Final answer:
The most likely cause of the engine's fan hitting the shroud when accelerating under a load is excessive fan blade flexing.
Step-by-step explanation:
The most likely cause of the engine's fan hitting the shroud when accelerating under a load is excessive fan blade flexing.
When the engine is under a load, the fan blades spin faster, causing them to flex. If the fan blades are not rigid enough or have become worn over time, they can contact the shroud. This can result in noise, damage to the fan, and reduced cooling efficiency.
To fix this issue, the fan blades can be replaced with stiffer ones or the shroud can be adjusted to provide more clearance for the fan blades.