Definition
On an energy-state diagram, curved lines that connect every combination of altitude and airspeed producing the same total mechanical energy per unit weight. A point on a higher line represents more total energy (the sum of potential energy from altitude and kinetic energy from airspeed); a point on a lower line represents less. Movement along a single line means trading altitude for airspeed, or airspeed for altitude, with no change in total energy.
Plain English
Lines on a chart that show all the altitude-and-airspeed combinations that add up to the same total energy. If you stay on one line, you can swap height for speed or speed for height, but you still have the same overall energy.
Context Anchor
Seen in energy management discussions and diagrams that show how an airplane can trade airspeed for altitude, or altitude for airspeed.
Derivation
Constant' means unchanging, and 'energy height' is a way of expressing total mechanical energy as if it were all converted into altitude. So a 'line of constant energy height' is a line along which that total stays the same, even though altitude and airspeed are shifting.
Why Pilots Care
Helps pilots understand safe energy trades during maneuvers, emergencies, or performance planning without unintended energy loss.
Grounding Statement
Picture climbing a hill on a bicycle without pedaling: as you go up, you slow down; as you come down, you speed up. The total energy stays roughly the same, just shifting between height and speed. Each line on the diagram represents one such 'total energy' level.
Intuition Check
Do not read “energy” here as electrical power or engine power only. Here it means the airplane’s combined ability to climb because of altitude and to keep moving because of speed. Do not read “height” as altitude alone. Here it means total energy expressed as an equivalent height.
Example Sentence 1
By pitching up smoothly, the pilot traded airspeed for altitude and moved along the same line of constant energy height.
Example Sentence 2
Using lines of constant energy height, the pilot determined the maximum zoom-climb altitude reachable from the current airspeed without adding power.