Under what condition can a lack of clearance in the valve-operating mechanism of an aircraft engine using hydraulic lifters be expected?

In an aircraft engine utilizing hydraulic lifters, a lack of clearance in the valve-operating mechanism can be expected during normal operation. Hydraulic lifters are designed to automatically adjust the valve clearance as the engine runs, using oil pressure to fill the lifter and maintain a zero clearance condition. This ensures that the valves open and close at the proper time, optimizing engine performance and preventing damage from excessive clearance.

During normal operation, the hydraulic lifters are fully engaged, and as the engine temperatures rise, the oil within the lifters allows them to expand and respond to the needs of the valve train. This automatic adjustment contributes to efficient engine operation as it compensates for thermal expansion of engine components.

In contrast, during start-up, the hydraulic lifters may not yet be fully pressurized, potentially leading to initial clearance until adequate oil flow is achieved. After an overhaul, the lifters may require some time to adjust to operational conditions again. Furthermore, in cold weather, oil viscosity can be an issue, which may prevent the lifters from operating efficiently until the engine warms up. Thus, the hydraulic lifters maintain their designed function best during the engine's normal operational state.