Powered vs unpowered descent
In a powered descent, the engine runs but thrust is reduced below what drag requires. The flight path angles downward, and the aircraft trades altitude for continued flight. Approach to landing is almost always a powered descent.
In an unpowered descent (glide), thrust is zero — the engine is at idle or off. Weight now has a component along the downhill flight path that replaces thrust, keeping the aircraft moving forward. The aircraft trades altitude for airspeed to sustain lift.
Glide ratio
Glide ratio is the horizontal distance traveled per unit of altitude lost. It equals the aircraft's lift-to-drag ratio at the best glide speed.
| Glide ratio formula | Horizontal distance ÷ altitude lost |
| Equals | Lift ÷ Drag at best glide speed |
| Cessna 172 (typical) | ~9:1 (9 ft forward per 1 ft of altitude) |
| Glider (typical) | 30:1 to 60:1 |
Glide range from altitude: Multiply your altitude (in feet) by the glide ratio to get maximum horizontal range (in feet) with no wind. From 10,000 ft in a Cessna 172: 10,000 × 9 = 90,000 ft ≈ 17 miles.
Related terms
- Level Flight — the balanced state
- Climb — the opposite state
- Lift and Drag — their ratio determines glide performance