Definition
The maximum manifold pressure value, marked by a red line on the manifold pressure gauge, that the engine manufacturer permits for continuous operation. Exceeding this value risks detonation, overheating, and engine damage.
Plain English
It is the highest engine power setting allowed, shown as a red mark on the gauge. Pushing the throttle past it can damage the engine.
Context Anchor
A pilot encounters this while setting takeoff or climb power in an airplane equipped with a manifold pressure gauge.
Derivation
The phrase comes from the practice of marking gauge limits with coloured arcs and lines. A red line means 'do not exceed.' Manifold pressure refers to the air pressure inside the engine's intake manifold, which indicates how much power the engine is producing.
Why Pilots Care
Exceeding this limit can cause detonation, structural damage, or sudden engine failure.
Intuition Check
Do not treat the red line as something to exceed briefly or as a general “high power” suggestion. In this context, the red line is a maximum limit on the manifold pressure gauge.
Example Sentence 1
During the takeoff roll, the pilot advanced the throttle smoothly until manifold pressure reached, but did not exceed, the red line.
Example Sentence 2
In the climb checklist the student confirmed they were not exceeding red line manifold pressure.