During warm weather operations, what condition makes the fuel system susceptible to vapor formation?

The condition that makes the fuel system susceptible to vapor formation during warm weather operations is associated with ground operations at low RPM. When an aircraft operates on the ground at low RPM, the fuel pumps may not circulate the fuel effectively. Insufficient fuel flow can lead to localized heating of the fuel, especially if the ambient temperature is high, which increases the likelihood of vapor formation. Vapor can create bubbles in the fuel lines, potentially leading to vapor lock, where the fuel cannot reach the engine due to the presence of gas instead of liquid fuel.

In contrast, high altitude flights often see decreased temperatures, which would support liquid fuel rather than vapor formation. Low ambient temperatures would similarly reduce the risk of vapor formation. Having heavy fuel loads can also help in keeping fuel temperatures down, as the larger mass of fuel absorbs heat more effectively, reducing the risk of vaporization.