An engine exhaust-stack plug for sealing an exhaust stack of an aircraft engine is provided. The exhaust-stack plug includes a desiccant and a flexible plug adapted to slide into an outlet of an engine exhaust-stack. The flexible plug includes a front panel that is substantially impermeable to water vapor and is accessible from the outlet of the engine exhaust-stack. An outer wall, which is also substantially impermeable to water vapor, is sealed with the front panel and shaped to fit snuggly within the outlet of the engine exhaust stack. A backside is coupled with the outer wall, opposite the front panel, for facing internal to the engine exhaust stack. The front panel, the outer wall, and the backside form an enclosure adapted to contain the desiccant. The backside is substantially air permeable such that the desiccant reduces humidity and prevents condensation inside the aircraft engine.