Definition
A conceptual line representing all combinations of altitude and airspeed that share the same total mechanical energy (potential plus kinetic). Moving along this line means trading altitude for airspeed, or airspeed for altitude, without changing the airplane's overall energy state.
Plain English
It's the set of altitude-and-airspeed pairs where the airplane has the same total energy. You can swap height for speed or speed for height, but the total stays the same.
Context Anchor
Seen in energy-management discussions in the Airplane Flying Handbook, especially when learning how altitude and airspeed trade with each other during climbs, descents, and speed changes.
Derivation
Constant' means unchanging; 'total energy' is the sum of the airplane's energy from height (potential) and motion (kinetic). The phrase describes a path along which that sum stays the same.
Why Pilots Care
Helps pilots visualize and control energy trades during descents, approaches, and go-arounds so the airplane neither gains nor loses total energy unintentionally.
Analogy
Think of a roller coaster on a frictionless track: at the top of the hill it's slow but high; at the bottom it's fast but low. Same total energy, just redistributed.
Grounding Statement
Picture an airplane descending slightly and speeding up: it is giving up height and receiving speed in return, rather than gaining new energy.
Intuition Check
Do not think of this as a physical line in the sky or on the ground. Also, “constant total energy” does not mean altitude and airspeed are both constant; one may change while the other changes in the opposite direction.
Example Sentence 1
By pitching down without adding power, the pilot traded altitude for airspeed and stayed on the same line of constant total energy.
Example Sentence 2
On the energy diagram the airplane remained on the same line of constant total energy while the pilot adjusted pitch to trade height for speed.