Definition
The amount of air, by weight, passing through the engine per unit of time. In turbine and turbocharged engines, mass flow — not just volume — determines the power available, because thinner (less dense) air contains fewer molecules to burn fuel with, even if the volume of air entering the engine is the same.
Plain English
How much actual air, measured by weight, is moving through the engine each second. Thin air weighs less, so even if the same amount of space is filled with air, there is less of it to make power.
Context Anchor
Seen in engine power, propeller thrust, and high-altitude performance discussions.
Derivation
Mass' refers to the weight or quantity of matter, and 'flow' to its movement over time. The phrase highlights that what matters to the engine is the weight of air passing through, not just the volume of space it fills.
Why Pilots Care
Mass flow directly determines how much thrust the engine can produce.
Grounding Statement
Two breaths of the same size — one at sea level, one on a mountaintop — fill your lungs equally, but the mountain breath weighs less and gives you less oxygen. The engine experiences the same thing.
Intuition Check
Mass flow is not the same as airspeed. Air can move quickly but still have low mass flow if the air is thin.
Example Sentence 1
At higher altitudes, mass flow through the engine decreases as the air becomes less dense, reducing available power.
Example Sentence 2
At higher altitudes the reduced air density lowers mass flow through the engine.