Definition
A line drawn on an energy state diagram that connects all combinations of altitude and airspeed which represent the same total mechanical energy (potential plus kinetic) for a given airplane. Points along the same energy height line can theoretically be exchanged for one another by trading altitude for airspeed, or vice versa, without adding or losing energy.
Plain English
It's a line on a chart showing all the height-and-speed combinations that add up to the same total energy. If you're on the line, you can climb by giving up speed, or speed up by giving up altitude, and end up at another point on the same line.
Context Anchor
Seen in the Airplane Flying Handbook’s discussion of energy management, especially when comparing altitude, airspeed, climbs, descents, and glide capability.
Derivation
From 'energy height,' a term used in performance theory to express an aircraft's total mechanical energy as an equivalent altitude — the height the airplane would reach if all its kinetic energy (speed) were converted into potential energy (altitude). The 'line' simply connects all points sharing that same total energy value.
Why Pilots Care
Helps pilots visualize how total energy remains constant when trading speed for altitude or altitude for speed, supporting precise energy management during maneuvers and approaches.
Grounding Statement
Picture a glider at 3,000 feet flying slowly versus the same glider at 1,000 feet flying fast — both can hold the same total energy, and both sit on the same energy height line.
Intuition Check
Do not read this as a real line in the sky or as altitude alone. It is a picture of total airplane energy, with speed converted into an altitude-like amount.
Example Sentence 1
By trading 500 feet of altitude for additional airspeed, the pilot moved along the energy height line without needing to add power.
Example Sentence 2
Maintaining position on the energy height line during the approach ensured enough energy remained to reach the runway threshold.