Question 69 AEL01 - Assistant Engineer - Limited

The Correct Answer is A **Explanation of Option A (Correct):** A safety shutdown due to excessive crankcase pressure (often detected by a specialized sensor, like a Graviner detector) indicates a potential internal problem, such as overheating of a bearing, piston skirt, or other moving part (a "hot spot"). When a hot spot exists, the sudden stoppage of the engine can cause the heat to dissipate into the surrounding crankcase atmosphere, vaporizing lubricating oil and creating a highly flammable, potentially explosive mixture of oil mist and air. If the crankcase is opened immediately, fresh air (oxygen) rushes in and mixes with this hot, rich oil mist, which could be ignited by the remaining hot spot, leading to a violent crankcase explosion. The mandatory 2-hour waiting period allows the engine to cool sufficiently, ensuring that any remaining hot spots have cooled below the auto-ignition temperature of the oil mist, thereby eliminating the risk of explosion when the crankcase is opened. This procedure is a critical safety requirement mandated by classification societies and regulatory bodies (e.g., SOLAS, IMO). **Explanation of Incorrect Options:** * **B) Opening the crankcase before 2 hours has elapsed may result in the engine spontaneously restarting.** This is incorrect. Once the engine control system initiates a safety shutdown, the fuel rack is zeroed and the engine cannot spontaneously restart without an external action (like turning the starting air valve). * **C) Opening the crankcase before 2 hours has elapsed may result in crankshaft rotation.** This is incorrect. While the crankshaft *might* rotate slightly due to thermal expansion/contraction or slight residual pressure, the risk of a major rotation is negligible and has no direct relevance to the mandatory 2-hour cooling period, which is purely based on explosion risk. * **D) Opening the crankcase before 2 hours has elapsed may result in excessively rapid cooling.** This is incorrect. While rapid cooling *can* cause thermal stress, the 2-hour wait is mandated to prevent an explosion, not to manage cooling rates. Opening the crankcase simply equalizes pressure and introduces cooler ambient air, which would actually facilitate cooling, but the primary concern is the potential for explosion due to hot spots.