Definition
The specific energy equation, expressing an airplane's total mechanical energy per unit weight as the sum of its height (h, potential energy per unit weight) and its kinetic energy per unit weight (V² divided by 2g, where V is true airspeed and g is the acceleration due to gravity). The result is expressed in units of height, allowing altitude and airspeed to be compared on the same scale.
Plain English
An airplane's total usable energy can be measured as a single 'height' number by adding its actual altitude to the extra height it could climb if it traded all its speed for altitude. Speed and altitude are just two forms of the same energy.
Context Anchor
Seen in energy management discussions, especially when learning how an airplane can trade speed for height or height for speed.
Derivation
ES stands for 'specific energy' — 'specific' here means 'per unit weight,' a common engineering convention that lets the equation produce a result in feet rather than foot-pounds. The h term is height; V²/2g comes from the standard kinetic energy formula (½mV²) divided by weight (mg), which cancels mass and leaves V²/2g.
Why Pilots Care
It shows pilots the total energy reserve they can use to climb, accelerate, or maintain flight without adding power.
Analogy
Think of height and speed as two forms of the same budget. You can spend speed to climb, or spend height to gain speed, but the total amount still matters.
Grounding Statement
An airplane at 3,000 feet flying fast has more total energy than the same airplane at 3,000 feet flying slowly — and this equation puts a number on that difference, in feet.
Intuition Check
Do not read this as simply “height plus speed.” The V²/2g part converts speed into a height-like value so it can be added to h correctly.
Example Sentence 1
Using ES = h + V²/2g, an instructor showed that diving to gain 20 knots costs roughly the same energy as descending a specific number of feet at constant speed.
Example Sentence 2
During the approach briefing the instructor pointed out that maintaining the proper energy state formula value kept the airplane on the correct glide path.