at sea level, the fuel-air mixture can be set to "full rich" for optimal performance, while adjustments are necessary as the aircraft climbs to higher altitudes to maintain the correct fuel-air ratio for efficient combustion.