Definition
The angle of attack and corresponding airspeed at which the airplane produces the greatest amount of lift relative to drag. At this point, the airplane is operating at its most aerodynamically efficient condition, which in a power-off glide produces the longest possible glide distance for a given altitude.
Plain English
It is the speed and angle that lets the airplane travel the farthest through the air for the least loss of altitude. In a glide, flying at maximum L/D gives you the most distance from the height you have.
Context Anchor
Seen in glide discussions, especially when learning best glide speed and power-off landing planning.
Derivation
L/D is simply the ratio of lift to drag, two of the four forces acting on an airplane. 'Maximum' marks the single point on the performance curve where this ratio is at its highest value. Every other airspeed produces a lower ratio and therefore a steeper glide.
Why Pilots Care
It identifies the airspeed that produces the longest glide distance, which is critical for reaching a suitable landing spot after total power loss.
Grounding Statement
At maximum L/D, each foot of altitude buys the most forward distance the airplane can get in a glide.
Intuition Check
Maximum L/D does not mean maximum lift, and it does not mean the airplane stays up for the longest time. It means the best balance of lift compared with drag, giving the greatest glide distance.
Example Sentence 1
When the engine quit, the pilot pitched for maximum L/D and turned toward the nearest airport.
Example Sentence 2
The handbook lists the indicated airspeed for maximum L/D at the airplane's current weight so the pilot can set the proper pitch attitude.