Definition
A condition in a reciprocating engine in which the valve springs are unable to close the intake or exhaust valves quickly enough to keep up with the camshaft at high RPM. The valves momentarily 'float' off their seats instead of following the cam profile, causing loss of power, rough running, and potential valve or piston damage.
Plain English
When an engine spins faster than the valve springs can handle, the valves don't close fully in time. They hang open for a split second when they shouldn't, which makes the engine run poorly and can damage parts.
Context Anchor
Seen in piston-engine operation and maintenance, especially when discussing engine overspeed, weak valve springs, rough running, or loss of power.
Derivation
Float' here borrows from the everyday sense of something hanging suspended rather than seating firmly. The valve, instead of snapping shut against its seat, hovers — it 'floats' above where it should be.
Why Pilots Care
It reduces engine power and can damage valves or seats if allowed to continue.
Analogy
It is like a spring-loaded door that is supposed to snap shut, but the spring is too weak or the door is moving too fast, so it bounces and stays partly open.
Intuition Check
Float does not mean the valve is floating in liquid or air. Here it means the valve is not being controlled firmly enough to open and close at the correct time.
Example Sentence 1
The mechanic warned that overspeeding the engine in a dive could cause valve float and damage the cylinder.
Example Sentence 2
During the post-flight inspection the mechanic looked for signs of valve float after the reported high-RPM incident.