What is the maximum allowed differential pressure during a compression check on a reciprocating engine?

The maximum allowed differential pressure during a compression check on a reciprocating engine is 25%. During a compression check, the purpose is to assess the condition of the engine's cylinders and valves. The differential pressure is calculated by comparing the maximum cylinder compression to the overall static pressure of the cylinder when the engine is cranked.

A differential pressure of 25% indicates that the engine's cylinders are generally in good condition, allowing for some variations due to factors like wear and tear. Exceeding this threshold may indicate significant issues such as worn piston rings, leaking valves, or other internal problems that could affect engine performance. This percentage helps maintain a standard for reliable operation and safety in aviation.

Understanding this limit is crucial for mechanics and inspectors as it directly impacts the engine's performance and reliability in the field.