Definition 1 of 2
Definition
The internal fluid pressure that builds up inside the housing (the case) of a hydraulic pump, motor, or similar component, formed by small amounts of fluid that leak past internal moving parts during normal operation. This pressure is normally relieved through a dedicated case drain line that returns the fluid to the reservoir.
Plain English
The pressure that builds up inside the outer shell of a hydraulic pump or motor from fluid that slips past its internal seals while it runs. A separate small return line carries that fluid away so pressure doesn't build up too much inside the housing.
Context Anchor
Seen in aircraft hydraulic system maintenance, especially when checking pumps, motors, case drains, and return-flow problems.
Derivation
Case' here means the outer housing or shell of the component (from Latin 'capsa', meaning box or container). So 'case pressure' is simply the pressure inside that housing — not the main system pressure the pump is producing.
Why Pilots Care
Incorrect case pressure produces erroneous instrument readings that can mislead a pilot about airspeed or altitude, directly affecting flight safety and decision-making.
Grounding Statement
Picture fluid inside a pump housing: if that fluid cannot drain away normally, pressure builds inside the housing.
Intuition Check
Case pressure does not mean pressure on the outside of a case. It means pressure inside the component’s housing.
Example Sentence 1
The technician suspected a restricted case drain line because the pump was showing higher than normal case pressure during the ground run.
Example Sentence 2
A leak that changes case pressure will cause the vertical speed indicator to show incorrect rates of climb or descent.