How much of the 90 pounds of oil on board the PA-28-235 is considered "unusable"?

In the context of the PA-28-235, the term "unusable oil" refers to the quantity of oil that cannot be drawn upon by the engine during operation, primarily due to the design of the oil system and the locations of the oil tanks. For this specific aircraft, the amount of unusable oil is stated to be 2.4 pounds.

This figure is important for pilots and maintenance personnel to understand, as it ensures that they account for only the usable oil when calculating pre-flight checks and maintenance procedures. Knowing the correct quantity of usable versus unusable oil helps in preventing issues such as oil starvation and ensures that the aircraft operates within safe limits.

This aspect of aircraft systems is critical for engine reliability and performance, and being aware of these specifics allows for better maintenance practices. Thus, the answer indicating that 2.4 pounds of oil is considered unusable for the PA-28-235 is aligned with standard practices in aviation maintenance.