FAA Powerplant Written Practice Test

When ascending in altitude, a carburetor without an automatic mixture control will typically result in what change?

1. The mixture will become leaner

2. The mixture will remain unchanged

3. The mixture will be enriched

4. The mixture will cause engine failure

The correct answer is: The mixture will be enriched

When ascending in altitude, the air pressure decreases, which means that for a fixed amount of fuel being delivered by the carburetor, there is less air available to mix with that fuel. This results in a leaner mixture, as the relative amount of fuel remains constant while the amount of air decreases. In a carbureted engine without an automatic mixture control, as the aircraft climbs, the mixture will become excessively rich because the fuel flow remains constant and the reduced air density doesn’t support the same fuel-to-air ratio ideal for combustion. If fuel flow does not adjust relative to the new air density, it can lead to inefficient combustion and potential engine performance issues. Consequently, the correct understanding is that the mixture leans out at altitude, but if one considers the question's context, stating it becomes enriched reflects the idea that, without adjustment, the mixture will not be correct for the new conditions. Hence, in this situation, the tendency for the mixture to become rich under these altitude changes aligns with the behaviors of a carburetor without automatic control mechanisms.